V51B-0522

Melting Behaviors of Recycled Subducted Crust in the Mantle: Constraints from Melting Phase Relations of Pyroxenites

* Kogiso, T (kogisot@jamstec.go.jp) , JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
Hirschmann, M M (Marc.M.Hirschmann-1@umn.edu) , Univ. Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States

Ocean island basalts (OIB) have isotopic and trace element signatures that are considered to originate in subducted mafic oceanic crust. Recent experimental studies on mafic lithologies (pyroxenites) revealed that diversity in major element composition of OIB can also be explained by partial melting of a variety of pyroxenites derived from subducted mafic crust. Most part of subducted oceanic crust has MORB-like (silica-excess) composition, but the uppermost part of the subducted oceanic crust suffers extraction of small degree partial melts or siliceous hydrous fluids during subduction, resulting in formation of bimineralic pyroxenite which consists solely of garnet and clinopyroxene. Partial melts from MORB-like pyroxenite at high pressures have silica-saturated compositions, which have similarities to silica-rich tholeiitic OIB [1], whereas partial melts from bimineralic pyroxenite have silica-undersaturated compositions similar to alkalic OIB lavas [2]. On the other hand, subducted oceanic crust is hybridized with surrounding mantle peridotite through mechanical and diffusive interactions with peridotite, yielding olivine-bearing pyroxenite. To investigate melting behaviors of such olivine-bearing pyroxenite, we have conducted partial melting experiments on a mixture of bimineralic eclogite (B-ECL1) with 5 % Fo60 olivine (bulk Mg\# = 61) at 5 GPa. The experimental results show that addition of olivine to B-ECL1 expands the stability of garnet in the residue, resulting in producing liquids richer in SiO$_{2}$ and alkalis and poorer in Al$_{2}$O$_{3}$ than those of B-ECL1. At $1600\deg$C, the degree of partial melting of the B-ECL1 + olivine mixture is higher than that of B-ECL1, suggesting that the solidus temperature is lowered by addition of olivine. However, experiments on a Mg-rich pyroxenite (MIX1G: bulk Mg\# = 79), which has more olivine component, have shown that the solidus of MIX1G is higher than $1600\deg$C [3]. These results suggest that bimineralic pyroxenite can produce larger amounts of partial melts when it is mixed with smaller amounts of olivine. This further indicates that upwelling mantle containing recycled crust with bimineralic composition would begin to melt at the boundary between pyroxenite and peridotite, producing strongly alkalic liquids first. [1] Pertermann and Hirschmann 2003, J.Petrol. 44, 2173. [2] Kogiso and Hirschmann 2002, EOS 83, V71C-12. [3] Kogiso et al. 2003, EPSL 216, 603. Back

V51B-0523

The Role of Garnet Pyroxenite in High-Fe Mantle Melt Generation: High Pressure Melting Experiments

* Tuff, J (jtuf02@esc.cam.ac.uk) , Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
Takahashi, E (etakahas@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Institute for Research on Earth Evolution, Jamstec, 2-15 Natsushima-Cho, Yokosuka, 237-0061 Japan
Gibson, S (sally@esc.cam.ac.uk) , Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom

Evidence for the existence of heterogeneous or 'marble cake' convecting mantle$^{1}$ is provided recently by rare, high MgO ($\sim$ 15 wt.%) primitive magmas with anomalously high abundances of FeO* ($\sim$ 13.5 to 16 wt. %$^{2,3}$; where FeO* = total Fe as FeO). These high-Fe mantle melts show a limited occurrence in the initial stage of magmatism in large igneous provinces (e.g. Deccan, Ethiopia and Paran\'{a}-Etendeka) and some have incompatible trace-element and radiogenic-isotopic ratios (Sr, Nd and Pb) that resemble those of ocean-island basalts. This suggests that they are predominantly derived from the convecting mantle$^{2}$. The ferropicrites are mildly- to sub-alkaline and have low contents of Al$_{2}$O$_{3}$ ($<$ 10 wt.%) and heavy rare-earth elements (e.g. Lu $<$ 0.18ppm) that are consistent with the increased stability of garnet, due to the high FeO* content in the ferropicrite mantle source. It has been proposed that the source of the high FeO* may be garnet-pyroxenite streaks derived from subducted mafic oceanic crust$^{2}$. We have undertaken melting experiments between 1 atmosphere and 7 GPa in order to determine the anhydrous phase relations of an uncontaminated ferropicrite lava from the base of the Early-Cretaceous Paran\'{a}-Etendeka continental flood-basalt province. The sample has high contents of MgO ($\sim$ 14.9 wt.%), FeO* (14.9 wt.%) and NiO (0.07 wt.%). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the host rock, assuming a K$_{d}$ of 0.32 and we believe that the sample is representative of a primary Fe-rich mantle plume derived melt. In total, 75 experimental runs were carried out. Melting phase relations as well as compositions and modal proportions of all coexisting phases were successfully determined in 60 run products. Phase relations indicate that the ferropicrite melt was generated either at $\sim$ 2.2 GPa from an olivine-pyroxene residue or $\sim$ 5 GPa from a garnet-pyroxene residue. A low bulk-rock Al$_{2}$O$_{3}$ content (9 wt.%) and high [Gd/Yb]$_{n}$ ratio (3.1) are consistent with residual garnet in the ferropicrite melt source and favour high-pressure melting of garnet-pyroxenite. The garnet pyroxenite may represent subducted oceanic lithosphere entrained by the upwelling Tristan mantle plume starting-head. During adiabatic decompression, intersection of the garnet pyroxenite solidus at $\sim$ 5 GPa would occur at mantle potential temperatures of $\sim$ 1550$\deg$C. Subsequent melting of peridotite at $\sim$ 4.5 GPa may be restricted by the thick overlying sub-continental lithosphere such that dilution of the garnet-pyroxenite component would be significantly less than in intra-plate oceanic settings. This model accounts for the limited occurrence of ferropicrite magma in the initial stage of continental large igneous provinces and its absence in ocean-island basalt successions. $^{1}$ All\`{e}gre {\it et al}., Philosophical Transactions of the Royal Society of London A297, 447-477 (1980). $^{2}$ Gibson {\it et al}., Earth and Planetary Science Letters 174, 355-374 (2000). $^{3}$ Gibson, Earth and Planetary Science Letters 195, 59-74 (2002). Back

V51B-0524

A Global Dataset of Noble gas Concentrations and Their Isotopic Ratios in Volcanic Areas

* Abedini, A A (aabedini@usgs.gov) , U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
Hurwitz, S (shaulh@usgs.gov) , U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States

The extent to which ocean islands are derived from the deep mantle (mantle plumes) or from chemical heterogeneities embedded within the mantle convective flow has long been debated. Noble gases have unique properties that provide significant information regarding this debate and make them important geodynamic tracers. The study of noble gas isotopic compositions in active tectonic and volcanic areas is central to understanding the origins of major volcanic anomalies. For example, helium isotope composition is considered to be the most unambiguous geochemical indicator of a lower mantle plume component in surface rocks, and its variability is often taken as the strongest evidence for a layered mantle. We have compiled all the published global noble gas data from MOR, ocean islands, seamounts, and volcanic arcs, extending existing datasets which are limited to samples from oceanic rocks (Farley and Neroda, 1998; Graham, 2002). Our data set contains information on helium, neon, argon, and xenon concentrations, as well as their isotopic ratios. Where available we also included the isotopic ratios of lead, strontium, neodymium, and carbon. Overall, there are more than 5,000 entries in the database, which is sub-divided both by material sampled (e.g., volcanic glass, different minerals, fumarole, spring) and by tectonic setting (MOR, ocean islands, volcanic arcs). Our extended dataset is consistent with earlier studies and shows that helium isotope ratios in MORB glass in the Pacific, Indian and Southern Atlantic Oceans have a sharp peak between 7 and 9 RA. The large peak in MORB samples correlates with the maxima in samples from volcanic arcs, probably implying that the upper mantle has a uniform helium isotope ratio. In the North Atlantic there is a broader distribution with a maximum at 11 Ra. In contrast, helium isotope ratios in ocean-island basalts (OIB) are highly variable. As noted by previous workers, the helium isotope ratios in OIB have a large peak at 8 RA and a second peak at 13 RA, separated by a pronounced minimum at about 10 RA. In most cases, higher-than-MORB He-isotope ratios coincide with deep mantle plumes revealed by seismic tomography (Montelli et al., 2004). The database will be available through the World Wide Web and will allow examination of some unresolved scientific problems. Farley K.A., Neroda E., Ann. Rev. Earth Planet. Sci., 1998. Graham, D. W., Rev. Mineral. Geochem., 2002. Montelli, R. et al., Science. 2004. Back

V51B-0525

THE CHEMICAL SURFACE SIGNATURE CREATED BY UPWELLING PLUMES

* Schmalzl, J (joergs@uni-muenster.de) , University Muenster Institute of Geophysics, Corrensstr. 24, Muenster, NRW 48149 Germany
Hansen, U (hansen@earth.uni-muenster.de) , University Muenster Institute of Geophysics, Corrensstr. 24, Muenster, NRW 48149 Germany

Convective flows govern much of the dynamics of the Earth. Examples of such flows are convection in the Earth's mantle, convection in magma chambers and much of the dynamics of the world oceans. Nowadays these time-dependent flows are often studied by means of three dimensional (3D) numerical models which solve the equations for the transport of heat and momentum alternatingly. These flows are often driven by a temperature difference. But for many flows there is also an active or passive chemical component that has to be considered such as entrainment of material from an underlying layer. One characteristics of these flows is that the chemical diffusivity is very small. Implementing such a chemical field with a very low diffusivity into a numerical model using a field approach is difficult due to numerical diffusion introduced by the Eulerian schemes. We have implemented a tracer-mesh method which tracks only the position of the interface between the two different components. While the thermal shape of a plume for conditions as appropriate for the Earth's mantle is cylindrical in an first order approximation the shape of the entrained chemical material also has a very strong sheet-like component. This can be important for the understanding of the observed isotopic variability of plumes. Back

http://earth.uni-muenster.de/~joergs

V51B-0526

Limitations on the Estimation of Parental Magma Temperature Using Olivine-melt Equilibria: Hotspots Not So Hot

* Natland, J H (jnatland@rsmas.miami.edu) , RSMAS/MGG University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149 United States

Estimates of temperatures of magmas parental to picritic tholeiites using olivine-melt equilibria and FeO-MgO relationships depend strongly on the assumption that a liquid composition, usually a glass, is related to the most magnesian olivine in the rock, or to an olivine composition in equilibrium with mantle peridotite, along an olivine-controlled liquid line of descent. The liquid Fe$^{2+}$/Fe$^{3+}$ also has to be known; where data exist, average values from wet chemical determinations are used. Crystallization histories of tholeiitic picrites from islands, spreading ridges, and large igneous provinces, however, usually reveal them to be hybrid rocks that are assembled by two types of magma mixing: 1) between a) differentiated magmas that are on olivine-plagioclase or olivine-plagioclase-clinopyroxene cotectics and b) crystal sludges with abundant olivine that may have accumulated from liquids crystallizing olivine alone; and 2) between primitive magma strains in which olivine crystallized either alone or with other silicate minerals at elevated pressure on separate liquid lines of descent. Many picrites give evidence that both types of mixing have occurred. If either type has occurred, the assumption of olivine-control linking a glass and an olivine composition can only circumstantially be correct. Oxidation state can also be underestimated and therefore FeO contents overestimated if basalts have degassed S, as at Hawaii. In Case 1, hybrid host glass compositions often have higher FeO at given MgO content than liquids which produced many olivine crystals in the rock. In Case 2, the separate parental melt strains are revealed by diversity of compositions of both melt inclusions and Cr-spinel and are most often interpreted to mean local heterogeneity of the mantle source. The inclusions do not always affirm an olivine-controlled liquid line of descent. Instead, inclusions with $<$13% Al$_{2}$O$_{3}$ are increasingly interpreted from both major oxides and trace elements to be derived from melt strains produced by partial melting of both depleted and enriched pyroxenite or recycled ocean-crust (eclogite) (e.g., refs.1 and 2). Some Icelandic picrites also contain large phenocrysts of plagioclase and clinopyroxene; their abundant olivine evidently resulted from mechanical processes of concentration of olivine such as flowage differentiation. Using compositions of low-Al2O3 melt inclusions and host liquids to estimate spinel compositions (ref. 3) reveals many instances of crystallization at higher oxidation states than occur during MORB crystallization, and successfully predicts presence of spinel with Cr/(Cr+Al) = 60-75 actually found in picrites from Hawaii, Iceland, elsewhere in the North Atlantic Igneous Province, and the komatiites of Gorgona, but not in MORB. Where fresh glass is lacking (e.g., Gorgona), bulk-rock compositions have been used to reconstruct conditions of crystallization of parental liquids; but this is greatly complicated by the type and extent of alteration of the rocks. The consequence of all of these factors is that FeO in presumed olivine-controlled liquids is often overestimated, thus many estimated temperatures of crystallization of primitive magnesian liquids are too high by as much as 50-100$^{o}$ absolute, and derived potential temperatures consequently are too high by more than this. (1) Hansteen, T., 1991. Contrib. Mineral. Petrol. 109, 225. (2) Sobolev, A., Hofmann, A., and Nikogosian, I., 2000. Nature, 404, 986. (3) Poustovetov, A., and Roeder, P., 2001, Canad. Mineral. 39, 309. Back

V51B-0527

Two-Stage Melting Of Mantle Plumes And The Origin Of Rejuvenescent Volcanism On Oceanic Islands

* White, W M (white@geology.cornell.edu) , Cornell University, Dept. of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States
Morgan, J P (jpm@geology.cornell.edu) , Cornell University, Dept. of Earth and Atmospheric Sciences, Ithaca, NY 14853 United States

Many mid-plate oceanic volcanoes experience a rejuvenescent, or "post-erosional" phase of volcanism that occurs hundreds of thousands or million years after the main shield-building phase of volcanism has ended. The Hawaiian Islands are the best-documented example, but rejuvenescent volcanism also occurs on the Society Islands, the Marquesas, the Australs, Samoa, and Mauritius. It does not occur on near-ridge islands such as the Galapagos, the Azores, and Iceland. Rejuvenescent lavas have a number of features in common: they are erupted in small volumes, they are highly enriched in incompatible elements, and they are highly alkalic, typically basanitic to nephelenitic. All these features suggest they are quite small degree melts. In addition, rejuvenescent magmas have more depleted isotopic signatures, implying they are melts of more depleted sources, despite their strong incompatible element enrichment. Although isotopic signatures of these lavas are more depleted that those of the corresponding shield stage lavas, they are nevertheless not as depleted as MORB. Furthermore, the isotopic compositions of the rejuvenescent magmas rule out their sources being mixtures of plume material and MORB-source material. Thus geochemical considerations rule out both the lithosphere and the asthenosphere surrounding the plume as the source of rejuvenescent magmas; this implies the plume itself must be the source of rejuvenescent magmas. This conclusion is consistent with geophysical models of plumes. Finite difference numerical models of plume-lithosphere interaction that include both temperature and compositional viscosity dependence reveal that while most melting is concentrated above the hot core of the plume, a melting "tail" extends hundreds of km downstream. In this tail region, lateral spreading of the plume results in a slight rising motion of the plume, and consequently, small extents of melting. The problem thus becomes that of deciphering why melts produced in this tail region are isotopically distinct from those produced in the main melting region. We propose the following model to explain this difference: Mantle plumes are lithologically heterogeneous, consisting of eclogite or pyroxenite "plums" that have a solidus temperature several tens of degrees lower than the more refractory peridotite "pudding" in which they are embedded. Complete isotopic equilibrium is not achieved during melting - either because the plums are large enough ($>$10-100m) or the extraction of plum melts is rapid after their generation. Both the plums and the peridotite are incompatible-element enriched relative to the average depleted upper mantle, but the plums are substantially more enriched. The plums melt entirely in the base of the main melting region and the heat so consumed initially suppresses melting of the peridotite pudding. Plum-derived melts mix as they rise with melts of the peridotite pudding produced higher in the main melting region. This mixture of eclogitic and peridotitic melts form the shield stage magmas. Material in the melting "tail" has had the plums melted out of it in the main melting region. Low degree melting of the plum-free peridotite in the melting tail gives rise to rejuvenescent magmas. Melt production in the tail is more or less continuous, but rejuvenescent volcanism is not. This suggests that some other factor is involved, such as lithospheric loading by adjacent volcanoes, that provides pathways to the surface for small degree tail melts. Back

V51B-0528

How Many Plumes In Africa ? The Geochemical Point of View

PIK, R (rpik@crpg.cnrs-nancy.fr) , CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 France
* Marty, B (bmarty@crpg.cnrs-nancy.fr) , CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 France
* Marty, B (bmarty@crpg.cnrs-nancy.fr) , ENSG, BP40, Vandoeuvre-Les-Nancy, 54501 France
Hilton, D (drhilton@ucsd.edu) , SCRIPPS, 8675 Discovery Way, La Jolla, CA 92093 United States

Since the Oligocene, volcanic activity in Africa was particularly important in the Horn of Africa where ~ 1 million km3 of continental flood basalts (the Ethiopian CFB) erupted 30 Ma ago in a time interval of 1-2 Ma or less. The Afar volcanic province which is still magmatically active is thought to represent the surface expression of a deep mantle plume, a view consistent with ultra-low velocity anomalies at the base of the mantle beneath the African superswell and the Ethiopia-Afar volcanic province. This plume origin is also supported by the occurrence of 3He/4He ratios up to 20 Ra (Ra is the 3He/4He ratio of atmospheric helium) much higher than those of mid-ocean ridge basalts (on average, 8,b1 Ra) and thought to characterize mantle material originating from below the 660 km discontinuity. However, a deep mantle origin for "high 3He" material is currently questioned by some models which rather ascribe a lithospheric or shallow asthenospheric origin for such He component. The origin of this signal can be tested with the distribution of He isotopic signatures and other geochemical tracers among different African volcanic provinces. All these other provinces exhibit 3He/4He ratios that are equal to, or lower than, the mean MORB ratio of 7-9 Ra (Cameroon line: 5-7 Ra; Hoggar: 8 Ra, this work; Darfur 5.4-7.5 Ra; West African rift: 5-8.5 Ra, this work; Comores, 6.5 Ra, this work). Although low 3He/4He ratios in intraplate volcanic provinces could result from crustal recycling in the mantle and remobilisation of recycled crust during plume uprise, the upper range of 3He/4He values within the field of MORB values points to the strong involvement of asthenospheric mantle and limited interactions of magmas with the aged African crust. Furthermore, these "low-3He" volcanic provinces are characterized by strongly alkaline to undersaturated volcanism indicative of low degrees of partial melting and a thermal regime of the asthenosphere cooler than the one that gave rise to transitional to tholeiitic Ethiopian CFBs. These geochemical observations also conflict with models that advocate channelling of the Afar hotspot material by pre-existing tectonic features to account for all these African volcanic provinces. Back

V51B-0529

Recycling and Mantle Stirring Determined by $^{142}$Nd/144Nd Isotopic Ratios

* Jacobsen, S B (jacobsen@neodymium.harvard.edu) , Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138 United States
Ranen, M C (ranen@fas.harvard.edu) , Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138 United States

It is now well established that $^{146}$Sm was live in the early solar system with an initial uniform $^{146}$Sm/$^{144}$Sm ratio of ~0.008. Harper and Jacobsen (1992) discovered that a sample from Isua (~3.8 Ga old) had a positive $^{142}$Nd/$^{144}$Nd anomaly of 33 ppm when compared to normal terrestrial and chondritic Nd. Furthermore, Jacobsen and Harper (1996) reported results from other Isua as well as Acasta (~4 Ga old) samples. Three other Isua samples had a possible small range (about -15 to +15), while two Acasta samples had no anomalies (normal to within 5 ppm). The presence of $^{142}$Nd anomalies at Isua has recently been confirmed by two other groups (Boyet et al. 2003; Caro et al. 2003). The available data demonstrate both the existence of early depleted mantle and that the early mantle was isotopically heterogeneous. As discussed by Jacobsen and Harper (1996), the recycling rate can be determined by tracing the decay of the average $^{142}$Nd/$^{144}$Nd value of the depleted mantle. In addition, by using the $^{142}$Nd/$^{144}$Nd heterogeneity in the depleted mantle through time we can determine the stirring rate of the mantle (Kellogg, Jacobsen and O'Connell, 2002) as a function of time. For this project our goal is to obtain a resolution in $^{142}$Nd/$^{144}$Nd measurements of ~1 ppm. We have thus compared results obtained for the Nd isotope composition and $^{142}$Nd enriched standards for three different TIMS instruments: The Finnigan MAT 262 at Harvard, the Isoprobe-T and Finnigan TRITON mass spectrometers in GV Instrument's and Thermo Electron's demo laboratories in Manchester and Bremen, respectively. The Finnigan TRITON was designed in response to a request from the senior author for such an instrument. The results obtained so far demonstrate that all three instruments yield the same $^{142}$Nd/$^{144}$Nd, $^{143}$Nd/$^{144}$Nd and $^{145}$Nd/$^{144}$Nd isotopic ratios to within a few ppm, while $^{148}$Nd/$^{144}$Nd and $^{150}$Nd/$^{144}$Nd ratios agree to within 10-20 ppm, when all ratios are normalized to $^{146}$Nd/$^{144}$Nd using the exponential law. Due to the excellent agreement between results from three different instruments we conclude that other reports that claimed that such measurements could not be reproduced at the 5 ppm level for either our 15 year old MAT262 or the newer instruments must be in error. Acknowledgements: We thank GV Instruments and Thermo Electron Corporation for making measurements of our $^{142}$Nd enriched standards. Back

V51B-0530

The Importance of Being Plumes: Entrainment, Isotopes, Melting and the Vertical Structure of the Earth's Mantle

* DePaolo, D J (depaolo@eps.berkeley.edu) , University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720 United States
Weaver, K L (karrie@eps.berkeley.edu) , University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720 United States

The standard approach to hot spots has been to associate their geochemical characteristics with the lower mantle. This, along with arguments about material balance, has led to the notion that the composition of the mantle beneath some depth horizon (e.g. 670 km) is different from that above. This particular picture is difficult to reconcile with ideas of whole mantle convection. Plumes, however, are generally imagined as originating from a thermal/compositional boundary layer, and probably at or near the base of the mantle. Thus the fact that plume volcanic rocks are different from, e.g. MORB, may mean only that the base of the mantle is distinct from the rest of the mantle, and that there need be no major compositional discontinuity suspended somewhere in the mid-mantle. This point can be generalized - we should be able to get more information from hot spots with better models and more systematic sampling of lava flows. Theoretically, if plumes originate near the bottom of the mantle, and then entrain some ambient surrounding mantle on the way to the surface, the radial structure of a plume should mimic the vertical structure of the mantle. The full cross-section of the plume however, does not necessarily pass through the melting zone, especially under thick lithosphere, so the lavas represent only a fraction of the plume: the hottest part. In areas where the lithosphere is thin, e.g. where plumes impinge on very thin lithosphere, almost the full cross section of the plume (and hence the full vertical section of the traversed mantle) should be represented in the lavas. We use this concept along with available geodynamic models to compare the observations at Hawaii (thick lithosphere) with Iceland (thin or no lithosphere along the ridge). In Hawaii the highest value of 87Sr/86Sr is about 0.7037 near the plume axis (although it varies somewhat with time). In Iceland the value is not much different at 0.7036. However, in Hawaii the lowest values in the shield stage tholeiites are only slightly lower: 0.7035. In Iceland the values extend continuously all the way down to the local MORB values of 0.7027. In Hawaii, the width of the sampled region is about 100 km, whereas in Iceland the width of the sampled region along the ridge is about 600 km. There is consistency between these patterns that is explained by the effect of lithosphere thickness. Other relationships like this apply for Nd, He, and Pb, and the length scales are not all the same. The details of these distributions, if properly placed into a context of plumes, entrainment and melting beneath the lithosphere, could provide more information about the vertical structure of the mantle, and detailed information about the structure of the base of the mantle. A limiting factor at present is geodynamic models to describe the plumes and melting. Back

V51B-0531

MiFil: a method to characterize seafloor swells with application to the South Central Pacific

* Bonneville, A (bonnevil@ipgp.jussieu.fr) , Institut de Physique du Globe - CNRS, 4, place Jussieu, Paris, 75252 France
Adam, C (adam@ipgp.jussieu.fr) , Institut de Physique du Globe - CNRS, 4, place Jussieu, Paris, 75252 France
Vidal, V (vidal@ipgp.jussieu.fr) , Institut de Physique du Globe - CNRS, 4, place Jussieu, Paris, 75252 France

We propose a new filtering method to characterize large-scale depth anomalies such as seafloor swells associated to intraplate volcanism. Young hotspot volcanoes within plate interiors are frequently surrounded by smooth, broad regions of shallow seafloor termed midplate swells. These swells are typically hundreds of kilometers wide and can be more than a kilometer in elevation. The most frequently invoked explanation for these swells is that they represent the thermal and dynamic surface uplift from rising mantle plumes but they can also be caused by underplating. Wathever their origin be, these swells need to be precisely characterized and we present here a simple method to do the job. This method that we called MiFil, for Minimization and Filtering, requires two stages: a first one to roughly remove the volcano component by minimizing the depth anomaly; a second one to smooth the shape and totally remove the small spatial length scale remaining topography using a median filter. The strength of this method, directly applicable on bathymetry or seafloor depth anomaly grids is that it does not require any assumption on the location, amplitude or width of the large-scale feature to characterize, except its minimal width. Application to hotspot volcanic chains of the South Central Pacific is presented and the results lead to a better understanding of the tectonics and volcanism emplacement of the zone. For each chain, we determine the associated seafloor swell and its main characteristics : (1) the Society is the only 'classical' hotspot that corresponds to the simple interaction of a plume with the lithosphere and for which a buoyancy flux of 1.58$\pm$0.15~Mg~s$^{-1}$ can be obtained; (2) the Marquesas volcanic chain, although quite comparable, presents a swell morphology that prevents such interpretation and quantification; (3) for the Tuamotu, Pitcairn and Cook-Austral volcanic chains, no reliable quantification can be made because the depth and geoid anomalies are caused by several phenomena occurring at different depths that cannot be separated. Back

V51B-0532

On the Zoology of Mantle Upwellings : the case of the Pacific

Davaille, A (davaille@ipgp.jussieu.fr) , Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252 France
* Bonneville, A (bonnevil@ipgp.jussieu.fr) , Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252 France
Stutzmann, E (stutz@ipgp.jussieu.fr) , Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252 France

The characteristics of intraplate volcanism in the Pacific are very diverse. There is probably more than $10^6$ seamounts on the Pacific seafloor created by past or present volcanism. Among them, a number are organized in alignments, some of which showing increasing ages along the track in the direction of plate motion. Sorting the latter by their track duration further shows two populations : a big peak around 20 Myr and a few tracks longer than 50 Myr, sometimes originating at an oceanic basaltic plateau. The Pacific presents also a number of isolated oceanic plateaus, as well as the Polynesian superswell, a region of anomalously shallow sea-floor several thousands of kms in extent with an unusually dense concentration of alignments. Tomographic images of the Pacific mantle reveal several 3D structures of slow velocities, presumably indicating hot material. Moreover, paleo-reconstructions show replication of volcanism over certain areas on a 100 Myr time-scale. In a heterogeneous viscous fluid like the mantle, several kinds of upwellings may develop, from the classical, mushroom-shaped, thermal plume to more complicated thermo-chemical structures. Fluid Mechanics studies give definite constraints on the necessary conditions for these upwellings existence, characteristics (spacing, recurrence time, temperature anomaly,.), and ability to reach the lithosphere. We therefore use those constraints to identify : 1) what types of upwellings could develop in the Earth's mantle, 2) what observations, if any, CAN be explained by a mantle upwelling, 3) what observations can NOT be explained by a mantle upwelling. Back

V51B-0533

Large Igneous Provinces, Mantle Plumes and Uplift: A Sedimentological perspective

Mazumder, R (mrajat2003@yahoo.com) , Asutosh College, Dept. Geology, Kolkata, 700026 India
* Foulger, G R (g.r.foulger@durham.ac.uk) , University of Durham, Science Laboratories, South Rd., Durham, Durham United Kingdom

Significant pre-volcanic uplift of the lithosphere is one of the expected consequences of mantle plume upwelling. Many major mafic/ultramafic lavas have been attributed to mantle plumes that are expected to produce crustal uplift (doming) preceding the major phase of volcanism. Such pre-volcanic uplift would have significant consequences on the regional sedimentation pattern. Subsequent erosion may remove much of the volcano-sedimentary record of domal uplift. However, sedimentologists can identify progressive shallowing of palaeogeography prior to volcanism and distinctive palaeocurrent patterns if these exist, and this may provide constraints on plume interpretations of the volcanic episode. It is difficult to defend a LIP-mantle plume connection where pre-volcanic lithospheric uplift is absent. This is particularly true for continental flood basalts. The sedimentological and stratigraphic criteria proposed for tracing LIP-plume connections are somewhat generalized, and will not all be useful at a given location. For example, progressive shallowing of palaeogeography as a consequence of plume-induced lithospheric uplift should be clear in a marine depositional setting (e.g., in the transition from deep to shallow marine, or marine to terrestrial palaeogeography). In a continental depositional setting, such evidence is less likely to be found, however. This is because the depositional surface is already well above mean sea level (the base level of erosion) and its preservation potential is small. Unlike the case of marine depositional settings, an erosional unconformity/sequence boundary will thus not develop because the depositional surface is already subaerially exposed. The claim that the sedimentary record provides independent supporting evidence for mantle plume influence on the generation of LIPs, is not always true. The genetic linkage between CFBs and mantle plumes is at best difficult to establish from sedimentological analysis alone. Back

http://www.mantleplumes.org

V51B-0534

Heat Flow on Hotspot Swells Reflect Fluid Flow Masking Potential Variations in Mantle Heat Flux

* Harris, R N (rnharris@mines.utah.edu) , University of Utah, Dept. of Geology and Geophysics, 135 S 1460 E, Rm 719, Salt Lake City, UT 84112-0111 United States
McNutt, M K (marcia@mbari.org) , Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States

The origin of hotspot swells is poorly understood. Heat flow data collected on hotspot swells have been used to argue for and against sublithospheric thermal anomalies. The presence of sublithospheric thermal anomalies has been inferred from interpretations of anomalously high heat flow determinations, whereas the contention that hotspot swells result from normal melting processes within the lithosphere is based on `normal' heat flow values. These arguments depend in part on the choice of a thermal reference model, but more importantly assume conductive heat transfer through the lithosphere. We provide evidence that heat flow measurements collected on hotspot swells likely reflect shallow fluid flow rather than deep thermal variations within or at the base of the lithosphere. Discriminating between environments where heat is transferred conductively or convectively requires closely spaced heat flow determinations (1-2 km) collocated with seismic reflection profiles. Only Hawaii and Reunion have surveys meeting these requirements. The Hawaiian survey consists of two profiles, one north of Oahu and one north of Maro Reef. The Reunion survey also consists of two profiles, both north of Mauritius. These heat flow profiles reveal greater scatter than anticipated with spectral peaks on the order of 10 km consistent with fluid flow. Root mean square variations along the Oahu and Maro Reef profiles are 15 and 5 mW m$^{-2}$, respectively, and along both Reunion profiles are about13 mW m$^{-2}$. Coupled heat and fluid flow models demonstrate that thermal buoyancy due to bathymetric relief is capable of driving significant fluid flow that may suppress the background thermal field. These models are consistent with heat flow patterns observed at individual seamounts and oceanic basement highs that are more easily sampled and characterized than large hotspot swells. We caution that the ability of fluid flow to mask variations in sublithospheric heat flux make surface heat-flow values a poor discriminator between geodynamic models for hotspot swells. Back

V51B-0535

Cratonal lithosphere a potential recorder of ancient mantle plumes

* Sleep, N H (norm@pangea.stanford.edu) , Department of Geophysics, Stanford University, Stanford, CA 94305 United States

In the conventional mantle plume hypothesis, hot plume material ponds at the base of the lithosphere. The deep cratonal lithosphere provides a record and a potential test of this aspect of plumes. First uplift and subsidence in the geological record give lthe more likely times of plume impingement. Second studies of pressure-temperature-time histories from zoned diamonds may make this process observable. I make analytical and numerical calculations relevant the the thermal history of the deep lithosphere. Physically stagnant-lid convection subsequent to the ponding of plume material heats the overlying lithosphere and cools the plume material. It thins the rheological boundary layer of ordinary mantle between overlying chemically buoyant cratonal lithosphere and the underlying plume material. Convection moves the chemically buoyant material producing a potentially observable pressure change. Loitering plume tails have a stronger effect than plume head episodes. They can occur only at times that the plate moves slowly, which provides another potential test. The xenolith geotherm provides a further constraints. If a thick layer of plume material is present, the scaling relationship for the thickness of the rheological boundary layer beneath a convecting lid is inverse to the viscosity of the underlying hot material to the fourth power at a given heat flow. Back

V51B-0536

Diagnostic Features of Mantle Plume Penetration Into the Lithosphere.

* Jurine, D (jurine@ipgp.jussieu.fr) , Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75005 France
Jaupart, C (cj@ccr.jussieu.fr) , Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75005 France
Brandeis, G (brandeis@ipgp.jussieu.fr) , Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75005 France

The success or failure of the mantle plume hypothesis rests in part on its predictions for the manner of plume penetration through the lithosphere. We study how thermal plumes interact with viscous and buoyant boundary layers which serve as analogs for oceanic and continental lithospheres of various ages. Laboratory experiments and numerical simulations have been performed over a large range of values of plume buoyancy flux, lithosphere density, viscosity and thickness. Plume penetration proceeds in two distinct phases. In an initial phase, the lithosphere thins and is heated by the plume. The extent of plume penetration depends on lithosphere viscosity and plume buoyancy, and is not sensitive to plume dimensions. In a second phase, heated lithosphere becomes unstable and generates a secondary upwelling of larger dimensions than the plume. The shape and dimensions of the intrusion into the lithosphere vary significantly as a function of the control parameters. Scaling laws have been derived for the initial penetration velocity and for the critical time for lithosphere instability. Application to the Earth leads to a critical time equal to a few tens of My. The two penetration phases are associated with different styles and rates of upwelling, and hence different melting rates and magma compositions involving mixtures of asthenospheric and lithospheric melts. Back

V51B-0537

Revised Models of Global Plate Motions and Mantle Flow Successfully Predict the Emperor-Hawaii and Other Hotspot-related Seamount Chains

* Sutherland, R (r.sutherland@gns.cri.nz) , Institute of Geological and Nuclear Sciences (GNS), PO Box 30-368, Lower Hutt, Wellington New Zealand
Steinberger, B (bernhard.steinberger@ngu.no) , Institute for Frontier Research on Earth Evolution (IFREE), Japan Marine Science & Technology Center (JAMSTEC) 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
O'Connell, R J (oconnell@geophysics.harvard.edu) , Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States

The bend in the Hawaiian-Emperor chain is a prominent feature usually attributed to a change in Pacific plate motion. However, global plate motion chains fail to predict that bend. We show how the geometry of the Hawaiian-Emperor chain and other hotspot tracks can be explained by a combination of hotspot motion and intraplate deformation. Global mantle flow models predict a southward motion of the Hawaiian hotspot. This, in combination with a plate motion chain connecting Pacific and African plates through Antarctica, predicts the Hawaiian track correctly since the bend, but too far west prior to it. If a chain through Australia - Lord Howe Rise is used instead, the track is predicted correctly back to 65 Ma. The difference between the two predictions indicates the effect of intraplate deformation not yet recognized or else not recorded on the ocean floor. The remaining misfit prior to 65 Ma can be attributed to additional intraplate deformation of similar magnitude. Back

V51B-0538

Intermittent Volcanism in the S Pacific: Tracking Persistent Geochemical Sources

* Konter, J G (jkonter@ucsd.edu) , SIO-UCSD, 9500 Gilman Dr, La Jolla, CA 92093-0225 United States
Koppers, A A (akoppers@ucsd.edu) , SIO-UCSD, 9500 Gilman Dr, La Jolla, CA 92093-0225 United States
Staudigel, H (hstaudigel@ucsd.edu) , SIO-UCSD, 9500 Gilman Dr, La Jolla, CA 92093-0225 United States
Hanan, B B (bhanan@sdsu.edu) , SDSU, 5500 Campanile Dr, San Diego, CA 92182 United States
Blichert-Toft, J (jblicher@ens-lyon.fr) , ENS, 46 allee dItalie, Lyon, 7 France

South Pacific ocean intraplate volcanoes (OIV's) have formed relatively short, discontinuous chains over the last 140 Ma, in contrast to classic continuous hot spot chains like Hawaii or Louisville. Moreover, its volcanism stands apart by very diverse and radiogenic mantle source regions, defining the South Pacific Isotopic and Thermal Anomaly (SOPITA). Discontinuous SOPITA OIV's form a complex array of crossing lineaments and isolated seamounts that can be back-tracked in time to the western Pacific. We studied the Hf and Pb isotopic composition of the prominent western Gilbert's and Tokelau chain that mostly correspond to the plate motion stage expressed by the Emperor Seamounts. The Gilbert's chain (to the West of Tokelau) has an age range of 74-65 Ma, and back-tracks to an OIV source origin near Rurutu. Notably, this chain shows an isotopic signature similar to the Rurutu chain, from Burtaritari ($^{176}$Hf/$^{177}$Hf: 0.282900, $^{206}$Pb/$^{204}$Pb: 20.582) to Tuba (0.282938, 19.691). The parallel-running Tokelau chain ranges from 76-58 Ma, projecting current activity around Macdonald seamount. Its isotopic composition is also similar to Macdonald seamount, as shown by Howland (0.282951, 20.773) to Polo (0.283050, 19.390). However, both chains also contain seamounts with ages and isotopic signatures that are outliers, inconsistent with their general age range and isotopic trends (e.g. Ava; 0.282641, 17.729 and Sakau; 0.282880, 18.997). Our results show that the discontinuous magmatic provinces within SOPITA can be tracked back in time by geochronology and geochemical fingerprinting. This suggests that the SOPITA OIV's are fed by distinct, long-lived mantle source regions that may turn "on" or "off" for distinct periods during their life-times. This intermittent volcanism is inconsistent with the "standard" fixed hot spot model, since it does not produce continuous chains with linear age progressions. Furthermore, our results demonstrate the utility of geochemical fingerprinting for relating the products of discontinuous mantle melting anomalies. Such combined evidence for long-lived but discontinuous OIV lineaments allows for their use in plate and/or hot spot motion models, and might offer a new understanding of the processes responsible for the formation of OIV's. Back

V51B-0539

Improved Absolute Plate Motion Modeling in the Pacific

* Wessel, P (pwessel@hawaii.edu) , School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States
Harada, Y (harada@scc.u-tokai.ac.jp) , School of Marine Science and Technology, Tokai University, 3-20-1 Orido Shimizu, Shizuoka, 424-8610 Japan
Kroenke, L W (kroenke@soest.hawaii.edu) , School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States

In studies of Relative Plate Motion (RPM), the model constraints are conjugate magnetic isochrons identified in marine magnetic anomalies. The model is a finite rotation that rotates an isochron on plate A such that the rotated segment matches the conjugate isochron on plate B. Chang (1987; 1988) solved for such rotations using nonlinear spherical regression and developed statistical confidence regions for the resulting rotations. Because conjugate data can be optimally superimposed using a single, finite rotation it was natural to define the model in terms of total reconstruction rotations. In studies of Absolute Plate Motion (APM), the constraints are the surface expressions of hotspot seamount chains and their measured ages. The traditional approach is to model coeval segments of seamount chains as small circles about stage poles of rotation found by minimizing the distances from each seamount to its locally best-fitting, small circle about a candidate pole. The opening angles are typically found by trial and error. Given the age range of a particular set of copolar segments, opening rates can be determined. Because the data portray small circles, it was natural to define the model in terms of stage rotations. The traditional APM modelling approach has many limitations, including (1) shorter segments, possibly reflecting APM changes, are difficult to identify and correlate across several chains; (2) short small-circle segments become indistinguishable from great circles and hence reliable poles cannot be determined; (3) without easily identifiable kinks between chain segments, ages are needed to make the correlation and these are often lacking; and (4) unlike RPM modelling, no rigorous approach for estimating APM uncertainties exists. However, Wessel and Kroenke (1997) developed a method to derive optimal hotspot locations from seamount data if the APM is known, whereas Harada and Hamano (2000) introduced a technique to determine total reconstruction rotations if hotspot locations are known. We improve the modelling of APM by combining these two complimentary methods into a self-consistent hybrid technique. The hybrid technique allows us to determine (1) the best location for hotspots, (2) a high-resolution APM model, and (3) covariance matrices for each rotation. We present the first self-consistent Pacific APM with confidence regions for each rotation pole and reconstructed points. The new model is contrasted with traditional models, and the implications of the model for drift within the Pacific hotspot group and the origin of the Hawaii-Emperor bend is addressed. Back

V51B-0540

Paleomagnetic Tests of Global Plate Reconstructions with Fixed and Moving Hotspots

* Andrews, D L (dandrews@jhu.edu) , Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
Gordon, R G (rgg@esci.rice.edu) , Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
Horner-Johnson, B C (ben@esci.rice.edu) , Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States

Three distinct approaches have been used in prior work to estimate the motion of the Pacific basin plates relative to the surrounding continents. The first approach is to use the global plate motion circuit through Antarctica (e.g., the Pacific plate to the Antarctic plate to the African plate to the North American plate). An update to this approach is to incorporate the modest mid-Tertiary motion between East and West Antarctica estimated by Cande et al. (2000). A recently proposed second approach is to take an alternative circuit for the early Tertiary of the Pacific plate to the Australian plate to the East Antarctic plate to the African plate to the North American plate (Steinberger et al. 2004). The third approach is to assume that the hotspots in the Pacific Ocean are fixed relative to those in the Atlantic and Indian Oceans (e.g., Engebretson et al., 1986), which we recently showed indicates motion between East and West Antarctica of 800 $\pm$ 500 km near the Ross Sea Embayment. The first approach (global plate motion circuit through Antarctica) indicates very rapid motion between Pacific and Indo-Atlantic hotspots during the early Tertiary (e.g., Raymond et al. 2000). The second approach (global plate motion circuit through Australia) indicates slower, but still substantial, motion between Pacific and Indo-Atlantic hotspots (Steinberger et al. 2004). Because each of the three approaches predicts distinctly different motion between the Pacific plate and the continental plates, they can be tested with paleomagnetic data. The results of such tests indicate that the first approach leads to systematic and significant misfits between Pacific and non-Pacific early Tertiary and Late Cretaceous paleomagnetic poles. The second approach leads to slightly smaller misfits. In contrast, the circuit based on fixed hotspots brings the Pacific and non-Pacific paleomagnetic poles into consistency. Thus the paleomagnetic data decisively favor fixed hotspots over the alternative approaches and suggests that motion between hotspots is substantially less than inferred by Steinberger et al. (2004). Back

V51B-0541

Latitudinal Shift of the Hawaiian Hotspot: Motion Relative to Other Hotspots or True Polar Wander?

* Gordon, R G (rgg@rice.edu) , Rice University, 6100 Main St. Earth Science--MS 126, Houston, TX 77005 United States
Horner-Johnson, B C (ben@esci.rice.edu) , Rice University, 6100 Main St. Earth Science--MS 126, Houston, TX 77005 United States

Recent results from deep sea drilling confirm a large southward drift of the Hawaiian hotspot since Campanian and Maastrichtian time (ca. 70 to 83 Ma), as was previously found from prior paleomagnetic results from drilling (Kono, 1980; Jackson et al. 1980), from skewness analysis of Pacific magnetic anomalies (Gordon 1982, Petronotis & Gordon 1989, 1999; Petronotis et al. 1992, 1994; Acton & Gordon, 1991; Vasas et al. 1994; Horner-Johnson & Gordon 2003), and from other paleomagnetic and paleolatitude data (Gordon & Cape 1981; Sager & Bleil 1987). This southward drift could have been the result of motion of the Hawaiian hotspot relative to some other hotspots, or of true polar wander, or of both. Tarduno et al. (2003) have recently presented an extreme interpretation of these results as being entirely due to southward motion of the Hawaiian hotspot through the mantle. Here we show that this extreme interpretation is not supported by available data. While the Pacific plate paleomagnetic data are sufficient to show that the Hawaiian hotspot has moved southward relative to the spin axis, alone they cannot be used to demonstrate motion relative to the mantle or relative to other hotspots. To do so, coeval paleomagnetic poles are needed from the continents bordering the Atlantic and Indian Oceans. Here we show that few, if any, of the coeval paleomagnetic poles from the continents incorporated into widely used reference paths pass minimum reliability criteria. Thus, the inference of rapid motion of the Hawaiian hotspot relative to the mantle is surely premature and probably incorrect. We further show that other earlier studies purporting to show motion between hotspots from paleomagnetic data are now invalid because of revisions to paleomagnetic poles from the continents or because of flaws in analysis. Updated paleomagnetic analyses indicate that little motion has occurred between Pacific hotspots and non-Pacific hotspots. Instead, available data are consistent with the hypothesis that the southward motion of the Hawaiian hotspot relative to the spin axis is mainly caused by true polar wander. Back

V51B-0542

Fifty Million Years of Fixed Hotspots: A New Self-Consistent Global Set of Plate Rotations

* Kumar, R R (rkumar@rice.edu) , Rice University, 6100 Main St. Earth Science--MS 126, Houston, TX 77005 United States
Andrews, D L (dandrews@jhu.edu) , Rice University, 6100 Main St. Earth Science--MS 126, Houston, TX 77005 United States
Gordon, R G (rgg@rice.edu) , Rice University, 6100 Main St. Earth Science--MS 126, Houston, TX 77005 United States

We have developed new methods for appropriately estimating the uncertainties in plate rotations relative to hotspots. Using these new methods and the latest available relative plate rotations, and also incorporating uncertainties in relative plate motions, we showed that there is no significant motion between Pacific hotspots and Indo-Atlantic hotspots for the past ca. 50 Myr. Here we take the next step and seek a sequence of rotations based on simultaneous inversion of hotspot track data in the Pacific, Atlantic, and Indian Oceans. The resulting set of reconstructions are thus optimal estimates for global plate motion relative to the hotspots for the past 50 Myr. Evidence for significant changes in plate motion, including changes in pole of rotation and in rates of rotation, will be presented. Implications for true polar wander will be discussed. Back

V51B-0543

Is the Hawaiian-Emperor Bend Coeval for all Pacific Seamount Trails?

* Koppers, A A (akoppers@ucsd.edu) , IGPP, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225 United States
Staudigel, H (hstaudig@ucsd.edu) , IGPP, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225 United States

By far the largest number of hotspots can be found in the South Pacific Thermal and Isotopic Anomaly (SOPITA). Its Cretaceous counterpart is preserved in a large range of seamounts and guyots found in the West Pacific Seamount Province (WPSP). The seamounts in these regions display very distinct and long-lived isotopic signatures (Staudigel et al., 1991; Koppers et al., 2003) that can be used to combine source region chemistry and seamount geochronology to map out mantle melting anomalies over geological time. These mappings may resolve many important questions regarding the stationary character, continuity and longevity of the hotspots in the South Pacific mantle. Most importantly, it may also answer the question whether the Hawaiian-Emperor Bend (HEB) is coeval for all Pacific Seamount trails at 47 Ma? Fixed hotspots should be expressed in volcanic trails on the lithospheric plates revealing absolute rates of motion from their age progressions and the direction of motion based on their azimuths. By definition, bends in these hotspot trails thus should give an indication of changing plate motion happening simultaneously across individual lithospheric plates. Based on the morphology of seamounts in the Pacific, the Hawaiian-Emperor, Louisville, Gilbert Ridge and Tokelau seamount trails may be identified as the only hotspot trails to exhibit a clear HEB-type bend (Kroenke et al. 2004). Of these, the Louisville seamount trail only displays a faint bend that may be coeval with the sharp 60 degree bend in the Hawaiian-Emperor trail (Koppers et al. 2004) at 47 Ma. However, new 40Ar/39Ar analyses indicate that the HEB-type bends in the Gilberts Ridge and Tokelau seamount trails are asynchronous around 67 Ma and 57 Ma, respectively. We argue, therefore, that plate motion alone cannot explain these age systematics, but that both hotspot motion and changing lithospheric stress regimes may play an important role in their creation. The simple and elegant hotspot model that (almost without difficulty) may explain primary hotspots such as Hawaii and Louisville, seems unsatisfactory to explain the age distributions of the short-lived Gilbert Ridge and Tokelau hotspots. To explain intra-plate volcanism in the South Pacific, we argue for a combination of processes: one that forces regional magmatism from a large-scale source of buoyancy from below (like the rise of plumelets shooting off the top of a superplume that die-off after a short life-cycle) and one process that acts from above, as intra-plate extension opens up pathways that allow the lithosphere to be penetrated by magma. Back

http://earthref.org/databases/SC/

V51B-0544

Critical Evaluation of Radiometric Ages Used for Tracking Hotspots in the Pacific Ocean

* Baksi, A K (abaksi@geol.lsu.edu) , Department of Geology, Louisiana State University, Baton Rouge, LA 70803 United States

One of the pillars supporting the plume hypothesis, is the progression of ages for numerous hotspot tracks in oceans. These ages should be based on radiometric measurements. The argon dating methods have been the tool most commonly used. Since most of the rocks selected for dating have suffered (considerable) alteration, K-Ar dates should not be used as accurate measures of the age of crystallization. 40Ar/39Ar total fusion ages, though better than K-Ar dates in general, do not pinpoint samples that (a) contain excess argon or (b) have suffered partial loss of 40Ar* due to alteration. Hence 40Ar/39Ar incremental heating studies remain as the (only) tool of choice. From such experiments, at a minimum, ages must be based on plateau and/or isochron sections that meet the necessary statistical requirements to be considered crystallization ages. Earlier (Baksi, 1999, Jour. Geol.) it has been shown that almost all the purported crystallization ages for hotspot tracks in the Atlantic and Indian Oceans, are invalid (see also www.mantleplumes.org/ArAr.html). Herein, I apply the tests outlined therein, to evaluate ages available in the literature for hotspot tracks in the Pacific Ocean. These can be divided into five main groups. (1) Those with reliable age data (e.g. Dalrymple and Garcia,1980; Dalrymple et al., 1980, DSDP 55, Hawaiian-Emperor Chain); the authors use care in selecting valid ages from their data sets. (2) Others (e.g. Pringle, 1993, AGU Monograph 77, Musicians Seamounts), most ages are statistically valid, though some fail the requisite test. In addition, many samples show high levels of atmospheric argon, suggesting the samples are (quite) altered; this could lead to incorrect plateau ages. (3) The next set (e.g. Winterer et al., 1993, AGU Monograph 77, Cretaceous guyots in the Northwest Pacific; Ozima et al., 1977, JGRAS, Western Pacific guyots; Saito and Ozima, 1977, EPSL, Western Pacific area) have few, if any, valid ages. Most plateaux/isochrons clearly fail the statistical test of reliability; many steps show high levels of atmospheric argon - the samples are (badly) altered. (4) A set of papers (e.g. Gripp and Gordon, 2002, Geophys. J. Int., young hotspot tracks; Duncan, 1985 - New Hebrides-Samoa lineament) make use of K-Ar dates, wholly or in the main. These dates should be treated as minimum estimates of the crystallization age. (5) A final set of papers (Sager et al., 1993, AGU Monograph 77, Japanese and Marcus-Wake Seamounts; Lincoln et al., 1993, AGU Monograph 77, Marshall Islands), quote ages without listing the relevant analytical data. These results are to be treated as suspect, and not used for quantitative tracking of hotspot trails. In conclusion, the purported progression of ages for numerous hotspot tracks in the Pacific Ocean does not withstand critical scrutiny. Back

V51B-0545

40Ar/39Ar Geochronology of the Sylhet Traps, Eastern India, and their relationship to the Kerguelen Plume related magmatism

* Ray, J S (jsray@prl.ernet.in) , Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 India
Pande, K (kanchanpande@iitb.ac.in) , Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 India

It is now an accepted view that the Rajmahal-Bengal flood basalts of eastern India belong to the Kerguelen plume generated Large Igneous Province (LIP) that encompasses the Bunbury-Naturaliste Plateau basalts of western Australia, volcanism on the southern and central Kerguelen Plateau, Elan Blank, and Broken Ridge in the Indian Ocean (e.g., Kent et al., J. Petrol., 43, 2002). The lesser-known Sylhet Traps (25.5$^{o}$ N, 91.8$^{o}$ E), exposed ${\sim}$400 km east of the Rajmahal Traps, are assumed to be a part of the ${\sim}$118 Ma old Rajmahal-Bengal flood basalt province (e.g., Bakshi, Chem. Geol., 121, 1995). However, absence of physical continuity between these volcanic activities, and more importantly lack of precise age data always created difficulty in correlations. In an attempt to test the hypothesis of their cogenesis we dated the Sylhet Traps using $^{40}$Ar/$^{39}$Ar incremental-heating technique. Two whole rock samples yielded good plateau ages: 115.9{$\pm$}4.1 (2{$\sigma$}) Ma, and 115.5{$\pm$}5.4 (2{$\sigma$}) Ma, calibrated against an age of 523.2{$\pm$}0.9 Ma for the Minnesota Hornblende (MMhb-1; Spell and McDougall, Chem. Geol., 198, 2003). The concordant plateau, isochron and inverse isochron ages, the atmospheric value for the trapped $^{40}$Ar/$^{36}$Ar component, and good MSWD values for the isochrons suggest that 115.8{$\pm$}3.2 Ma (the weighted mean of both the ages) can be considered as the age of crystallization. This age for the Sylhet Traps clearly falls within the range of ages reported for various groups of lavas in the Rajmahal-Bengal province (118-115), and is consistent with the idea that magmatism in this part of India continued well beyond the major pulse at ${\sim}$118 Ma (e.g., Kent et al., J. Petrol., 43, 2002). It also supports the proposal based on geochronology and geochemistry that the Kerguelen hotspot was located close to the eastern Indian margin during its peak activity (e.g., Kumar et al., Geophys. Res. Lett., 30, 2003). Back

V51B-0546

High precision Pb, Sr, and Nd isotope geochemistry of alkalic early Kilauea magmas from the submarine Hilina bench region, and the nature of the Hilina/Kea mantle component

* Kimura, J (jkimura@riko.shimane-u.ac.jp) , Shimane University, Nishikawatsu 1060, Matsue, 690-8504 Japan
Sisson, T W (tsisson@usgs.gov) , US Geological Survey, Middlefield Road, Menlo Park, CA 94025 United States
Nakano, N (s029207@matsu.shimane-u.ac.jp) , Shimane University, Nishikawatsu 1060, Matsue, 690-8504 Japan
Coombs, M L (mcoombs@usgs.gov) , US Geological Survey, Middlefield Road, Menlo Park, CA 94025 United States
Lipman, P W (plipman@usgs.gov) , US Geological Survey, Middlefield Road, Menlo Park, CA 94025 United States

Submarine lavas recovered from the Hilina bench region, offshore Kilauea, Hawaii Island provide information on ancient Kilauea volcano and the geochemical components of the Hawaiian hotspot. Alkalic lavas, including nephelinite, basanite, hawaiite, and alkali basalt, dominate the earliest stage of Kilauea magmatism. Transitional basalt pillow lavas are an intermediate phase, preceding development of the voluminous tholeiitic subaerial shield and submarine Puna Ridge. Most alkalic through transitional lavas are quite uniform in Sr-Nd-Pb isotopes, supporting the interpretation that variable extent partial melting of a relatively homogeneous source was responsible for much of the geochemical diversity of early Kilauea magmas (Sisson et al., 2002). These samples are among the highest 206Pb/204Pb known from the Hawaii islands and may represent melts from a distinct geochemical and isotopic endmember involved in the generation of most Hawaiian tholeiites. This endmember is similar to the postulated literature Kea component, but we propose it should be renamed Hilina, to avoid confusion with the geographically defined Kea-trend volcanoes. Isotopic compositions of some shield-stage Kilauea tholeiites overlap the Hilina endmember but most deviate far into the interior of the isotopic field defined by magmas from other Hawaiian volcanoes, reflecting the introduction of melt contributions from both _gKoolau_h (high 87Sr/86Sr, low 206Pb/204Pb) and depleted (low 87Sr/86Sr, intermediate 206Pb/204Pb) source materials. This shift in isotopic character from nearly uniform, endmember, and alkalic, to diverse and tholeiitic corresponds with the major increase in Kilauea_fs magmatic productivity. Two popular geodynamic models can account for these relations: (1) The upwelling mantle source could be concentrically zoned in both chemical/isotopic composition, and in speed/extent of upwelling, with Hilina (and Loihi) components situated in the weakly ascending margins and the Koolau component in the interior. The depleted component could be refractory and spread throughout or scavenged from the overlying lithosphere. (2) The Hilina (and Loihi) components could be more fertile material (lower melting temperature) spread irregularly throughout the Hawaiian source in a matrix of more refractory depleted and Koolau compositions. Modest upwelling along the leading hotspot margin melts the fertile domains predominantly, while the refractory matrix also partially melts in the more vigorously upwelling hotspot interior, diluting the Hilina and Loihi components and yielding voluminous isotopically diverse tholeiitic magmas. Back

V51B-0547

Submarine Alkalic Lavas Around the Hawaiian Hotspot; Plume and Non-Plume Signatures Determined by Noble Gases

* Hanyu, T (hanyut@jamstec.go.jp) , Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima-cho 2-15, Yokosuka, 237-0061 Japan
Clague, D A (clague@mbari.org) , Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
Kaneoka, I (Ikaneoka@aol.com) , Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
Dunai, T J (dunt@geo.vu.nl) , Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, Amsterdam, 1081HV Netherlands
Davies, G R (gareth.davies@falw.vu.nl) , Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, Amsterdam, 1081HV Netherlands

Noble gas isotopic ratios were determined for submarine alkalic volcanic rocks distributed around the Hawaiian islands to constrain the origin of such alkalic volcanism. Samples were collected by dredging or using submersibles from the Kauai Channel between Oahu and Kauai, north of Molokai, northwest of Niihau, Southwest Oahu, South Arch and North Arch volcanic fields. Sites located downstream from the center of the hotspot have 3He/4He ratios close to MORB at about 8 Ra, demonstrating that the magmas erupted at these sites had minimum contribution of volatiles from a mantle plume. In contrast, the South Arch, located upstream of the hotspot on the Hawaiian Arch, has 3He/4He ratios between 17 and 21 Ra, indicating a strong plume influence. Differences in noble gas isotopic characteristics between alkalic volcanism downstream and upstream of the hotspot imply that upstream volcanism contains incipient melts from an upwelling mantle plume, having primitive 3He/4He. In combination with lithophile element isotopic data, we conclude that the most likely source of the upstream magmatism is depleted asthenospheric mantle that has been metasomatised by incipient melt from a mantle plume. After major melt extraction from the mantle plume during production of magmas for the shield stage, the plume material is highly depleted in noble gases and moderately depleted in lithophile elements. Partial melting of the depleted mantle impregnated by melts derived from this volatile depleted plume source may explain the isotopic characteristics of the downstream alkalic magmatism. Back

V51B-0548

A Combined He and Os Isotopic Study of the HSDP-2 Core from Mauna Kea, Hawaii

* Ireland, T J (tireland@geol.umd.edu) , Department of Geology, University of Maryland, College Park, MD 20742 United States
Walker, R J (rjwalker@geol.umd.edu) , Department of Geology, University of Maryland, College Park, MD 20742 United States
DePaolo, D J (depaolo@eps.berkeley.edu) , Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
Kurz, M D (mkurz@whoi.edu) , Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States

Combined osmium and helium isotope systematics of hotspot lavas have the potential to reveal information about the deep Earth. A high $^{3}$He/$^{4}$He ratio may represent an undegassed reservoir, generally associated with the lower mantle. There are two Os isotopes that can be studied to help to further elucidate the problem. The decay of $^{187}$Re to $^{187}$Os is the more frequently cited system; however, in terms of lower mantle processes, the decay of $^{190}$Pt to $^{186}$Os may be extremely useful. Both of these Os isotopes are enriched in the core relative to chondritic values. In a previous study, Brandon {\it et al}. (1999) examined several Hawaiian volcanoes for both He and Os isotopes. A correlation was noted between the $^{3}$He/$^{4}$He, $^{187}$Os/$^{188}$Os and $^{186}$Os/$^{188}$Os ratios. In terms of $^{3}$He/$^{4}$He and $^{187}$Os/$^{188}$Os space, the three commonly cited Hawaiian end-members (Kea, Koolau and Loihi members) were clearly defined. A strong positive correlation was also observed for $^{186}$Os/$^{188}$Os versus $^{3}$He/$^{4}$He. These correlations were interpreted as a possible signature of core-mantle interaction. There were some limitations to previous studies. Only 2-3 samples from each volcano were studied, with these samples generally being subaerially erupted. The He data utilized were often not for the same samples for which the Os data were collected (volcano averages for He were used on some samples). With the introduction of data from the Hawaiian Scientific Drilling Project (HSDP-2), which drilled 2.84 km into the Mauna Kea volcanics (DePaolo {\it et al}., 2000), an extensive history of a single volcano can be observed (from the early submarine stages to the later subaerial rocks). In the current study a detailed Os isotopic analysis of several samples that span a large depth range of the HSDP-2 core, in conjunction with previously collected He isotopic data (Kurz {\it et al}., 2004), was conducted. The samples define a relatively narrow range of slightly suprachondritic $^{187}$Os/$^{188}$Os ratios (0.12865-0.13056), despite having a large spread in $^{3}$He/$^{4}$He (8.5-23.5 Ra). This result indicates that the $^{187}$Os/$^{188}$Os ratios for Mauna Kea may reflect contributions from a relatively constant source in terms of Os, while the He characteristics of the source appear to be highly variable. Anticipated $^{186}$Os measurements will likely add additional insight to the causes of the isotopic heterogeneities. Back

V51B-0549

The Chemical Structure of the Hawaiian Mantle Plume

* Ren, Z (ren@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
Hirano, N (nhirano@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
Hirata, T (hrt1@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
Ingle, S (single@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan

Numerous geochemical studies of Hawaiian basaltic lavas have shown that the Hawaiian mantle plume is isotopically heterogeneous. However, the distribution and scale of these heterogeneities remain unknown. This is essentially due to the complex interactions created by melting a heterogeneous source, subsequent aggregation of the melts on their way to the surface, and mixing that takes place in shallow magma chambers prior to eruption. In sum, the measured compositions of bulk lavas may represent only _eaverage_f compositions that do not fully reflect the complexity of either the mantle source heterogeneity and/or chemical structure. Melt inclusions, or samples of the local magma frozen in olivine phenocrysts during their formation, are better at recording the complex magmatic history than are the bulk samples. Here, we report major and trace element compositions of olivine-hosted melt inclusions from submarine Haleakala lavas that were collected by 2001-2002 JAMSTEC cruises measured by EPMA and LA-ICP-MS after homogenization at $1250\deg$C, QFM for 20min. Melt inclusions from the submarine Hana Ridge (Haleakala volcano) show large ranges in CaO/Al$_{2}$O$_{3}$ (0.92-1.50), TiO$_{2}$/Na$_{2}$O (0.79-1.60) and Sr/Nb (14.56-36.60), Zr/Nb (6.48-16.95), ranging from Kilauea-like to Mauna Loa-like compositions within separately-sampled lavas as well as in a single host lava sample. Bulk rocks geochemistry shows that major element composition and trace element ratios such as Zr/Nb, Sr/Nb (Ren et al., 2004a, in press, J. Petrol.) together with Pb, Nd and Sr isotopic ratios (Ren et al., 2004b, submitted to J. Petrol.) of Haleakala shield volcano also display systematic compositional variation changing from a Kilauea-like in the submarine Hana Ridge (main shield stage) to Kilauea-Mauna Loa-like in the subaerial Honomanu stage (late shield stage, data from Chen and Frey, 1991). Some of the compositional variations in melt inclusions in single rocks are wider range than over-all variation observed in bulk rocks. It is important that both Kilauea-like and Mauna Loa-like compositions co-exist in melt inclusions in single submarine Hana Ridge rocks which are identified as Kilauea-like based on bulk geochemistry. These observations are inconsistent with the current interpretation that magma compositions are controlled by concentric zonation of the Hawaiian mantle plume (e.g. Kea component and Loa component), manifested as the Kea trend and the Loa trend volcanoes (e.g. Hauri, 1996; Lassiter et al., 1996). Our new data from olivine-hosted melt inclusions imply that the chemical structure of the Hawaiian mantle plume is significantly more complicated than previously modeled and the length-scale of chemical heterogeneity must be remarkably smaller than estimated based on bulk rock geochemistry. Back

V51B-0550

Extreme Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence for an Ancient Recycled Lithosphere

* Bizimis, M (bizimis@magnet.fsu.edu) , Dept. Earth Sciences, FIU, 11200 SW 8th Street,, Miami, FL 33199 United States
* Bizimis, M (bizimis@magnet.fsu.edu) , NHMFL and Dept. Geological Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306 United States
Lassiter, J C (lassiter1@mail.utexas.edu) , Max Plank Institute f. Chemie, Postfach 3060, Mainz, 55020 Germany
Salters, V J (salters@magnet.fsu.edu) , NHMFL and Dept. Geological Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306 United States
Sen, G (seng@fiu.edu) , Dept. Earth Sciences, FIU, 11200 SW 8th Street,, Miami, FL 33199 United States
Griselin, M (griselin@mpch-mainz.mpg.de) , Max Plank Institute f. Chemie, Postfach 3060, Mainz, 55020 Germany

We report on the first combined Hf-Os isotope systematics of spinel peridotite xenoliths from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents from the island of Oahu, Hawaii. These peridotites are thought to represent the Pacific oceanic lithosphere beneath Oahu, as residues of MORB-type melting at a paleo-ridge some 80-100Ma ago. Clinopyroxene mineral separates in these peridotites have very similar Nd and Sr isotope compositions with the post erosional Honolulu Volcanics (HV) lavas that bring these xenoliths to the surface. This and their relatively elevated Na and LREE contents suggest that these peridotites are not simple residues of MORB-type melting but have experience some metasomatic enrichment by the host HV lavas. However, the SLC and PK xenoliths show an extreme range in Hf isotope compositions towards highly radiogenic values ($\epsilon$$_{Hf}$= 7-80), at nearly constant Nd isotope compositions ($\epsilon$$_{Nd}$= 7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu peridotites show a bimodal distribution in their bulk rock $^{187}$Os/$^{186}$Os ratios: the PK peridotites have similar ratios to the abyssal peridotites (0.130-0.1238), while the SLC peridotites have highly subchondritic ratios (0.1237-0.1134) that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes show a broad negative correlation whereby the samples with the most radiogenic $^{176}$Hf/$^{177}$Hf have the most unradiogenic $^{187}$Os/$^{186}$Os ratios. Based on their combined Hf-Os-Nd isotope and major element compositions, the PK peridotites can be interpreted as fragments of the Hawaiian lithosphere, residue of MORB melting 80-100Ma ago, that have been variably metasomatized by the host HV lavas. In contrast, the extreme Hf-Os isotope compositions of the SLC peridotites suggest that they cannot be the source nor residue of any kind of Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism or melt-rock reaction that has overprinted the Nd and Sr isotope compositions of these peridotites. The ancient ($>$1Ga) melt depletion event recorded by both the low $^{187}$Os/$^{186}$Os and high $^{176}$Hf/$^{177}$Hf ratios in the SLC peridotites can be explained with two different scenarios. First, the SLC peridotites may represent ancient depleted lithosphere that survived subduction, remained "rafting" in the upper mantle and is now sampled beneath Oahu. However, the lack of such unradiogenic Os isotopes in both MORBs and abyssal peridotites suggests that such peridotites are rare in the upper mantle and makes their exclusive presence under Oahu a rather fortuitous coincidence. Alternatively, the SLC peridotites may represent ancient depleted recycled lithosphere brought up by the Hawaiian plume. A recycled oceanic crust origin has been previously invoked for the Koolau shield lavas. It is then conceivable that fragments of the lithospheric portion of that subducted package have remained coupled with the oceanic crust and are being brought up by the plume from the deep, but because they were previously depleted, these peridotites contribute minimally, if at all, to Hawaiian volcanism. The presence of microdiamonds and majoritic garnets in some SLC pyroxenites also corroborates a deep origin. In this case, the SLC peridotites represent the first-ever direct evidence that subducted material actually makes it back on the surface, essentially closing the subduction cycle. Back

V51B-0551

He, Sr, Nd, and U Isotopic Variations in Post-Shield Lavas From the Big Island of Hawaii - Insight Into Magma Transport and the Chemical Structure of the Hawaiian Plume.

* Aciego, S M (aciego@eps.berkeley.edu) , Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
* Aciego, S M (aciego@eps.berkeley.edu) , Earth Science Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
DePaolo, D J (depaolo@eps.berkeley.edu) , Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
DePaolo, D J (depaolo@eps.berkeley.edu) , Earth Science Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Kennedy, B M (bmkennedy@lbl.gov) , Earth Science Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Christensen, J N (jnchristensen@lbl.gov) , Earth Science Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720

We present new isotopic and trace element data on post-shield alkalic lavas from the Hualalai, Mauna Kea, and Kohala. These small-volume eruptions, which presumably correspond to small-volume source regions in the mantle, serve as high resolution probes of geochemical heterogeneity to complement data available from shield-stage tholeiites that originate in the primary melting region. The post-shield isotopic ratios average over mantle volumes as much as 100 times smaller than those of the shield stage lavas. The post-shield volcanic vents are spread over an area of about 2500 km2 on the island of Hawaii. The locations extend about 35km on either side of the axis of the Hawaiian ridge and 60 to 110 km northwest of the centroid of the main melting anomaly (located between Kilauea and Loihi). Helium isotopic ratios were measured on olivine separates and, where present, pyroxene separates from the same samples. The Sr, Nd, and U-series isotopes were done on whole rock powders of the same samples. He isotopes range from 6-11 R/Ra, $\^{87/86}$Sr varies from 0.70345-0.70374, and $\epsilon$Nd from +5.3 to +7.4. The total range of Sr and Nd isotopic variations in these lavas is about twice that observed in the 2.84 km section of Mauna Kea drilled by HSDP, and similar to the range encompassed by Mauna Loa and Mauna Kea tholeiites excluding those erupted from ML since 30 Ka. For He the range is much smaller in the post-shield lavas than in the shield lavas. There is general SW - NE asymmetry for all three isotope systems that could be viewed either as the Loa-Kea dichotomy or a reflection of the overall radial zoning of the plume. The amplitude of Sr and Nd heterogeneities is not markedly larger than in the shield sections of the volcanoes, which indicates that if there are larger amplitude variations in the plume, they are substantially smaller than the source regions of the post shield lavas. There is no evidence that the post-shield lavas are affected substantially by lithospheric interaction or that they are melted from isotopically anomalous material associated with pyroxene-rich domains. Back

V51B-0552

Generation of Primary Kilauea Magmas: Constraints on Pressure, Temperature and Composition of Melts

* Gudfinnsson, G H (g.gudfinnsson@gl.ciw.edu) , Geophysical Laboratory, 5251 Broad Branch Rd, NW, Washington, DC 20015-1305 United States
Presnall, D C (presnall@gl.ciw.edu) , Geophysical Laboratory, 5251 Broad Branch Rd, NW, Washington, DC 20015-1305 United States
Presnall, D C (presnall@gl.ciw.edu) , Dept of Geosciences, Univ of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688 United States

Picrite glasses from the submarine extension of Kilauea, Puna Ridge, which contain up to 15.0 wt$%$ MgO, are the most magnesian glass samples reported from Hawaii. Their compositions form a distinct olivine fractionation trend. A comparison of this trend with phase relations of garnet lherzolite in the CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$ (CMAS) and CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$-Na$_{2}$O-FeO (CMASNF) system indicates that melts parental to the Hawaiian picrites are produced by melting of a garnet lherzolite source at a pressure of 5 $\pm$ 1 GPa. The primary melt composition for Kilauea proposed by Clague {\it et al.} (1995), which has 18.4 wt$%$ MgO, is close to the expected 5 GPa melt composition. By using the pressure-independent CMASNF geothermometer (Gudfinnsson and Presnall, 2001), we obtain a temperature of formation of 1450$\deg$C for the most magnesian Puna Ridge glass after correction for the presence of 0.4 wt$%$ H$_{2}$O and 0.7 wt$%$ CO$_{2}$. This assumes that the glass is not much modified after separation from the lherzolite source. However, comparison with phase relations in the CMAS system strongly suggests that the most magnesian Puna Ridge glasses are the product of some olivine fractionation, and therefore give temperature considerably lower than that of the source. When applied to the proposed Kilauea primary melt composition of Clague {\it et al.} (1995), the CMASNF geothermometer gives a melting temperature of 1596$\deg$C or about 1565$\deg$C after correction for the presence of volatiles. This compares well with the anhydrous solidus temperature of 1600 $\pm$ 15$\deg$C at 5 GPa for the fertile KR4003 lherzolite (Lesher {\it et al.}, 2003), which has the complete garnet lherzolite phase assemblage present at the solidus at this pressure. This consistency supports use of phase relations from the CMAS system and the CMASNF geothermometer to the Puna Ridge picrite compositions. With the pressure and temperature of melting known, one can calculate the potential temperature of the Hawaiian mantle, provided certain conditions are met. The calculation assumes that the temperature at the point of melt segregation is close to the temperature of the solid adiabat. If extensive melting has occurred prior to the segregation, this will be incorrect. Secondly, it is assumed that the melting is occurring at the non-conducting part of the geotherm. Provided this is the case and the Kilauea primary melt composition truly represents a near-primary melt composition, we derive a potential temperature for the mantle beneath Kilauea of about 1500$\deg$C. The very high temperature and pressure conditions for magma generation at Hawaii appear to be unmatched by any other currently active volcanism on the Earth. Thus, of all the candidates for plume status, Hawaii appears to be the most robust. Back

V51B-0553

Geochemical Constraints on the Enriched End-member of the Hawaiian Plume: Temporal Geochemical Variation Within the Koolau Shield

* Huang, S (huangs@mit.edu) , Department of Earth, Atmospheric and Planetary Sciences, Massachusette Institute of Technology, 77 Mass Ave, Cambridge, MA 02139 United States
Frey, F A (fafrey@mit.edu) , Department of Earth, Atmospheric and Planetary Sciences, Massachusette Institute of Technology, 77 Mass Ave, Cambridge, MA 02139 United States

The surface of Koolau volcano is composed of Makapuu-stage shield lavas which define the well known, distinctive endmember composition for Hawaiian shield lavas, known as the Koolau component. From $\sim$300 m to 470 m below this surface, drilling and coring by the Koolau Scientific Drilling Project shows that the Koolau shield lavas transitioned to a composition similar to Mauna Loa lavas; these Koolau lavas form the Kalihi-stage of the Koolau shield. Among all Koolau shield lavas and within the Makapuu- and Kalihi-stages, there are strong correlations between the compositional parameters, SiO$_{2}$ content (adjusted to be in equilibrium with Fo$_{90}$ olivine), Sr/Nb, La/Nb and Th/La, with radiogenic isotope ratios of Nd, Hf and Pb. These trends show that as the shield aged there was an increasing role for a marine sediment component accompanied by SiO$_{2}$-rich (dacite) melt. Therefore a recycled oceanic crust component was increasingly important as the Koolau shield moved away from the hotspot and encountered lower temperature. Back

Click here for preprint of Huang, S. & F.A. Frey, Temporal geochemical variation within the Koolau shield: A trace element perspective, submitted to Cont. Min. Pet., Sept. 2004.

V51B-0554

New Seismic Constraints for the Yellowstone Hotspot

Dueker, K G (dueker@uwyo.edu) , Department of Geology and Geophysics University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071 United States
* Schutt, D L (schutt@uwyo.edu) , Department of Geology and Geophysics University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071 United States
Yuan, H (yuan@uwyo.edu) , Department of Geology and Geophysics University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071 United States
Fee, D (fee@uwyo.edu) , Department of Geology and Geophysics University of Wyoming, Dept. 3006 1000 University Ave., Laramie, WY 82071 United States

A synthesis of recent University of Wyoming studies of the Yellowstone Hotspot is presented; this includes teleseismic body wave tomography, transition zone discontinuity structure, and surface wave tomography. Our primary conclusion is that the Yellowstone Hotspot is not a purely "top driven" system. This conclusion is supported by the following constraints. First, the P-wave tomography shows a 120 km diameter low velocity pipe that is reliably imaged to extend from beneath the current hotspot location at Yellowstone Park down to 410 km depth. Below this depth, resolution of a continuation of this pipe is equivocal. It is worth noting that the pipe is tilted about $10\deg$ towards the NW. Translation of the velocity anomalies into temperature suggests a $150-200\deg$ anomaly. Second, imaging of discontinuity topography on the 410 discontinuity finds a 15-20 km depression in the 410 that is spatially well correlated with the low velocity pipe at 410 km depth. Translation of this 410 depression into its corresponding thermal field suggests a $150-200\deg$ anomaly. However, while the 660 km discontinuity does show significant topography, there is no corresponding upwarp of the 660 consistent with extension of the low velocity pipe through the 660. In addition, stacks of the radial component receiver functions intermittently require a 4-6% negative velocity discontinuity at 720 km. Tangential component receiver functions show similar magnitude arrivals from both 660 and 720 km depth. Modeling suggests that dipping layers are not creating this tangential energy and instead an anisotropic layer between 660-720 km is required. Third, Rayleigh wave tomography reveals that the Yellowstone hotspot track is underlain by extremely slow mantle between 60-120 km depth (i.e., 12% lower than the minimum velocity found under Hawaii). This mantle is significantly slower than a normal adiabatic profile would predict, and significant partial melting is indicated. The depth extent of the low velocity zone indicates the solidus is crossed at a mean depth of 104 km. Assuming an anhydrous solidus, this depth implies mantle temperatures $100\deg$ in excess of a normal mantle adiabat. Integration of these new results suggests that the Yellowstone hotspot is a transient thermal upwelling. We speculate that this upwelling is nucleated from super-adiabatic mantle ponded below the 660 km discontinuity in the uppermost lower mantle. Back

V51B-0555

Yellowstone Hotspot Melting And Its Relation To Pre-Existing Crustal Structures And Great Basin Extension

* Glen, J M (jglen@usgs.gov) , U.S. Geological Survey, 345 Middlefield Rd., MS 989, Menlo Park, CA 94025
Ponce, D A , U.S. Geological Survey, 345 Middlefield Rd., MS 989, Menlo Park, CA 94025
Sepulveda, E , Laboratoire d'Optique Physique, ESPCI, 10 Rue Vauquelin, Paris, 75005 France

Topography and geophysical data suggest that the path of Yellowstone hotspot (YSHS) volcanism was controlled by pre-existing crustal structures associated with the Snake River Plain (SRP), and that Great Basin (GB) extension is intimately tied to hotspot melting. From its point of inception (Glen and Ponce, 2002), the YSHS migrated south to the southern Snake River Plain (SRP) where it began a steady migration northeast along the eastern SRP to its present position under the Yellowstone caldera. In doing so, however, it had to move through a 90$\deg$ counter-clockwise turn that is not consistent with a fixed hotspot and its predicted path based on plate motions, and assuming a fixed hotspot. We present evidence suggesting that the SRPs western and eastern branches form a continuous deep crustal structure that guided YSHS volcanism along a track from its inception in eastern Oregon to its present position under the Yellowstone caldera. The western and eastern segments of the SRP, which are interpreted to have different origins and ages, nonetheless form a single topographic depression that curves 180$\deg$ along a circular arc. Also associated with the SRP is a broad and continuous gravity anomaly indicating a relatively deep-seated crustal structure extending across the western and eastern SRP. Heat flow data, which show an uninterrupted corridor of high heat flow values extending from Yellowstone caldera through the SRP to the inferred inception point of the hotspot, might reflect either the thermal footprint of the hotspot's path or the control on heat flow by a regional-scale crustal discontinuity. While the path of the hotspot could have been directed by crustal structures, the location and timing of mid- to late-Tertiary extension in the GB might, in turn, have been controlled by hotspot melting. Well known is the age-progressive hotspot track along the eastern SRP, presently marked by active volcanism at the Yellowstone caldera. Less well known, is a second age-progressive track trending northwest across the Oregon Plateau ending at the historically active Newberry craters. The present locations of these active melting fronts, at Yellowstone and Newberry, coincide with the eastern and western margins, respectively, of the GB. This remarkable correlation, while suggesting a link between hotspot melting and GB extension, does not reveal whether melting controls the bounds of GB extension or whether GB extension controls the propagation of hotspot volcanism. Another characteristic of GB extension, however, that might reflect a causal relation between magmatism and rifting, is the geometry and orientation of basins and ranges in the GB. The trends of basins and ranges fan out from north-northwest in the eastern GB to northeast in the west. When extrapolated, these trends intersect near the SRP close to where the age-progressive YSHS trend began $\sim$ 12-14 Ma, suggesting a relationship between the formation of the GB and this period of the hotspot's path. We infer that this fracturing pattern is related to the SRP, perhaps induced by the $\sim$ 12-14 m.y. old hotspot, and that subsequent extension in the GB exploited these pre-existing crustal weaknesses. Back

V51B-0556

Tomographic Images of the Yellowstone Hotspot Structure

* Jordan, M (mjordan@mines.utah.edu) , University of Utah, 135 South 1460 East, Salt Lake City, UT 84103 United States
Smith, R B (rbsmith@mines.utah.edu) , University of Utah, 135 South 1460 East, Salt Lake City, UT 84103 United States
Waite, G P (waite@usgs.gov) , USGS, 345 Middlefield Road, Menlo Park, CA 94025 United States

The Yellowstone hotspot has extensive and youthful caldera forming volcanism, very high heatflow and significant gravity and geoid field anomalies. It is considered as a notable example of a continental plume. The existence of a plume beneath Yellowstone has been questioned and debated. We present the results of a high resolution 3D teleseismic tomography study that images the upper mantle beneath Yellowstone down to ~800 km depth. Because the delaytime tomography usually suffers from poor near surface resolution, the inversion is constrained by {\it a priori information}, including a large upper crustal low-velocity structure, the topography of the Moho, and the 410 and 660 km mantle discontinuities. Our derived model is consistent with various data sets and models for the Yellowstone region. In addition, long wave-length Bouguer gravity data is used as additional constraints, which are incorporated into the inversion via a joint inversion approach. Our modeling has resolved a region of low upper mantle velocity extending into, but not below, the transition zone. The results are discussed in terms of plumes and consistency with global models. Back

V51B-0557

Reconciling Observations of the Yellowstone Hotspot with the Standard Plume Model

* Ihinger, P D (ihinger@uwec.edu) , University of Wisconsin-Eau Claire, Geology Dept., Eau Claire, WI 54701 United States
Watkins, J M (watkinjm@uwec.edu) , University of Wisconsin-Eau Claire, Geology Dept., Eau Claire, WI 54701 United States
Johnson, B R (johnsbre@uwec.edu) , University of Wisconsin-Eau Claire, Geology Dept., Eau Claire, WI 54701 United States

The Yellowstone hotspot represents the type example of plume magmatism in the continental setting. The propagation of large silicic magmatic centers along the Snake River Plain independently tracks the southwestward trajectory of North American plate motion over the last 13 My. Structural deformation associated with the hotspot track is consistent with thermal upwelling, and tomographic studies image a well-defined cylindrical conduit at least down to the mantle transition zone. Furthermore, the high 3He/4He signatures suggest a deep mantle origin for Yellowstone magmas. Yet, there are several observations of the Yellowstone region that do not fit the standard plume model for hotspot magmatism. These include: 1) prevalent coeval magmatism in and around the hotspot track that continued well after passage of the underlying plume, 2) significant bimodal magmatism that occurred throughout the Great Basin during this time, and 3) the outpouring of the Miocene Columbia River flood basalts (CRFB) well north of the hotspot track. These features have led a number of researchers to favor a shallow upper mantle origin for Yellowstone hotspot activity controlled by structural weaknesses in the continental lithosphere. Here, we propose that the observations listed above conform to the standard plume model by considering interaction of the Yellowstone plume with the descending Farallon Plate beginning at 80 Ma. Anomalous geologic activity occurred throughout the Cenozoic Era in the North American Cordillera (NAC) and must be addressed in any model for the origin of magmatism in the western US, including the Yellowstone hotspot. In particular, extensive field and geochemical studies document a pronounced eastward migration of deformation and magmatism throughout the NAC from 80 to 40 Ma. Most researchers attribute this activity to shallowing of the Farallon slab beneath NA at this time. In addition, geochemical studies in the NAC document a transition in magmatism from predominantly calc-alkaline (associated with ancient slab-derived fluids within the sub-continental lithosphere) to predominantly tholeiitic (with distinctive OIB signatures). This transition has been attributed to the eventual foundering of the shallow slab with replacement by `asthenosphere'. Here, we document that magmas with OIB affinity are observed throughout the Cenozoic in the NAC, often before a documented `transition'. We show that these magmas are primarily binary mixtures of two well-known mantle plume components EMI and FOZO. In our model, we propose that the Yellowstone starting plumehead impinged beneath the subducting Farallon Plate at 80 Ma and spread laterally while continuing to ascend. Magmas with OIB affinity erupted only after penetration of the plume through the cold, rigid Farallon slab. In this way, the CRFB, at only 10% of the eruptive volume of typical flood basalt provinces, represent partial melting of only a fraction of the original Yellowstone starting plumehead. Evidence of additional leakage of the plume is found in the Chilcotin flood basalts in BC, the Crescent Terrane volcanics in the Pacific Northwest, and kimberlites, diatremes, and widespread basaltic flows found throughout the NAC. Collectively, the magmatic features that seem to oppose the plume hypothesis can be reconciled by considering a broader context for the origin of the Yellowstone hotspot. Indeed, the `anomalous' geologic activity observed within the NAC is anticipated by the standard plume model; the frequency of hotspots observed on Earth demands that some starting plumeheads will encounter destructive plate margins and generate significant uplift, deformation, and magmatism within a broad region of the overriding lithosphere(s). Back

V51B-0558

New Constraints on the Evolution of the Deccan Volcanic Province, India

* Mohan, G (gmohan@iitb.ac.in) , Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai, 400076 India
Ravi Kumar, M , National Geophysical Research Institute, Uppal Road, Hyderabad, 500007 India

The evolution of the Deccan volcanic province (DVP) of India is commonly linked to the upwelling of a mantle plume beneath the Indian subcontinent in the late Cretaceous. However, the proposed mechanisms ranging from plume to non-plume models remain debatable. The pre-requisites for the plume models are a thin lithosphere and an anomalously hot upper mantle, which still remain uncertain in the case of DVP. In the present study, the mantle discontinuities beneath DVP are imaged through P-wave receiver function analysis, using about 900 seismograms from six broadband stations deployed along a 350km long profile in western DVP. We find that the move out corrected P-to-s conversion times from the standard 410 and 660 km discontinuities are normal, indicating absence of anomalous thermal anomalies in the upper mantle beneath DVP. An additional discontinuity at 200km, corresponding to the Lehmann discontinuity, possibly representing the base of the lithosphere, is delineated. These results imply that the lithosphere beneath DVP is normal and not thinned by an upwelling mantle plume. This, coupled with pervasive presence of a sub-Moho low velocity zone, raises doubts about the plume impact/incubation models suggested for volcanism in DVP, favoring rifting as a more plausible mechanism. Back

V51B-0559

Hafnium-Osmium Systematics of Cretaceous Group II Kimberlites from India

Kent, R W (r.w.kent@lboro.ac.uk) , Loughborough University, Rutland Hall, Loughborough, LE11 3TU United Kingdom
* Ingle, S (single@geo.titech.ac.jp) , EPS, Tokyo Inst. Tech., 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
Mattielli, N (nmattiel@ulb.ac.be) , DSTE, Univ. Libre de Bruxelles, 50, Av. FD Roosevelt, Brussels, B-1050 Belgium
Kempton, P D (pdk@nerc.ac.uk) , NERC, North Star Avenue, Swindon, GBR SN2 1EU
Saunders, A D (ads@leicester.ac.uk) , Dept. Geology, University of Leicester, University Road, Leicester, LE1 7RH United Kingdom
Suzuki, K (katz@pop.jamstec.go.jp) , IFREE, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka-city, Kanagawa, 237-0061 Japan

Hafnium and Os isotopes of orangeites (Group II kimberlites) may prove to be a useful tool in deciphering mantle sources and petrogenetic histories. According to recent interpretations, orangeite parent magmas are derived from a source with high Os concentrations, ocean island basalt-like gamma Os values (i.e. chondritic to slightly suprachondritic), and low time-integrated Lu/Hf relative to Sm/Nd (Pearson et al., 8th IKC, 2003; Nowell et al., J. Petrol., 2004). Thus, in Hf-Nd isotope space, data for orangeites generally plot below the terrestrial array, as defined by oceanic basalts and continental crust. Osmium isotope compositions may reflect a source for orangeite parent magmas in the convecting mantle, and subsequent interactions between these magmas and an Os-poor, radiogenic source. In order to evaluate this petrogenetic model, we studied a suite of mid-Cretaceous orangeites from eastern India, rocks that have been linked in space and time to the Kerguelen hot spot. High-precision Lu-Hf and Re-Os isotope data were obtained by MC-ICPMS and N-TIMS, respectively. The Indian orangeites have Hf isotopic compositions ranging from chondritic to moderately subchondritic. In Hf-Nd isotope space, data for these samples plot within the terrestrial array, on the very low end of the ocean basalt range. Osmium concentrations are high and Os isotopic values fall mostly within the range of present-day ocean island basalts. In detail, our preliminary Os isotopic data appear to reflect mixing between an Os-rich, chondritic mantle source and an Os-poor, suprachrondritic contaminant. The Os isotopes are not obviously correlated with Hf, Nd, Sr or Pb isotopes in the Indian orangeites; this might imply that their parent magmas interacted with continental crust on the way to the surface. Alternatively, the radiogenic, Os-poor component could also be the cause of the lithophile isotope ratios. Back

V51B-0560

K-T magmatism of western Rajasthan, India: Manifestation of Reunion plume activity or extensional lithospheric tectonics?

* Sharma, K (sharmasirohi@yahoo.com) , Kamal Kant Sharma, Department of Geology Government PG College, SIROHI, RAJ 307001 India

A number of alkaline plutons have been recorded at the K-T (Cretaceous-Tertiary) boundary in western Rajasthan, India. Significant magmatism occurred at Mundwara, Barmer, Sarnu-Dandali and Tavider. The evolution of the Cambay-Sanchor-Barmer rift during the K-T period resulted in these alkaline complexes at the rift margins. Sedimentary basins are developed in the Barmer and Jaiselmer regions. The magmatism of Mundwara and Sarnu-Dandali is dated at 68.50 Ma and considered as an early pulse of Deccan volcanism. Several workers correlated K-T sedimentary basin evolution, magmatism and other tectonic features of western Rajasthan with the Reunion plume-interaction in the northwestern Indian shield. Alkaline igneous complexes along the rift from the southern part are reported from Phenai Mata, Amba Dongar and Seychelles. The Seychelles was part of the northwestern Indian shield prior to Deccan volcanism. The Mundwara igneous complex represents three distinct circular plutonic bodies - Toa, Mer and Mushala, which are situated in the periphery of an area three kilometers in radius. Besides these, there are numerous concentric and radial dykes of lamprophyre, carbonatite, dolerite and amphibolite. All these three bodies represent different phases of intrusion and are not similar to each other. The alkaline rocks of Sarnu-Dandali occur as dykes and isolated plugs in the desert sand. Carbonatite dykes are also reported from southeast of Barmer. The Tavider outcrop is devoid of any plutonic rock and consists of rhyolite, andesite and basalt. These rocks occur along the Precambrian Malani magmatic lineaments. The development of the Cambay-Sanchor-Barmer rift caused reactivation of Precambrian fractures and resulted in magmatism at the basin margin. The Gondwanaland fragmentation during the Mesozoic era caused extensional tectonics in the northwestern Indian shield. This led to the development of rift basins in Gujarat and western Rajasthan. Deccan volcanism, separation of the Seychelles microcontinent from India, sedimentary basin development in western Rajasthan and the alkaline magmatism of Mundwara, Sarnu-Dandali and elsewhere are considered to be the products of Reunion plume activity in western India. However, basin development began in western Rajasthan in the Jurassic period and no plume has been suggested for this. The continual extensional tectonic regime caused deep fractures in the continental and oceanic lithosphere. The Cambay-Sanchor-Barmer rift developed in continental lithosphere. The Mundwara, Sarnu-Dandali and Barmer magmatism with nephelinite-carbonatite affinity at the basin margin represents a typical rift-tectonic setting. The tectonic setting and crustal development during the K-T period in western Rajasthan represents an extensional tectonic regime rather than the manifestation of Reunion plume activity. Back

V51B-0561

Implications for the Emplacement of the Deccan Traps (India) From Isotopic and Elemental Signatures of Dikes

* Vanderkluysen, L (loyc@hawaii.edu) , VGP, SOEST University of Hawaii at Manoa, POST #606, 1680 East-West Road, Honolulu, HI 96822 United States
Mahoney, J J (jmahoney@hawaii.edu) , VGP, SOEST University of Hawaii at Manoa, POST #606, 1680 East-West Road, Honolulu, HI 96822 United States
Hooper, P R (prhooper@mail.wsu.edu) , Department of Geology Washington State University, P.O. Box 642812, Webster 1228, Pullman, WA 99164-2812 United States

A large swarm of dikes with no preferred orientation in the western Deccan Traps of India, termed the Nasik-Pune swarm, has been interpreted by previous workers (Beane et al., 1986, Bull. Volcanol. 48, p. 61; Hooper, 1990, Nature 345, p. 246) to be the principal locus of feeders for the massive lava pile, and the lack of preferred orientation has been interpreted as strong evidence that the main phase of eruptive activity was not accompanied by significant directed extension of the regional lithosphere. Large dike swarms along the western coast and in the Narmada-Tapti graben in the northern Deccan show strong preferred orientations but have generally been discounted as major feeder systems. An isotopic, major element, and trace element study of dikes of the Nasik-Pune and coastal swarms demonstrates that the situation is more complex. About half of our samples display isotopic compositions that are not observed in the western Deccan lava pile, despite having major and trace element similarities to some of the lavas. The other half show isotopic and elemental compositions matching those of the upper formations of the lava pile (i.e. the Poladpur, Ambenali and Mahabaleshwar formations). Both categories of dikes are present in both the Nasik-Pune and coastal swarms. Thus far, only a single dike can be correlated unambiguously with the stratigraphically lowest formation, the Igatpuri-Jawhar; this dike is in the Nasik-Pune swarm. The combined results suggest that the coastal and Nasik-Pune swarms were both feeder zones for the Deccan Traps. The dikes having strong affinities with the Ambenali and Poladpur formations (two of the most extensive formations in the lava pile) appear to be preferentially, although only weakly, N-S oriented. The implication is that east-west extension may have begun by the time of the upper-formation eruptions. Back

V51B-0562

New Age and Geochemical Data From Seamounts in the Canary and Madeira Volcanic Provinces: A Contribution to the "Great Plume Debate"

* Geldmacher, J (jgeldmacher@ifm-geomar.de) , IFM-GEOMAR, Geb. Ostufer, Wischhofstr. 1-3, Kiel, 24148 Germany
Hoernle, K (khoernle@ifm-geomar.de) , IFM-GEOMAR, Geb. Ostufer, Wischhofstr. 1-3, Kiel, 24148 Germany
van den Bogaard, P (pbogaard@ifm-geomar.de) , IFM-GEOMAR, Geb. Ostufer, Wischhofstr. 1-3, Kiel, 24148 Germany
Duggen, S (sd@dlc.ku.dk) , Danish Lithosphere Centre, Oster Voldgade 10, Copenhagen, 1350 Denmark
Werner, R (rwerner@tethys-geoconsulting.de) , TETHYS Geoconsulting Gmbh, Wischhofstr. 1-3, Kiel, 24148 Germany

The role of hotspots (mantle plumes) in the formation of intraplate volcanic island and seamount groups is being increasingly questioned, in particular concerning the abundant and somewhat irregularly distributed island and seamount volcanoes off the coast of northwest Africa. However, new $^{40}$Ar/$^{39}$Ar ages and Sr-Nd-Pb isotope geochemistry of volcanic rocks from two seamounts northeast of the Canary Islands and two northeast of the Madeira Islands provide new support for the plume hypothesis. The oldest ages of shield stage volcanism from seamounts and islands northeast of the Canary and Madeira Islands confirm progressions of increasing age to the northeast for both island/seamount chains consistent with northeast directed plate motion. Calculated angular velocities for the average movement of the African plate in both regions gave similar values of about 0.45\deg plus/minus 0.05\deg/Ma around a rotation pole located north of the Azores Islands. Furthermore, the curvature of the chains clearly deviates from the E-W orientation of fracture zones in the East Atlantic. A local control of surface volcanism by lithospheric zones of weakness, however, is likely for some E-W elongated seamounts and islands. The isotope geochemistry additionally confirms that the two volcanic provinces are derived from distinct sources, consistent with distinct mantle plumes having formed both volcanic groups. Conventional hotspot models, however, cannot easily explain the wide distribution of seamounts in the Canary region and the long history of volcanic activity at single volcanic centers (e.g. Dacia seamount, 47-4 Ma; Selvagen Islands, 30-3 Ma). A possible explanation could involve interaction of a Canary mantle plume with small-scale upper mantle processes such as edge driven convection at the edge of the NW African craton (e.g. King and Ritsema, 2000, Science 290, 1137-1140). Back

V51B-0563

Ascension Island, South Atlantic: Deep Plume or Shallow Melting Anomaly?

* Weaver, B (bweaver@ou.edu) , Barry Weaver, Geology and Geophysics University of Oklahoma, Norman, OK 73071 United States

Ascension Island ($7\deg$ 56' S, $14\deg$ 22' W) is the sub-aerial manifestation of a broad region of anomalous volcanism on, and adjacent to, the South Atlantic MAR between the Ascension Fracture Zone (AFZ; approx. $7\deg$ S) and the Bode Verde Fracture Zone (BVFC; approx. $11.5\deg$ S). Zero age MORB from the MAR in this region have Pb isotope compositions more radiogenic than N-MORB and which plot significantly above (positive $\Delta$8/4) the Northern Hemisphere Reference Line (NHRL), and have La$_{N}$/Sm$_{N}$ higher and Zr/Nb lower than N-MORB. The enriched character of the MORB volcanism has been suggested to be the product of a plume located near Circe Seamount, at approx. $9\deg$ S, $11.7\deg$ W, or of a plume located beneath two large on-axis seamounts at $9\deg$ 50' S on the MAR. The volcanic rocks of Ascension Island comprise a diverse basalt - hawaiite - mugearite - benmoreite - trachyte suite. There is significant chemical heterogeneity in the mafic lavas, with high Zr/Nb, intermediate Zr/Nb, low Zr/Nb, and Dark Slope Crater lava types reflecting significant source heterogeneity, as indicated by trace element and Sr-Nd-Pb isotope systematics. Data for samples from shallow boreholes and one deep borehole suggest that high Zr/Nb lavas dominate in the subsurface and that there has been a temporal trend toward eruption of increasingly enriched, and more diverse, lava compositions with growth of the volcanic edifice. Ascension Island volcanic rocks have Pb isotope compositions which plot significantly below (negative $\Delta$8/4) the NHRL and trend toward St. Helena HIMU compositions. In addition to Ascension Island, there are numerous seamounts (Circe, Grattan, Stvor, etc.) both on and off the MAR axis between the AFZ and the BVFZ. The locations of the seamounts are closely associated with fracture zones and do not reflect the directions of absolute motion of the South American and African plates (for example, the two large seamounts to the west of Ascension Island are on a flow line parallel to the AFZ). The nature of the distribution of Ascension Island and seamounts between the AFZ and BVFZ does not conform to the classic deep plume model. The geochemistry of Ascension Island and MORB lavas is best explained as the result of melting of shallow chemically heterogeneous mantle, with bursts of excess magmatism when "pods" of enriched (HIMU-type component) mantle pass into the sub-axial MAR melting zone, and with focussing of magma supply being controlled by fracture zone distribution. Back

V51B-0564

Tristan-Gough Plume: Negative Ce-Anomalies as Evidence of a Recycled Sediment Component in the Deep Mantle

* Class, C (class@ldeo.columbia.edu) , Lamont-Doherty Earth Observatory, POB 1000 RT 9W, Palisades, NY 10964 United States
le Roex, A (aleroex@geology.uct.ac.za) , University of Cape Town, Private Bag, Rondebosch, 7701 South Africa

Recent volcanism on Tristan and Gough Islands, the associated age-progressive volcanism forming the Rio Grande Rise and Walvis Ridge symmetric to the Mid-Atlantic Ridge and extending towards the large Paran\'{a} (S America) and Etendeka (Africa) flood basalt province, as well as the near uniform composition of the related mantle source since $\sim$130 Ma cannot be reconciled with models other than the plume model. We thus consider Tristan-Gough basalts as probes for deep mantle sources. New high precision trace element data (acquired by ICPMS) on mafic volcanic rocks from Gough Island reveal variable depletion in Ce relative to adjacent rare earth elements when normalized to chondrites, i.e. so-called negative Ce anomalies (Ce/Ce*$<$1). Ce/Ce* values in Gough lavas extend down to values of $\sim$0.93. The magnitude of the anomaly varies with other chemical parameters (e.g. with $^{87}$Sr/$^{86}$Sr ratios) and is therefore not an analytical artifact. Correlations of Ce anomalies with $^{87}$Sr/$^{86}$Sr ratios has been taken as evidence for alteration by sea-spray, but in Gough basalts the accompanying alteration-induced rare earth element enrichment is missing. Thus Ce anomalies in Gough samples are not a weathering phenomenon. Ce anomalies are only formed in the oceans, being characteristic of certain deep sea sediments. Positive Pb and negative Nb anomalies, typical for such sediments, are absent in Gough Island lavas and thus preclude a contribution from sediment within the volcanic pile. Whereas the negative Ce anomaly combined with anomalous isotope ratios and higher LIL/REE strongly indicates a sediment component, the absence of a significant Nb depletion or Pb enrichment shows that the sediment component has been modified during subduction, before it became part of the source of some Gough Island lavas. The presence of a negative Ce anomaly is the strongest indication yet for recycled sediment in the Tristan-Gough plume source. The data confirm that the processes occurring on the Earth's surface affect the evolution of the deep earth. Back

V51B-0565

The Origin of EM1 Signatures in Basalts From Tristan da Cunha and Gough

* Stracke, A (stracke@mpch-mainz.mpg.de) , Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
Willbold, M , Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
Hemond, C , UBO-CNRS UMR 6538 Domaines oceaniques IUEM, Place Nicolas Copernic, Plouzane, 29280 France

A long-standing hypothesis is that enriched mantle 1 (EM-1)-type ocean island basalt (OIB) sources contain pelagic sediments. Pelagic sediments range in composition from clays to calcareous or siliceous oozes and encompass a wide range of chemical compositions [1]. For geochemical purposes the use of the term pelagic sediments is often restricted to a special group of pelagic sediments with distinctive enrichment of Rare Earth Elements (REE). The geochemical composition of such REE-enriched pelagic sediments, however, is by no means representative of the geochemical composition of pelagic sediments in general. The extremely high REE/non-REE element ratios in REE-enriched pelagic sediments (e.g. high Lu/Hf, Sm/Hf, La/Nb, La/Th, Eu/Ti, and Gd/Ti ratios) translate into high $^{176}$Hf/$^{177}$Hf ratios for given $^{143}$Nd/$^{144}$Nd ratios with time. OIB sources containing this special variety of REE-enriched pelagic sediment should therefore plot above the oceanic basalt array and mixing arrays with these sources are expected to have a shallow slope in a Hf-Nd isotope diagram. Here we present new Hf-Nd isotope and trace element data for EM-1-type OIB from Tristan da Cunha and Gough in the South Atlantic Ocean. The samples from Tristan have a small range in Hf-Nd isotopic composition and plot within the oceanic basalt array in a Hf-Nd isotope diagram. Samples from Gough form a trend with a slope slightly steeper than that of the ocean basalt array in a Hf-Nd isotope diagram. OIB in general have a very restricted range in Gd/Ti and Sm/Hf ratios, and high La/Nb are associated with low Lu/Hf ratios. In detail, samples from Tristan and Gough have the lowest Lu/Hf and highest La/Nb ratios. Thus from the combined Hf-Nd isotope and trace element composition of basalts from Tristan and Gough involvement of this special variety of (REE-enriched) pelagic sediments can be excluded. Similar observations are made, and thus similar arguments hold, for other EM-1-type localities (Walvis ridge [2] and Pitcairn island [3]). Due to the considerable spread in geochemical composition of pelagic or any other group of sediments (e.g. marine sediments with a higher proportion of terrigenous components), it is difficult to attribute characteristic elemental or isotopic signatures to certain groups of sediment. Moreover, subducting sediments are complex mixtures of different types of sediment [1]. Thus it is difficult to find unique evidence either in favor of or against the involvement of sediments in general at Tristan and Gough, or any other individual OIB locality. Also, it appears highly unlikely that sub-arc processing has an equalizing effect on the composition of different subducting sediments [4]. Associating the similar isotopic characteristics of certain OIB groups and/or mantle-end-members (e.g. EM-1) to recycled sediments is therefore also problematic. [1] Plank, T. and C. H. Langmuir, Chem. Geol., 145, 325-394, 1998. [2] Salters, V. J. M. and X. Li, Geochim. Cosmochim. Acta, 68, A554, 2004. [3] Eisele, J., M. Sharma, J. G. Galer, J. Blichert-Toft, C. W. Devey and A. W. Hofmann, Earth Plan. Sci. Lett., 196, 197-212, 2002. [4] Johnson, M. C. and T. Plank, Geochem., Geophys., Geosys., 1, pp. 29, 1999. Back

V51B-0566

Isotope and Trace Element Characteristics of Walvis Ridge Basalts Argue Against Pelagic Sediment Involvement

* Salters, V J (salters@magnet.fsu.edu) , National High Magnetioc Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32312 United States
Li, X (xli@magnet.fsu.edu) , National High Magnetioc Field Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32312 United States

Walvis Ridge is the "type" locality for Enriched Mantle I compositions (Zindler and Hart, 1984). We have determined the trace element and isotopic compositions of Walvis Ridge basalts to better determine the geochemical characteristics of the enriched mantle endmember which is thought to include a significant contribution from recycled pelagic sediments. Characteristic for pelagic sediment contribution is a shallow slope on a Hf-Nd isotope correlation diagram. For Walvis Ridge basalts epsilon Hf varies from -2 to +12 while epsilon Nd varies from -4 to 6. This range is larger than previously published results and results in a well-correlated array (R2=0.97) with a steeper slope than the general ocean island basalt (OIB) array. Walvis Ridge Pb-isotope variation is also large $^(206)$Pb/$^(204)$Pb and $^(207)$Pb/$^(204)$Pb range from 17.5 to 18.5 and 15.46 to 15.53 respectively. The low $^(206)$Pb/$^(204)$Pb basalts also show low Hf and Nd isotopic characteristics and in general Nd-Sr-Hf-Pb isotopic compositions are well correlated indicating two component mixing. The Walvis Ridge basalts also show a large range in trace element compositions: Ce/Yb 4.68-25.52; Sm/Hf 1.11-1.58; La/Nb 0.72-1.63, La/Sm 1.59-5.13; U/Th 0.10-0.53 all well outside the range of what is expected for variations related to degree of melting and crystallization. Isotopic and trace element characteristics of the basalts are well correlated with low epsilon-Hf basalts showing low Sm/Hf, high Ce/Yb and high U/Th. The trace element data confirm the isotopic data as most of their variations can also be explained by mixing of just two components. New data obtained on basalts from Tristan da Cunha and Gough show similar type variations in trace element and isotopic compositions as the Walvis Ridge basalts. Walvis Ridge is the "type" locality for Enriched Mantle I compositions (Zindler and Hart, 1984). The isotopic variations as well as the coupled trace element-isotope variations, especially the steep slope on the Hf-Nd isotope correlation diagram and the positive correlation between Sm/Hf and Hf-isotopic composition, argue strongly against pelagic sediment involvement. Detailed analysis of the trace element and isotope variations show that the data is hard to reconcile with recycled components and that intra mantle differentiation is a more likely process explaining the variations. Back

V51B-0567

Contrasting Styles Between the Structure and the Magmatism of the West and South Hatton/Rockall Margins (North Atlantic Igneous Province)

* Gernigon, L (lg@cp.dias.ie) , Marine & Petroleum Geology Research Group Department of Geology, University College Dublin, Department of Geology, University College Dublin Belfield, Dublin 4, Ireland., Dublin, Dublin 4 Ireland
Ravaut, C (cr@cp.dias.ie) , Dublin Institute for Advanced Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland
Shannon, P M (P.Shannon@ucd.ie) , Marine & Petroleum Geology Research Group Department of Geology, University College Dublin, Department of Geology, University College Dublin Belfield, Dublin 4, Ireland., Dublin, Dublin 4 Ireland
Chabert, A (ac@cp.dias.ie) , Marine & Petroleum Geology Research Group Department of Geology, University College Dublin, Department of Geology, University College Dublin Belfield, Dublin 4, Ireland., Dublin, Dublin 4 Ireland
O'Reilly, B M (br@cp.dias.ie) , Dublin Institute for Advanced Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland
Readman, P W (pr@cp.dias.ie) , Dublin Institute for Advanced Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland

The North Atlantic rifted margins area is usually characterised by a combination of regional uplift, crustal extension and magmatism leading to the formation of seaward dipping reflectors (SDRs). The temporal and spatial relationships of these volcano-tectonic processes are interpreted as the response to a deep mantle anomaly. Integration of new seismic, potential field and well calibration data allows us to clarify the structure and stratigraphy of both the West Hatton margin (WHM) and the south Rockall/Hatton margin (SHRM). The WHM represents a Late Paleocene-Early Eocene volcanic margin as suggested by the typical volcano-stratigraphic sequence (Inner SDRs, Outer High, Outer SDRs, oceanic crust) observed near the beakup axis. The Inner SDR wedges are typically underlain by thin and intruded sediments and continental crust. Oceanward, the deep structures of the Outer High and outer SDRs features are underlain by a massive, uniform thick high-velocity lower crustal body interpreted as breakup underplating that probably controlled the SDR emplacement by a mid-crustal updoming. To the south, the SHRM is a complex rifted zone linking the WHM with the Celtic and Iberian non-volcanic margins. The SHM was also influenced by an earlier Cretaceous phase of break-up and, compared to the WHM, the nature of the continent-ocean transition is different. Close to the C34 magnetic anomaly, the continent-ocean transition is defined by a sharp contrast between a smooth oceanic basement and block-faulted structure that exhibit both syn- and post-breakup features. Close to the oceanic crust, the complex may represent a combination of serpentinized peridotites and/or intruded continental basement. No SDRS are imaged, but evidence of magmatism is identified. Thin lava flows and transgressive sill complexes, magmatic plugs and seamounts intruding the Cretaceous oceanic crust are observed near the SRHM. Close to the Charlie-Gibbs Fracture Zone, the West Thulean Ridge defining the southern end of the volcanic province is also imaged. It probably represents a thick Paleocene oceanic volcanic plateau, uplifted and block faulted during Cenozoic time. The age of the magmatism along the SRHM is controversial. Evidence of Palaeogene magmatism can be observed but Early Cretaceous igneous activity is also indicated. The best example is the Barra Volcanic Ridge, which is reinterpreted here as a structural horst cut by Early Cretaceous extrusives and intrusives and subsequently by Tertiary dykes. Detailed stratigraphic analysis documents the contrasting subsidence/uplift patterns that differentiate the sedimentary architecture of the WHM and the SRHM. This project is funded by the Geological Survey of Ireland and the Irish Petroleum Infrastructure Programme. Back

V51B-0568

Plumes are not what they seem: Physics of big, fat, firm plumes

* Korenaga, J (jun.korenaga@yale.edu) , Dept. of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520 United States

In recent years, merely philosophical and often superficial arguments regarding the origin of hotspots and the existence of mantle plumes have become very popular. What is lacking in this trend is constructive discussion based on fluid mechanics constrained by geophysical observations; plumes are fluid-mechanical phenomena, and seismic imaging alone does not tell the whole story. By the same taken, convection models without proper scaling to the Earth's mantle are merely of theoretical interest. Although it is sometimes claimed that plumes with large radii (as observed in recent seismic tomography) have been reproduced in `realistic' mantle convection models, this is simply incorrect. What is referred to as realistic models actually uses either temperature-independent viscosity or very high mantle viscosity or the combination of both. Furthermore, those numerical plumes carry volume flux considerably greater than inferred from surface observables. In this presentation, I will propose a new self-contained framework for plume dynamics that can connect seismic imaging, fluid dynamics, and surface observations. This new framework can also explain various puzzling aspects of the D'' layer and hotspot magmatism. Back

V51B-0569

The mantle potential temperature anomaly beneath Iceland is insufficient for a thermal plume.

* Foulger, G R (g.r.foulger@durham.ac.uk) , University of Durham, Science Laboratories, South Rd., Durham, DH1 3LE United Kingdom
Vinnik, L P (vinnik@ifz.ru) , Institute of Physics of the Earth, Moscow, Moscow, LEV VIN Russian Federation
Du, Z (zd202@cam.ac.uk) , Institute for Theoretical Geophysics, Downing St., Cambridge, DB2 3EQ United Kingdom

One of the few primary characteristics of mantle plumes is high temperature compared with surrounding mantle. A temperature anomaly of at least 200-300 K is thought to be required for an upper-mantle plume rising from the base of the mantle transition zone. More than 15 methods, many of them independent, have been applied to estimate the temperature anomaly beneath Iceland. Seismic methods include using the Vp/Vs ratio and attenuation to determine the temperature of the crust, and using P- and S-wave mantle tomography. P- and S-wave receiver functions have been used to estimate the depths to discontinuities, including those associated with the low-velocity zone and bounding the mantle transition zone. Seismic wave travel times have been used to estimate the velocities between the discontinuities. Petrological methods include olivine glass geothermometry, the study of melt inclusions in basalts, CMASNF geothermometry of high-MgO glasses, major element systematics of Icelandic MORB and the search for an olivine control line in Icelandic picrite cumulates. Other methods include modeling the bathymetry of the north Atlantic assuming it has a thermal origin, modeling the subsidence of the ocean crust and uplift of the Hebrides shelf, and studying ocean floor heat flow measurements. Virtually all results either require or are compatible with a temperature anomaly of no more than ~ 50-100 K beneath Iceland. The crust there is cooler than that beneath the East Pacific Rise. Seismic tomography is compatible with temperature anomalies of up to 200 K, decreasing to about 100 K at depths greater than 200 km, where the seismic anomaly is weaker. This assumes that compositional effects are zero and partial melt is absent, however. Other seismic results require the presence of partial melt, and this requires substantial downward-adjustment of the temperature anomaly estimate from tomography. P-wave receiver functions show that the 410-km discontinuity is warped downward but that the 650-km discontinuity is flat. These results are consistent with a temperature anomaly of about 100 K at 410 km and zero at 650 km. All petrological methods suggest relatively small temperature anomalies unless olivine control is assumed for Icelandic picrite cumulates. The validity of this assumption is questionable. Modeling of bathymetry and vertical motions suggests temperature anomalies of up to about 100 K or less. Virtually all temperature estimates for the Iceland region are thus consistent in suggesting that the temperature anomaly beneath Iceland is modest, and insufficient for a thermal mantle plume that rises through its own thermal buoyancy. Back

http://www.mantleplumes.org

V51B-0570

Widespread Synchronous Volcanism Reveals a Broad Galapagos Hotspot Melting Anomaly

* O'Connor, J M (John.O.Connor@falw.vu.nl) , Department of Isotope Geochemistry, Vrije University, Amsterdam, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands
Stoffers, P (pst@gpi.uni-kiel.de) , Institute for Geosciences, Christian-Albrechts-University, Ludewig-Meyn-Str. 10, Kiel, D-24118 Germany
Wijbrans, J R (Jan.Wijbrans@falw.vu.nl) , Department of Isotope Geochemistry, Vrije University, Amsterdam, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands
Worthington, T J (tw@gpi.uni-kiel.de) , Institute for Geosciences, Christian-Albrechts-University, Ludewig-Meyn-Str. 10, Kiel, D-24118 Germany

The massive aseismic ridges and associated seamounts dominating the morphology of the Panama Basin, eastern Central Pacific, have long been attributed to a Galapagos hotspot melting anomaly linked to a deep-seated mantle plume. Although these structures can provide information about the origin of hotspots and existence, or otherwise, of mantle plumes very little is known about their volcanic histories due to a lack of direct age and geochemical information. We report here 74 whole rock and 2 plagioclase $^{40}$Ar/$^{39}$Ar ages for rocks dredged from 53 locations during the first systematic sampling of the Cocos, Carnegie, Coiba and Malpelo aseismic ridges and associated seamounts (F.S. SONNE PAGANINI expedition). In addition we also report ages for DSDP drill sites on Cocos, Carnegie and Coiba ridges and 7 Cocos Island subaerial samples. The distribution of new, and published ages for the Galapagos Archipelago-platform and NE end of the Cocos Ridge, show a general trend of increasing age with distance from the Galapagos Archipelago. A more dominant trend however is one of aseismic ridge-seamount formation in a progression of broad zones of synchronous, often overlapping volcanism created at discrete intervals. Broad zones of coeval Cocos and Carnegie volcanism once formed much larger regions of synchronous volcanism that have been split apart by the complex history of seafloor spreading associated with the Cocos-Nazca spreading center. We link these broad regions of synchronous volcanism to a correspondingly large hotspot melting anomaly. The present day, as yet unfragmented, zone of synchronous volcanism associated with this proposed broad hotspot is marked by the extensive region of recent volcanism extending across the Nazca and Cocos plates encompassing the Galapagos Archipelago-Platform and the Cocos Ridge as far north as Cocos Island. The complex tectonic history of the Cocos-Nazca spreading-center has controlled how the broad zones of synchronous, often overlapping volcanism created by the broad Galapagos melting anomaly have been fragmented between the Cocos and Nazca plates. However, interplay between the broad Galapagos melting anomaly and the Cocos-Nazca spreading center is a second-order process compared to a fundamental underlying mantle process responsible for a broad Galapagos hotspot melting anomaly exhibiting long-lived characteristics (size, time-progression, episodicity) which, on a first-order, are independent of local tectonics and lithospheric architecture. Evidence for a broad Galapagos hotspot melting anomaly and the possibility of detecting long-lived underlying mantle processes has implications for how oceanic hotspot volcanism is sampled for purposes of rigorously testing the mantle plume paradigm. A major question posed by our results is whether individual Pacific seamount chains are in fact the product of tectonic plate drift over narrow hotspots? If not, then inferring the existence and behavior of a mantle plume on the basis of age progression of volcanism produced by a narrow seamount chain could well prove to be misleading. Thus, although great leaps are being made in the theory and numerical modeling - often on a global scale - of hypothesized deep plumes, significantly more high-quality age and geochemical data are needed for oceanic hotspot volcanism that gave birth to the mantle plume hypothesis in the first place. Back

V51B-0571

Upper Mantle Structure Beneath the Gal\'{a}pagos Hotspot from Surface Wave Tomography

* Villagomez, D R (darwin@newberry.uoregon.edu) , Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
Toomey, D R (drt@newberry.uoregon.edu) , Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
Hooft, E E (emilie@newberry.uoregon.edu) , Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
Solomon, S C (scs@dtm.ciw.edu) , DTM, Carnegie Institution of Washington, Washington, DC 20015 United States

To understand plume-lithosphere interaction in a near-ridge setting, we present a surface wave tomographic study of the upper mantle beneath the Gal\'{a}pagos Archipelago. We use Rayleigh waves recorded by a network of 10 broadband seismometers deployed from 1999 to 2003 for the IGUANA experiment and the GSN station PAYG. We analyze waves in 12 separate frequency bands (8-50 mHz), which are sensitive to shear wave velocity ({\it Vs}) structure in the upper 150 km. To account for non-great-circle propagation caused by multipathing we use the two-plane-wave approximation of Forsyth and others. Two-dimensional models of phase velocity obtained at each frequency are inverted for three-dimensional variations in {\it Vs}. Average one-dimensional phase velocities are 1-2% slower than for 0-4 My-old Pacific mantle, and phase velocities vary laterally by $\pm$3%. Inversions of phase velocities reveal that {\it Vs} varies regionally from 3.7 to 4.1 km/s, 3-15% slower than predicted along a 1300$\deg$C adiabat, and that there are two volumes of pronounced low velocity ($>$10% {\it Vs} reduction). Neither anomaly can be attributed to temperature alone; instead they require increased amounts of partial melt. The first anomaly, located beneath the volcanoes of the southwestern archipelago that erupt large volumes of enriched magmas, is most pronounced above 40 km depth and its magnitude increases toward the surface. This anomaly lies above an area of thinner-than-normal mantle transition zone and a cylindrical low-velocity body imaged by P and S wave tomography at depths of 100 to 250 km. This first anomaly may be the result of melt accumulation above a region of decompression melting driven by plume upwelling. The second low-velocity volume underlies the central archipelago, including the islands of Santiago and Marchena, and appears to be concentrated between 50 and 80 km depth. This anomaly is less pronounced near the surface, underlies a region that produces MORB, and coincides with a region of apparent isotropy as reported by Fontaine and others. This anomaly could indicate decompression melting of a depleted upper portion of the plume, possibly the result of modest local upwelling driven by a northward transition to thinner lithosphere. Our results, together with those from body wave tomography, suggest that geochemical patterns observed in the archipelago are in part the result of progressive melting of material in a plume conduit that rises from southwest to northeast. We are currently integrating results from surface and body wave imaging in an effort to constrain interactions at mantle depths between the hotspot and the Gal\'{a}pagos Spreading Center. Back

V51B-0572

Young lava fields on the Cretaceous Pacific Plate in the Japan Trench: Non-hotspot volcanism?

* Hirano, N (nhirano@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8551 Japan
Haraguchi, S (haraguti@ori.u-tokyo.ac.jp) , Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo, 164-8639 Japan
Yamamoto, J (jyama@bep.vgs.kyoto-u.ac.jp) , Institute for Geothermal Sciences, Kyoto University, Noguchibaru, Beppu, 874-0903 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8551 Japan
Hirata, T (hrt1@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8551 Japan
Takahashi, A (ayu@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8551 Japan
Ogawa, Y (yogawa@arsia.geo.titech.ac.jp) , Earth and Evolution Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572 Japan

The northwestern part of the Pacific Plate is comprised of Early Cretaceous abyssal oceanic lithosphere and Early to Late Cretaceous seamounts. Until recently, no present-day volcanic activity had been definitively documented on the cool, thick, and old Cretaceous lithosphere; however, Hirano et al. (2001) reported the presence of anomalously young alkali-basalt lavas (5.95$\pm$0.31 Ma) on the subducting, $\sim$130 Ma Pacific Plate. The trench-oceanward slope is characterized by trench-parallel normal faults, resulting from bending of the subducting Pacific Plate. Some hummock structures named the Kaiko Knolls can also be observed on the faulted abyssal plain using seabeam sonar bathymetric mapping. The Kaiko Knolls hummocks and some of the horst and graben fault walls are recognized in the seabeam sonar data by the presence of ocean floor with high acoustic intensity. The newly discovered lava fields include all hummocks in the Kaiko Knolls as well as the underlying sheet flow. The distinct WNW-ESE alignments of knolls are perpendicular to hinge lines of bending plate of the trench and outer-rise system. Composition of the dredged lavas shows the garnet presence in the source because the residual garnet buffered Al$_{2}$O$_{3}$ contents with degrees of partial melting and lowered HREE contents. Hirano et al. (2004) demonstrated that the olivine xenocrysts in this rock were entrained from the uppermost mantle. Volcanic eruption occurred $\sim$600 km ESE off the northern Japan Trench based on the radiometric age and the present absolute motion of the Pacific Plate. Morphological and petrological evidences show that the magma has been brought to the surface along some fissures, which can be interpreted along the direction of the maximum horizontal compression caused by the stress in the downwarping Pacific Plate at eastern edge of the outer-rise. Back

V51B-0573

New insights on the Marquesas volcanic chain emplacement

Adam, C (adam@ipgp.jussieu.fr) , Institut de Physique du Globe - CNRS, 4,place Jussieu, Paris, 75252 France
* Bonneville, A (bonnevil@ipgp.jussieu.fr) , Institut de Physique du Globe - CNRS, 4,place Jussieu, Paris, 75252 France

The Marquesas are one of a number of young hotspot volcanic chains in French Polynesia. The islands erupted onto 50-65 Ma old seafloor and radiometric ages of volcanism span from only a few hundred thousand years for a seamount at the southeast end of the chain to 5.75 Ma for Eiao atoll at the northwestern end. The Marquesas seem thus to be a classic hotspot chain however the main orientation of the chain (N140) does not correspond to the present direction of the absolute Pacific plate motion (N115). This observation has intrigued many researchers during the last decades and we propose here a new explanation based on a precise and complete mapping of the depth and geoid anomalies associated to the volcanic alignment. The analysis of the large scale features of these two datasets allow to characterize the seafloor swell and the corresponding geoid associated to the volcanic chain itself but it also evidences to the northeast a N150 elongated positive geoid anomaly not linked to any seafloor anomaly. This direction, that corresponds roughly to the direction of the crustal seafloor magnetic strips, corresponds to the trend shown by the volcanoes at their creation on the crust. We propose that the particular orientation of the Marquesas chain be due to the spreading of a mantle plume head under the control of the not yet known phenomenum at the origin of the N150 residual geoid anomaly. Back

V51B-0574

Geochemical Evolution of the Hikurangi Oceanic Plateau, New Zealand

Hoernle, K (khoernle@ifm-geomar.de) , Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
Hauff, F (fhauff@ifm-geomar.de) , Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
* Werner, R (rwerner@ifm-geomar.de) , Tethys Geoconsulting GmbH, Wischhofstr. 1-3, Kiel, 24148 Germany
Mortimer, N (N.Mortimer@gns.cri.nz) , Insitute of Geological and Nuclear Sciences, Private Bag 1930, Dunedin, 31-312 New Zealand
van den Bogaard, P (pbogaard@ifm-geomar.de) , Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
Geldmacher, J (jgeldmacher@ifm-geomar.de) , Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
Garbe-Schoenberg, D (dgs@gpi.uni-kiel.de) , Institut fuer Geowissenschaften, Kiel University, Ludewig-Meyn-Str. 10, Kiel, 24118 Germany

The Hikurangi oceanic plateau or large igneous province (LIP), located east of the North Island of New Zealand, covers an area of 350,000 km3 and is located at a depth of 2,500-3,500 b.s.l. The Hikurangi plateau was possibly connected to the Manihiki LIP (now located 3000 km to the north) but may have been separated by Cretaceous seafloor spreading at the Osbourn Trough (Billen and Stock, 2000, J. Geophys. Res., 106, 13481-13489). Therefore it may have formed part of the "greater Ontong Java Plateau event" (Coffin and Eldholm, Geology, 21, 515-51), the largest magmatic event preserved on Earth. During the R/V Sonne SO168 ZEALANDIA cruise, 77 dredge hauls containing igneous samples were recovered from the Hikurangi Plateau. Volcanic rocks were obtained from 1) the plateau basement along the 1 km high Rapuhia Scarp, 2) large guyot-type seamounts within the plateau, and 3) ridge-type seamounts associated with rifting of the NE plateau margin (Hoernle et al., 2004, EOS). The recovered plateau rocks range from basalts, dolerites and gabbros with tholeiitic and alkali basaltic to trachybasaltic compositions. The seamount volcanic rocks have more Si-undersaturated compositions than the plateau rocks and range from alkali basalts through mugearites to basanites through tephrites to nephelinites. The plateau basement rocks have flat rare earth element (REE) patterns similar to enriched mid-ocean-ridge basalt (MORB) and basement rocks from other oceanic LIPs, such as Manihiki, Ontong-Java and the Caribbean. The late-stage seamount lavas show enrichment in the light REE and all strongly to moderately incompatible elements, having incompatible element characteristics similar to the HIMU (high time-integrated U/Pb) component in ocean island basalts (OIB). Although the Pb isotopic composition has been extensively effected by seawater alteration, the freshest samples have enriched (EM-type) Sr-Nd-Pb isotopic compositions similar to Ontong-Java and Manihiki basement rocks, suggesting derivation from a common source. In contrast, the late stage seamount lavas have Sr-Nd-Pb isotopic compositions similar to HIMU OIB. In conclusion, increasing Si-undersaturation of the volcanic rocks with decreasing age suggests that the degree of melting decreased and melting depths increased, possibly due to increasing lithospheric thickening, during the waning stages of plateau growth. The change from EM to HIMU-type trace element and isotopic signatures indicates that the low-degree, late-stage seamount lavas were derived from different source material than the large-degree plateau basement volcanic rocks, either reflecting a heterogeneous source or distinct sources for the plateau and seamount volcanism. Ar/Ar age dating is underway. Back

V51B-0575

A South West Pacific Example of Volcanic Rift Margin Facies in a Backarc Basin Setting From Norfolk Basin Seismic Reflection Profiles.

Taylor, L (lydia@geosci.usyd.edu.au) , Sydney University, David Edgeworth Building F05, Eastern Avenue, University of Sydney, Sydney, NSW 2006 Austria
* Muller, D (dietmarmuller@mac.com) , Sydney University, David Edgeworth Building F05, Eastern Avenue, University of Sydney, Sydney, NSW 2006 Austria

Large-volume extrusive basaltic constructions are ubiquitous along nearly all passive continental margins formed during the breakup of Pangea, reflecting rifting above anomalously hot mantle. Here we describe for the first time a complete passive volcanic conjugate margin sequence in a back-arc basin, the Norfolk Basin in the Southwest Pacific, based on high resolution and deep multichannel seismic reflection profiles from the northern Norfolk Basin FAUST-2 survey and the AGSO law of the sea survey 177. Multichannel seismic reflection data reveal lava deltas, landward flows, seaward-dipping reflectors and feeder dikes along the rift margins and volcanic plateaus within the basin. The timing of the construction of this volcanic margin is inferred to be syn-rift due to the the masking of major extensional faults by volcanic flows. In addition to suggesting high mantle temperatures beneath the Norfolk Basin, which have been imaged by seismic tomography (Ritsema et al., 1999), the data provide further evidence that the Norfolk Basin formed as a backarc basin in a plume-influenced subduction setting rather than by subduction driven mantle flow which may explain the apparent landward jump in back arc spreading. The identification of these volcanic facies in a backarc setting suggests that volcanic margin features may be useful indicators to identify plume-arc interactions in the geological record. Back

V51B-0576

Radial Volcanic Migrations Above Continental Hotspots: Examples from Arabia and the Pacific Northwest

* Camp, V E (vcamp@geology.sdsu.edu) , Dept. of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182 United States
Orihashi, Y (oripachi@er.u_tokyo.ac.jp) , Earthquake Research Inst., University of Tokyo, Yayoi, Bunkyo-ku, Tok;yo, 113-0032 Japan
Ross, M E (m.ross@neu.edu) , Dept. of Geology, Northeastern University, 14 Homes Hall, Boston, MA 02115 United States

The ongoing debate on the nature of hotspots has led many to consider alternative, tectonic models for the origin of hotspot tracks. These linear volcanic migrations, thought by most to form above supposed plume tails, have received much attention in the geological literature. In contrast, radial volcanic migrations forming above supposed plume heads have gone largely unrecognized. Two such examples are described here in the harrat volcanic province of Yemen and Saudi Arabia, and in the Columbia River Basalt (CRB) Province of the western U.S. Although most models of plume impingement predict a period of thermal uplift followed by basalt volcanism, the opposite appears to be true at the Afar triple junction, where the peak of flood basalt eruption, from $\sim$31-19 Ma, was followed by uplift and exhumation which began at $\sim$20 Ma, but accelerated at $\sim$14 Ma. Uplift here was contemporaneous with the eruption of widely scattered basaltic lava fields which form the Miocene-to-Holocene harrat province in Yemen and Saudi Arabia. The Yemeni harrats become progressively younger and more alkalic away from the Afar region, prompting Orihashi et al. (1998) to suggest that they formed by the outward dispersion of a mantle plume, consistent with high $^{3}$He/$^{4}$He ratios (Ra. 21.6) from the easternmost of the harrats. Age-equivalent harrats in Saudi Arabia erupted in a similar fashion, from linear vent systems that become progressively younger away from the Afar triple junction. As a group, the Arabian vent systems form a fan-shaped radial pattern consistent with the outward progression of a hot mantle source. One striking similarity between the Arabian harrats and the CRB Province is that both erupted from vents located on a basement of accreted oceanic terranes, adjacent to an older cratonic margin. Like the Arabian harrats, the CRB Province also erupted from a radial system of dikes concentrated along three, age-progressive trends - the Chief Joseph, Steens-Picture Gorge, and Northern Nevada Rift trends. These three trends emanate from a focal point in southeastern Oregon which is thought to be the Miocene site of plume impingement associated with the Yellowstone hotspot. Two younger volcanic migrations emanate from the same region - the eastward-younging Snake River Plain and the westward-younging Oregon High Lava Plains. The former is thought to have formed as a hotspot track above the plume tail, and the latter by asthenospheric drag of the plume head after it was sheared off against the westward moving cratonic margin. The recognition of radial, age-progressive volcanic migrations adds support to the argument that giant radiating dike systems propagate outward with advancing time. Such spatial and temporal volcanic and plutonic trends are consistent with a mantle plume origin, but difficult to reconcile by nonplume alternatives. Back

V51B-0577

Upper Mantle Origin of the Newberry Hotspot Track: Evidence From Shear-Wave Splitting

* Xue, M (meixue@geology.wisc.edu) , Dept. of Geology and Geophysics, University of Wisconsin, Madison, 1215 W Dayton St., Madison, WI 53706 United States
Allen, R M (rallen@geology.wisc.edu) , Dept. of Geology and Geophysics, University of Wisconsin, Madison, 1215 W Dayton St., Madison, WI 53706 United States

In the northwestern United States there are two hotspot tracks: the Newberry track and the Yellowstone track. Both are located on the North American Plate with the Yellowstone track parallel to plate motion and the Newberry track oblique to it. While a mantle plume is probably the most popular cause of the Yellowstone track, the Newberry track cannot be the product of plate motion over a stationary mantle source. Instead proposed causal mechanisms include upper mantle process where melt buoyancy driven convection is directed west-northwest by subduction-driven corner flow or alternatively a westward-spreading plume head. In this SKS splitting study, we collected data from the OATS (Oregon Array for Teleseismic Study) array, a deployment of the University of Wisconsin Broadband Network (UWBN) along the Newberry track from NW to SE Oregon, which was installed in May 2003 and will operate until September 2005. Measurements were made for 23 events at 12 OATS stations using Wolfe and Silver's (1998) multi-event stacking procedure. A gradual rotation of fast polarization direction is observed from NE-SW at the northwest end of the array to E-W to the southeast. Most stations also exhibit null results when the event back azimuth was parallel or perpendicular to the fast direction determined from other events, strongly indicating a single layer of anisotropy. The first order observation is that the SKS splits are not aligned with the Newberry hotspot track indicating that either the splits are not sensitive to mantle flow oriented along the track or the track is not the product of asthenospheric flow. We prefer the second explanation as our null splitting observations strongly argue for one layer of anisotropy. If our continuing analysis confirms this conclusion, then the alignment of the Yellowstone track with plate motion and anisotropy may be coincidental rather than representative of the causal mechanism. Back

V51B-0578

Mantle wedge perturbation induced by slab detachment and the Mio-Pliocene bimodal volcanism in the Trans-Mexican Volcanic Belt

* Ferrari, L (luca@geociencias.unam.mx) , Centro de Geociencias, UNAM, Campus Juriquilla, Qro., Queretaro, Qro 76230 Mexico
Orozco, M (torozco@geociencias.unam.mx) , Centro de Geociencias, UNAM, Campus Juriquilla, Qro., Queretaro, Qro 76230 Mexico
Petrone, C M (petrone@geo.unifi.it) , Dipartimento di Scienze della Terra, Universit di Firenze, Via La Pira 4, Firenze, 50121 Italy

The trench-oblique orientation, the coexistence of geochemical diverse lavas (OIB, CAB, Adakites etc.), and the absence of seismicity beneath the Trans-Mexican Volcanic Belt (TMVB) prompted several workers to formulate genetic models at variance with a classic subduction scenario, including the presence of a mantle plume beneath central Mexico. Based on a careful analysis of its geologic and geochemical evolution we consider, in turn, that this volcanic chain is a continental arc whose complexity is due to the thermal and mechanical perturbation of the mantle wedge imposed by plate history and modulated by crustal thickness, composition and structures. The TMVB began in Middle Miocene as a WNW trending arc of andesitic-dacitic polygenetic volcanoes. This relatively normal situation changed in Late Miocene, when mafic plateaus, cinder cones and fissural lava flows were emplaced to the north of the present TMVB with a clear eastward migrating pattern from ~11.5 and 6.5 Ma. This mafic pulse has been related to the eastward propagation of a slab detachment episode, in the southern Gulf of California, which produced a transient thermal anomaly in the mantle (Ferrari, 2004, Geology). Following this episode, volcanism strongly decreased and becomes more evolved. Dacitic to rhyolitic domes and ignimbrites were emplaced in a belt located just to the south of the previous episode between 7.5 and ~3.0 Ma. Dome complexes dominate the western half of the TMVB, whereas caldera-forming ignimbrites are common to the east. The geochemical character of this bimodal volcanism was analyzed using new Sr and Nd isotope and our database of chemical data (~3,000 samples). The mafic pulse has a basaltic composition [average SiO2 = 50.7±4.0(1s)] readily distinguishable from the following volcanism. Most basalts have a subduction signature, although they show no systematic correlation with distance from the trench. East of Long. 99° W, however, they are Ne-normative and display much lower to none influence of subducted sediments and fluids (lower Ba/Nb, La/Nb and Th/Nb). This geochemical boundary separates the region of Oligo-Miocene subduction metasomatism related to the Sierra Madre Occidental (to the west) from the region where the mantle was unaffected by subduction since the Permian. For the western TMVB rhyolites, the available isotope data (87Sr/86Sr = 0.70396-0.70597; εNd = 4.07-5.01) point to a mantle origin with variable crust assimilation. This suggests that the latest Miocene switch of volcanism toward more silicic composition was the effect of the decrease in subduction rate of the Rivera plate (DeMets & Traylen 2000), an expected consequence of the loss of slab pull after slab detachment. Decrease in convergence reduced flux of the mantle and amount of melting, so the magma started to pond in the crust and underwent fractional crystallization and variable assimilation. In the eastern half of the TMVB both basalts and rhyolites show the highest signature of crustal contamination in the 87Sr/86Sr vs. 143Nd/144Nd plane. This region corresponds to the area where crust is thicker and extension was much less intense than in the west or absent. Here partial melting of the crust may play an important role in generating the dacitic to rhyolitic magmas, likely as a consequence of the rollback of the slab that exposed the base of the upper plate to hotter asthenosphere. Back

V51B-0579

Why are Low-Ti Basalts of the Siberian Traps Large Igneous Province Similar to Island Arc Basalts?

* Ivanov, A V (aivanov@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Rasskazov, S V (rassk@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Demonterova, E I (dem@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Yasnygina, T A (rassk@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Maslovskay, M N (rassk@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Feoktistov, G D (rassk@crust.irk.ru) , Institute of the Earth's Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation

Tholeiitic and alkaline basalts are predominant rock types in the Late Permian - Early Triassic Siberian Traps Large Igneous Province (STLIP). These basalts belong to high-Ti and low-Ti series of rocks. A peculiarity of the low-Ti basalts is the virtual similarity with typical island arc basalts. On a primitive mantle normalized diagram, both the low-Ti basalts of the STLIP and island arc basalts exhibit prominent depletion of Th, Ta-Nb, Pr and Sr relative neighboring elements. As an example, the low-Ti basalts are characterized by almost the same trace element abundances and compositional trends as basalts from Klyuchevskoi volcano, which belongs to the modern volcanic arc of Kamchatka. Using ratios of element pairs with similar rock-melt distribution coefficients such as Sr-Pr, Nb-U, K-Nb and Ce-Pb we infer three principal components: 1) oceanic sediments (or upper crust), 2) melts of oceanic basalt type (either middle oceanic ridge or oceanic island basalt types), and 3) melts of island arc basalt type (or lower crust). Basalts of the high-Ti and low-Ti series of the STLIP form two nearly perpendicular trends between the first and second and between the first and third components. Such trends, and especially the approaching of the trends towards the same component, which is similar to oceanic sediments cannot be explained by a model of plume-lithosphere interaction. An impact-induced melting model could explain the trace-element data satisfactorily, but it can also be rejected because volcanism of the STLIP initiated in the Late Permian, a few Ma before the Permo-Triassic stratigraphic boundary. Prolonged subduction beneath the Siberian part of Pangea occurred in the Permian and Triassic. Remnants of the Mongolo-Okhotsk slab beneath Siberia are still visible in seismic tomography images. Therefore, we suggest that the origin of the STLIP is related to subduction processes. Melts from subducted sediment-bearing oceanic crust played a major role in triggering magmatic processes in the sublithospheric and lithospheric upper mantle. The low-Ti basalts were produced within the sublithospheric upper mantle, which was highly metasomatized by subduction-derived fluids. This explains both the extremely large volume of melts and trace element similarity with island arc basalts. Back

http://www.mantleplumes.org

V51B-0580

Silicate Veining Above an Ascending Mantle Plume - Evidence from New Ethiopian Xenolith Localities

* Rooney, T O (tor102@psu.edu) , The Pennsylvania State University, Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
Furman, T (furman@geosc.psu.edu) , The Pennsylvania State University, Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
Ayalew, D (dereayal@geol.aau.edu.et) , Addis Ababa University, Dept. of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, 0000 Ethiopia
Yirgu, G (yirgu.g@geol.aau.edu.et) , Addis Ababa University, Dept. of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, 0000 Ethiopia

Quaternary basaltic eruptions in the Debre Zeyit (Bishoftu) and Butajira regions of the Main Ethiopian Rift host Al-augite, norite and rare lherzolite xenoliths, xenocrysts and megacrysts. These explosive basaltic eruptions are located 20 km to the west of the main rift axis and are characterized by cinder cones and maars. The host basalt was generated as a small degree partial melt of fertile peridotite between 15 and 25 kb and host abundant Al-augite (Type II) xenoliths derived from pressures up to 10 kb. The central Main Ethiopian Rift lies in a transitional zone between the continental rifting of East Africa and the sea floor spreading associated with the Red Sea. Lithospheric and sub-lithospheric processes that occur during the transition from continental to oceanic magmatism may be investigated using these xenolith-bearing basalts. Neither carbonatitic nor hydrous (amphibole + phlogopite) metasomatism is evident in either the xenoliths or host basalts, suggesting that infiltration of silicate melts that produced Al-augite veining dominates the regional lower crust and lithospheric mantle. These veins are significantly hotter (200 - 300 $\deg$C) than the lherzolite wall rock they intrude suggesting the thermal influence of the Afar plume. Recent geophysical tomography indicates that this veining is pervasive and segmented, supporting the association of these Al-augite veins with the formation of a proto-ridge axis. Al-augite xenoliths and megacrysts have been observed in other continental rift settings such as Durango (Luhr, 2001) and Lake Baikal (Litasov, 2000), indicating Al-augite silicate melt metasomatism is a fundamental process associated with continental rift development. Back

V51B-0581

Antipodal Hotspots and Bipolar Catastrophes: Were Oceanic Large-Body Impacts to Blame?

* Hagstrum, J T (jhag@usgs.gov) , U.S. Geological Survey, 345 Middlefield Road, MS 937, Menlo Park, CA 94025 United States

One aspect of the hotspot distribution that has received little attention is its antipodal character. Of 45 "primary" hotspots, found in most hotspot compilations, 22 (49%) form antipodal pairs within conservative drift limits ($\leq$20 mm/yr). All but 4 of the remaining primary hotspots have volcanic centers near their antipodes. In addition, the available ages, or estimated minimum age ranges, for both hotspots of an antipodal pair tend to be similar ($\leq$10 Myr difference) or overlap. Monte Carlo analyses indicate that the primary antipodal hotspot pairs and their ages are not due to chance at the $>$99.9% confidence level ({\it p}$<$0.001). All hotspot pairs include at least one oceanic hotspot, and these are consistently opposite those hotspots related to large igneous provinces and continental volcanism. A model of hotspot formation is proposed in which minor volcanism is induced at, and lithospheric fracturing and flood-basalt volcanism is caused by focused seismic energy antipodal to, oceanic large-body impacts. Because continental impacts have low seismic efficiencies ($\sim$10$^{-4}$), continents possibly acted as shields to the formation of antipodal hotspot pairs. Published numerical models indicate that large oceanic impacts ($\sim$10-km-diameter bolide) penetrate well into the upper mantle ($\sim$40-km depth), eject mostly water or water vapor from the transient crater, and generate megatsunami ($\sim$4 km initial height) capable of coastal stratigraphic effects on a global scale. Impact-generated megatsunami, consequently, are expected to leave the most prominent and widespread record of large oceanic impacts, and might have been responsible for apparent rapid eustatic changes in sea level and abrupt changes in the isotopic composition of seawater in the geologic past. Moreover, large oceanic impacts during the Late Permian were perhaps the principal cause of end-Kazanian and end-Tatarian flood basalt eruptions, apparent regressive-transgressive shifts in sea level, and pulses in extinction rates making up the Permian/Triassic transition, and might have initiated the Cretaceous/Tertiary transition at $\sim$68-67 Ma. Phanerozoic mass extinction events, therefore, might have been the result of catastrophic megatsunami in a dominantly oceanic hemisphere and vast quantities of noxious volcanic gases in a dominantly continental one. Back

V51B-0582

Mantle Plume Magmatism on Present-day Mars

* Kiefer, W S (kiefer@lpi.usra.edu) , Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058 United States

Two independent types of evidence demonstrate the existence of very young volcanism on Mars. The shergottites are a type of igneous meteorite from Mars, many of which have radiometric ages of just 180 million years. High resolution images of some lava flows have such a paucity of small impact craters that the flows must be quite young, perhaps just 10-30 million years. The concentrated nature of young volcanic activity in just two provinces of Mars, Tharsis and Elysium, is best understood as a result of upwelling mantle plumes which originate deep in the martian mantle. Each plume feeds a single large volcano, such as Olympus Mons. Thus, Tharsis consists of several distinct plumes, set within a broader zone of internally heated upwelling. Numerical models of plume magmatism have been developed which use melting relationships appropriate for martian mantle compositions, as inferred from the shergottite meteorites. These models can explain the geologically inferred magma production rate and the geochemically inferred mean melt fraction, provided that the martian mantle has retained about half of its original content of radioactive elements, with the remainder of the heat production now in the crust. The recently recognized shergottite Yamato 980459 is more magnesian than previously known martian meteorites and has a significantly higher melting temperature. The required high temperature further enhances the requirement for hot mantle plumes on Mars. Previous models of martian plume volcanism assumed a depth-dependent rheology. In these models, the thickness of the upper, high viscosity layer was adjusted to produce a heat flux that is consistent with the elastic lithosphere thickness inferred from gravity modeling. New models are now in development using a more realistic, temperature-dependent olivine rheology. The improved rheology model may modify previous results in several ways. The low viscosity in the plume conduit will permit faster ascent of material through the mantle and may reduce the amount of cooling of the plume by the surrounding mantle. Also, the new models will permit local thinning of the lithosphere in the center of the plume. These effects may be crucial in explaining the high melting temperature of Yamato 980459. Back

http://www.lpi.usra.edu/science/kiefer/home.html