V43G-01

Constraints on Mantle Thermal Variations from the Sedimentary Record of Large Igneous Provinces

Clift, P D (p.clift@abdn.ac.uk) , University of Aberdeen, Department of Geology and Petroleum Geology, Kings College, Aberdeen, AB24 3UE United Kingdom

One of the characteristics of all models for mantle plumes, whether they are deep or shallow rooted is the presence of anomalously hot asthenosphere underlying the lithospheric plate, the melting of which is believed to produce large igneous provinces (LIP). Surface uplift is driven by the upward flow of mantle, as well as a lower density caused by excess heat and melt extraction. Shallowing of the seafloor around Iceland, Hawaii and regions of French Polynesia are characteristic of modern plume activity and should be preserved in the sediment cover of old LIPs. Removal of the LIP from over the plume would result in more subsidence than observed in regular oceanic crust. However, studies of the sedimentary cover from a range of seamounts, plateaus and ridges of various ages from all major ocean basins do not always show this greater than expected subsidence. Drill sites from the North Atlantic margins show the clearest consistent evidence for subsidence anomalies that could be caused by mantle temperature anomalies of around 100?C for a 100 km thick plume layer. However, several examples (e.g., Walvis Ridge, Rio Grande Ridge, Iceland-Faeroe Ridge, MIT Guyot, Hess Rise, 90East Ridge) show no resolvable differences with the normal oceanic crust and preclude major thermal anomalies under them at or shortly after the time of their emplacement. Most unusually in some LIPs (e.g., Ontong Java, Manihiki, Magellan and Shatsky Rises) subsidence is slower than for normal oceanic lithosphere, suggesting either colder than normal mantle temperatures, or more likely the emplacement of a buoyant lithospheric root under the magmatic province at the time of its formation caused by melt extraction. Gradual emplacement of the LIP crust may also slow the net basement subsidence. In these slow subsiding examples the sediments do not preclude or require hotter than normal mantle involved with LIP generation, but instead indicate that the lithosphere in these provinces was formed by processes that are quite different from those operating at mid ocean ridges. It is not clear why the buoyant depleted root under Hawaii disperses with time after magmatism, yet those under the Pacific plateaus do not, although this implies differences in viscosity that could be temperature related.
http://www.mantleplumes.org/SedTemp.html Back

V43G-02

Hot, warm, cold; wet, damp, dry; peridotite, pyroxenite, eclogite; do petrologists know anything about mid-ocean ridge and ocean island basalt sources?

* Asimow, P D (asimow@gps.caltech.edu) , California Institute of Technology, 1200 E. California Blvd. M/C 170-25, Pasadena, CA 91125 United States

Igneous petrology can offer essential constraints on models of spreading center and intraplate volcanism to complement information drawn from seismology, geophysics, and geochemistry. However, as with all these other disciplines, inferences from petrological data are often non-unique and model-dependent. Petrology will be most useful to the general plume debate when and if it can uniquely invert for the temperature, volatile content, and major element composition of the mantle sources of erupted basaltic lavas. When instead there are ambiguities it is important to acknowledge these lest preferred models be taken as fact. Or, rather than attempting to invert for source information, petrologists might content themselves with running forward models to test hypotheses proposed by others. This is a well-defined task, free from ambiguity, and consistent with a conservative falsification approach to science. It is also of the first importance for all parties to avoid over-generalization of their arguments and false grouping of different localities into one category; proposed mantle plumes must be evaluated one at a time, rather than collectively. Furthermore, in order to be generally accepted, models must be able to explain all the observable features of a volcanic chain: the longevity and fixity (or not), the magma and buoyancy fluxes, the trace element and isotopic (lithophile, noble gases, and stable) character, and the distribution (in time and space) of major-element lava types. Such models must also be consistent with mass and energy conservation and known phase equilibria. Two tasks that are quite straightforward at present are (1) inference of the MgO content of the most primitive demonstrable parental lava in a suite from observed liquid and olivine phenocryst compositions and (2) the estimation of source parameters assuming dry peridotite melting. Although in principle an arbitrary amount of $H_{2}O$ in the primary magma might depress the liquidus temperature at any particular MgO as much as desired, in practice there are generally limits on parental $H_{2}O$ contents from melt inclusions or submarine eruptions. Among the remaining model-dependent uncertainties are the estimation of potential temperature from liquidus temperatures and the estimation of source parameters when wet melting, eclogite sources, or mixed lithology sources are considered. It is important to emphasize that a parental liquidus temperature can never be anything but a lower bound on source potential temperature. Any number of processes, most notably adiabatic melting and near-surface cooling, lower the temperature and may leave no record in the phenocryst population. To actually determine the potential temperature it is necessary to find a unique and self-consistent forward model that generates the appropriate parental melt compositions and at the same time sufficient melt volume. In this talk, I will focus on wet melting and the maximum effect that water might have on increased magma production and on over-estimation of liquidus temperatures. I will use the mid-Atlantic ridge near the Azores, the Reykjanes ridge and Iceland, the Galapagos Spreading Center, and Hawaii as examples. Back

V43G-03

Hot Hawaii, Cold Ridges, Mantle Heterogeneity, and Plumes

* Presnall, D C (d.presnall@gl.ciw.edu) , Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
Gudfinnsson, G H (g.gudfinnsson@gl.ciw.edu) , Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States

We use model-system phase relations 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) systems at 1 atm to 6 GPa to compare melt generation and crystallization processes of tholeiitic basalts at Hawaii and oceanic ridges. At both localities, erupted melt compositions are strongly controlled by low-pressure fractional crystallization of magmas generated at greater depths. Also, the Mg numbers of the most primitive melts from each locality are nearly the same (MORB, 72.1; Hawaii, 72.4 - when Fe$^{2+}$/(Fe$^{2+}$ + Fe$^3+}$) = 0.91). However, in other respects, the compositions of these most primitive basalts are quite different, and the phase relations indicate that in both cases they are only slightly less primitive than their respective parental primary melts. At Hawaii, the phase relations support generation of picritic tholeiitic melts at $\sim$5 GPa and $1565\deg$C (Gudfinnsson and Presnall, 2004), whereas at ridges, the conditions are $\sim$0.9-1.5 GPa and 1260-$1280\deg$C (Presnall {\it et al}., 2002). In Hawaii, the trend of picritic melt compositions indicates olivine-controlled fractionation, not a polybaric melting column like that suggested by Klein and Langmuir (1987) for MORBs. For the MORB modeling of Klein and Langmuir (1967) and Langmuir {\it et al}. (1992), which employs polybaric melting columns extending to 4 GPa, the phase relations show that aggregate melts would be produced that require significant low-pressure olivine-controlled fractionation in order to reach the field of observed MORB glasses. No trace of this fractionation has ever been observed in MORBs, even at Iceland. Furthermore, because the phase relations show that an inverse correlation of Na8 with Fe8 can be produced by melting of a heterogeneous mantle in the 0.9-1.5 GPa pressure interval (Presnall{\it et al}., 2002), this correlation cannot be used as an indicator of widely varying temperature. Mantle heterogeneity produced by recycling of oceanic crust and underlying depleted peridotite back into the source region for ridge volcanism would produce little change in the temperatures required for MORB generation in the plag/sp lherzolite transition. However, strong variations in melt productivity would be expected and the compositional range of basalts erupted would be expanded. No petrological evidence for ascending plumes driven by high temperatures appears to exist anywhere along the oceanic ridge system. However, some volcanic centers ({\it e. g}. Galapagos) may be caused by diapirism of low-density, major-element depleted peridotite recycled into the mantle at subduction zones (Presnall and Helsley, 1982). Low-velocity regions extending to depths $>$200 km beneath Iceland, Afar, and Easter (Ritsema and Allen, 2003) could be caused by carbonate-induced melting at low melt-fractions in an eclogite-enriched source rather than by elevated temperature. If temperatures in the central Pacific are generally high due to lithospheric blanketing, the high temperature indicated at Hawaii may not indicate a plume. Back

V43G-04

How Many Hotspots are on Present-day Earth, and are all Plumes hot?

* Herzberg, C (herzberg@rci.rutgers.edu) , Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 United States

The petrological characteristics of primary magmas that exit the melting regime are sensitive indicators of mantle potential temperature. However, most primary magmas partially crystallize some olivine during transit to the surface, and erupted lavas are typically hybrid mixtures of olivine and solidified liquid. Primitive glass on the surface can have an MgO content that is lower than a parental magma from which it was derived, and a parental magma can differ from its primary magma by partial crystallization of olivine in a crustal magma chamber. However, the parental magma composition can be restored using a simple petrological procedure when olivine is the sole phenocryst phase. On Kilauea the most primitive magnesian glass has been reported to contain 15% MgO, and the most magnesian olivines contain Fo 90-91. The exchange coefficients (Kd) for FeO and MgO between these olivine and glass compositions are 0.25-0.28, much lower than 0.33-0.34 for olivine equilibrated with liquid in melting experiments. The only way to obtain the correct Kd is by computing the effects of dissolving olivine into a 15% MgO liquid composition. This procedure results in a crustal parental magma with 17-19% MgO and a mantle primary magma with 18-20% MgO. The potential crystallization temperature for Kilauea is 1400C, an estimate that includes the effects of 0.34% H2O. Hawaii is therefore a hotspot. This is the most fundamental geological constraint that all models are required to satisfy. It is independent of ongoing questions concerning the role of subducted crust and pyroxenite in the melting regime. A primary magma with 18-20% MgO is successfully reproduced by decompression melting in a hot plume with potential temperatures in excess of 1550C. Hawaii is the only hotspot Earth at the present time. The mantle below Iceland is comparatively cooler, warmer than oceanic ridges, but it was hotter during the early Tertiary. A preliminary analysis of volcanics in and around the African and South Pacific superplumes indicates low extents of wet melting and potential temperatures that may be comparable to oceanic ridges (~1300-1400C). More work is needed for a quantitative petrological evaluation, but it is clear that these volcanoes cannot be hotspots even though they are associated with broad and narrow regions of slow seismic velocities that extend deep into the mantle. The implication is that most plumes or superplumes are buoyant for compositional reasons at or close to ambient mantle temperatures, and they are distinct from the Hawaiian hotspot. Back

V43G-05

Plumes or Not? Yes, and Plenty!

* Montelli, R (montelli@princeton.edu) , Dept of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544 United States
Nolet, G (nolet@princeton.edu) , Dept of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544 United States
Dahlen, F (fad@princeton.edu) , Dept of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544 United States
Masters, G (guy@igpp.ucsd.edu) , Institute of Geophysics and Planetary Physics, U.C. San Diego, La Jolla, CA 92093 United States

We present confirmation of the detection of deep mantle plumes, earlier imaged using P waves (Montelli et al., Science 2004) using a finite-frequency inversion of long period S waves. Our data set comprises 69079 S traveltimes, 26337 SS-S and 13856 ScS-S differential traveltimes. We invert for both velocity anomalies, origin times, and the relocation of the 6834 hypocenters, using the banana-doughnut kernels derived by Dahlen et al. (GJI, 2000). The S-wave images confirm the presence of well resolved deep mantle plume beneath Ascension, Azores, Canary, Easter, Samoa and Tahiti. Among the deep plumes that were not very well resolved in the earlier P-wave study, the S wave inversion shows a robust extension all the way to the CMB of the plumes beneath Cape Verde, Cook Island and Kerguelen. The presence of plumes rising from the base of the mantle but not reaching yet the surface in the Coral Sea, East of Solomon and South of Java is validated. Plumes such as Bowie, Eifel, Etna and Seychelles remain mostly confined to the upper mantle. However, the new S-wave images reopen the question on the depth extent of Iceland and Galapagos plumes. The weakening of the plume in the mid-mantle beneath Iceland is confirmed, but the S inversion clearly shows the presence of a low velocity zone at greater depth that was not visible in the P-wave images. For Galapagos, the new S-wave images show more clearly a possible connection of the plume with a broad low velocity anomaly in the lowermost mantle that feeds Easter as well. We will present the final S-wave plume images and will provide a synthesis of our findings in the light of existing ideas about plume characteristics and their superficial signature. Back

V43G-06

The Boundary Between the Upper and Lower Mantle

* Dziewonski, A M (dziewons@eps.harvard.edu) , Deapartment of Earth and Planetary Sciences, Harvard University, 20 Oxford St. , Cambridge, MA 02138 United States

The debate on the style of mantle convection continues partly because information from seismic tomography appears to be contradictory. There are strong arguments in favor of a major obstacle to the flow across the 650 km discontinuity. These include a change in the spectrum of lateral heterogeneity above and below the boundary, with the "red" spectrum of the velocity anomalies in the transition zone changing, either abruptly or very rapidly, to the white spectrum in the middle mantle. There are large wavelength (2,000-3,000 km) perturbations in the topography of the 650 km discontinuity with an amplitude of up to 20 km; the pattern of these perturbations matches well that of the velocity anomalies in the transition zone, with fast velocities being underlaid by depressions. The topographies of the 410 km and 650 km discontinuities are uncorrelated, indicating that they are not caused by a large scale mantle flow. Ponding of the subducted material in the transition zone seems to be consistent with all these facts. In addition, there is information on the deep earthquakes that supports the hypothesis of slab accumulation. The most outstanding example is a 500-600 km long zone of earthquakes under the Fiji Plateau with focal depths greater than 600 km. Isolated events, distant by 200-300 km from the main Wadati-Benioff zone, are also observed in South America, Tonga, Kuriles, Izu-Bonin and Banda Sea. These events are rare on the decadal time scale, but may be widespread over thousands or millions of years. Also, there is a frequent change in the orientation of the principal stress axes as the slab approaches the 650 km boundary. On the other hand, the tomographic images of the lowermost mantle bear resemblance to the near-surface tectonics. The circum-Pacific ring of fast velocities correlates well with the past locations of the subduction zones. There is also a significant concentration of the hot-spots in the regions of slow velocities in the lowermost mantle: in particular, above the Equatorial Pacific Plume Group and the Great African Plume (Dziewonski {\it et al.}, 1993). It appears that some connection between the upper and lower mantle must exist; whether it is direct or indirect cannot be determined from seismic tomography alone. Back

V44B-01

Where Plumes Live

* King, S D (sking@purdue.edu) , Department of Earth and Atmospheric Science, 550 Stadium Mall Dr. Purdue University, West Lafayette, IN 47907-2051 United States

From the perspective of fluid dynamics, `Plumes or not?' might be the wrong question. Let me begin by defining a few terms. Plume with a `P' is the well-known thermal structure with thin (order 100 km) tail and large, bulbous head that originates at the core-mantle boundary. The thin tail/large, bulbous-head morphology has been generated in a number of laboratory and numerical experiments. It can be seen, for example, on the cover of the famous fluid dynamics text by Batchelor. There is a clearly-defined range of parameters for which this structure is the preferred solution for instabilities arising from a bottom boundary layer in a convecting fluid. For example, a strong temperature-dependent rheology is needed. By contrast, plume with a `p' is any cylindrical or quasi-cylindrical instability originating from a thermal (or thermo-chemical) boundary layer. In fluid dynamics plume is sometimes used interchangeable with jet. Unless there is a very small temperature drop across the core-mantle boundary or a rather remarkable balance between temperature and composition at the base of the mantle, there are almost certainly plumes. (Note the little p.) Are these plumes the thermal structures with thin (order 100 km) tails and large bulbous heads or could they be broad, hot regions such as the degree 2 pattern seen in global seismic tomography images of the lower mantle, or the disconnected droplets seen in chaotic convection? To study this question, I will present a sequence of numerical `experiments' that illustrate the morphology of instabilities from a basal thermal boundary layer, i.e., plumes. Some of the aspects I will present include: spherical geometry, temperature-and pressure-dependence of rheology, internal heating, pressure-dependent coefficient of thermal expansion, variable coefficient of thermal diffusivity, phase transformations, and compositional layering at the base of the mantle. The goal is to map out the parameters and conditions where Plumes live (note the big P) and to provide insight into the structures that boundary layer instabilities at the base of the mantle may take. Back

V44B-02

Implications of Heat Flow in the Triaxial Earth on Layered Convection and Plume Formation

Criss, R E (criss@wustl.edu) , Washington U., Dept. Earth and Planet. Sci., 1 Brookings Dr., St. Louis, MO 63130 United States
* Hofmeister, A M (hofmeist@wustl.edu) , Washington U., Dept. Earth and Planet. Sci., 1 Brookings Dr., St. Louis, MO 63130 United States

Perception of Earth as vigorous arises from the discrepancy between model-dependent estimates of global heat flux (Q) and bulk radiogenic content, which necessitate additional sources and large secular delay. Weak, layered mantle convection is instead indicated by downward revision of these parameters, and by new theoretical models and measurements on the variation of thermal conductivity (k) with temperature. Hydrothermal circulation has been used to justify Q=44 TW derived from the half-space cooling model, rather than 31 TW obtained directly from measurements, yet MOR magmatism provides at most 4 TW. The half-space cooling model assumes inappropriate 1-D boundary conditions, resulting in infinite flux along the ridge centers over all time. Geological observations, inferred mantle overturn rates, estimated mantle cooling rates, and recent geodynamic models independently suggest that neither delayed secular cooling nor primordal heat are currently significant sources, necessitating that current heat production predominately originates in radioactive decay and is quasi-steady-state. Models of Earth's bulk composition based on enstatite chondrites are sufficiently radioactive to supply Q=31 TW, contain enough iron metal to account for Earth's huge core, and have the same oxygen isotopic ratios as the bulk Earth. That Earth is now quasi-steady state is further supported by nearly uniform release of heat over the entire surface. Weak mantle convection, suggested by quasi-steady state, is compatible with circulation within a chemically distinct mantle layers, as thinner layers mean lower Rayleigh numbers. Different dynamical styles above and below 670 km are required by k(T) variations and a change from vibrational to radiative transport mechanisms. Finally, the surface expression of mantle convection is compatible with layering: Geodesic and tomographic studies indicate that lower mantle flow is dominated by a double torus. We propose that the upper mantle system is organized in response to the non-hydrostatic triaxial stress field arising from convective motions of the lower mantle. Simple conjugate shears in the lithosphere that result from triaxial deformation are occupied by oceanic ridges and make a striking "X" pattern in polar projection. Their orientation creates alternating thermal and mechanical couplings between the upper and lower mantle systems, leading to largely E-W continental drift, and to longitudinal concentration of continents and subducting slabs. Hot-spot and ridge magmatism is attributed to thermal runaway and near-solidus temperatures, rather than to material exchange with lower mantle, which is strongly impeded. Back

V44B-03 16:35h

Dynamics of Thermochemical Plumes

* Farnetani, C G (cinzia@ipgp.jussieu.fr) , Institut de Physique du Globe, LDSG, boite 89 4, pl. Jussieu, Paris, FRA 75252 France
Samuel, H (henri.samuel@yale.edu), Yale University, Dept. of Geology and Geophysics PO Box 208109 , New Haven, CT 06511 United States

We investigate the dynamics of thermo-chemical plumes to enlighten the fundamental differences with purely thermal plumes. The key features of our 3D numerical model include: (1) a compressible mantle with an endothermic phase transition at 670km depth, (2) a mantle 'wind' induced by the imposed surface plate motion, (3) twenty million active tracers simulate denser material initially in the lowermost mantle, (4) plumes form naturally i.e., without imposing any temperature perturbation. First, we investigate the widely accepted head-tail structure of plumes. Our results show that thermo-chemical plumes reaching the surface may or may not have a head since, in some cases, only a narrow 'tail' of hot material is able to ascend in the upper mantle. Therefore, we suggest that the existence of a large igneous province at the onset of hotspot volcanism is not a valid prerequisite for a deep plume origin. Second, we investigate the entrainment of deep heterogeneities. Our results show the generation of narrow, long lasting, distinct filaments in the plume's tail. Therefore, the plume conduit is laterally heterogeneous, rather than concentrically zoned. Third, we calculate the shear wave velocity anomalies in the lower mantle, using the temperature field and the distribution of chemical heterogeneities provided by the convection model. The great variety of plume's shapes and sizes differs strikingly from the expected 'mushroom' shape of purely thermal plumes, bearing important implications for the interpretation of seismologically detected plumes. Finally, our model predictions will be compared with a variety of observations in the Central Pacific. Back

V44B-04

Do unradiogenic noble gases in oceanic basalts indicate undegassed deep mantle?

* Kurz, M D (mkurz@whoi.edu) , Woods Hole Oceanographic Institution, Mail Stop #25 Clark Laboratory WHOI, Woods Hole, MA 02543 United States

Unradiogenic helium and neon isotopic compositions, found in some oceanic island volcanoes, have been interpreted by geochemists as evidence for undegassed reservoirs deep in the earth. This presentation will provide a brief review of some of the evidence for and against this standard model. The deep undegassed mantle hypothesis has been challenged by geochemists and geophysists who would prefer to explain geochemical variations by recycling and non-plume processes. One alternative explanation is that helium is more compatible than Th and U during silicate melting, which could result in unradiogenic helium isotopes (high 3He/4He ratios) in ancient depleted sources. Crystal/liquid noble gas partition coefficients are not well known, but recent laboratory studies (e.g., Brooker et al., 2003; see also Parman et al., this meeting) yield values significantly lower than earlier studies which had suggested that noble gases might behave as compatible elements during melting. Estimates based on natural basaltic phenocrysts, compared to co-existing submarine glasses, strongly suggest that helium is more incompatible than Th and U (D for olivine/melt less than 0.001). Therefore, existing data do not support the hypothesis that helium is more compatible than Th and U. Recent studies suggest that helium and neon isotopes are well correlated in submarine oceanic basalt glasses, which suggests coherent evolution of mantle (Th+U)/Ne and (Th+U)/He ratios. The correlations of helium with neon, and helium with solid radiogenic isotopes (Sr,Nd,Pb), provide important arguments against models involving complete decoupling of helium from other elements by storage in ancient lithosperic reservoirs or melting processes. The most unradiogenic helium and neon isotopic signatures are routinely found in the most active volcanic regions, also suggesting a relationship between noble gases and excess heat and melting. The global isotopic data show that unradiogenic helium and neon are most often associated with Sr, Nd and Pb isotopic compositions that are intermediate between depleted mantle and hypothetical bulk earth mantle; this demonstrates that the undegassed reservoirs are not totally primitive in geochemical composition. The early earth most likely had a huge inventory of light noble gases and a partially degassed/depleted terrestrial mantle could yield the observed isotopic characteristics. The existence of relatively undegassed mantle reservoirs in the deep earth still provides the simplest explanation for all the observations. Undegassed reservoirs could exist within the low viscosity lower mantle or the core/mantle boundary. Back

V44B-05

The Standard Model for Noble Gases in Mantle Geochemistry: Some Observations and Alternatives

* Meibom, A (meibom@pangea.stanford.edu) , Department of Geological and Environmental Sciences, 320 Lomita Mall, Stanford University, CA 94305 United States
Sleep, N H (norm@pangea.stanford.edu) , Department of Geophysics, Mitchell Building, Stanford University, CA 94305 United States
Zahnle, K (kzahnle@mail.arc.nasa.gov) , NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 United States
Anderson, D L (dla@gps.caltech.edu) , Seismological Laboratory, MS 252-21 California Institute of Technology, Pasadena, CA 91125 United States

We evaluate the Standard Model of noble gases against a number of observational constraints of relevance to the distribution of noble gases in the Earth's mantle. These constraints include: $1$) the lack of evidence for high $^3$He/$^4$He ratios correlating with high (initial) He concentrations, $2$) that MORB and OIB $^3$He/$^4$He data do not represent two different distributions [$1$], $3$) that systematic global correlations between $^3$He/$^4$He ratios and lithophile isotopic systems are lacking, $4$) that the correlations we do observe are broadly linear, $5$) that large, local geographical $^3$He/$^4$He variations are observed, which are inconsistent with a strongly localized (i.e. plum-stem) flux of high-$^3$He/$^4$He material, and $6$) that dramatic temporal $^3$He/$^4$He variations are observed on very short time scales ($10-100$ years). Non-layered noble gas mantle models, in which the carrier of unradiogenic He is a relatively noble gas-poor phase scattered in the mantle, are more consistent with this set of constraints. We propose that the carrier of unradiogenic noble gases is primarily olivine [$2$]. Olivine-rich lithologies, produced in previous partial melting events, are a natural part of the Statistical Upper Mantle Assemblage (SUMA); a highly heterogeneous assemblage of small-to-moderate scale ($1-100$ km) enriched and depleted lithologies with a wide range in chemical composition, fertility, age and isotopic signatures. The isotopic signatures of oceanic basalts, including noble gases, are obtained by partial melting of the SUMA under slightly different P-T conditions; i.e. different degrees of partial melting and different degrees of homogenization prior to eruption [$3-5$]. Noble gas isotopic systematics do not trace deep mantle components in the source materials of oceanic basalts. They may, however, indirectly indicate potential temperature, as the order in which different mantle lithologies melt depends on pressure. References: [$1$] Anderson, EPSL $193$, $77-82$ ($2001$). [$2$] Brooker et al., Lithos, $73$, S$15$ ($2004$). [$3$] Morgan and Morgan, EPSL $170$, $215-239$ ($1999$). [$4$] Meibom and Anderson, EPSL $217$, $123-139$ ($2003$). [$5$] Ito and Mahoney, EPSL submitted ($2004$). Back

V44B-06 17:20h

Mantle Plume Upwelling Rates: Evidence from U-Series in Young Ocean Island Basalts

* Bourdon, B (bourdon@ipgp.jussieu.fr) , Lab. Geochimie Cosmochimie IPGP-CNRS, 4 Place Jussieu, Paris, 75252 France
Turner, S P (sturner@els.mq.edu.au) , GEMOC, Dept of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109 Australia
Stracke, A (stracke@mpch-mainz.mpg.de) , Abt. Geochemie, Max-Planck-Institut fur Chemie, Mainz, 55128 Germany
Saal, A E (Alberto_Saal@brown.edu) , Dept of Geological Sciences, Brown Universite Providence, RI 02912 United States

U-series disequilibria measured in recent lavas at intraplate volcanoes provide a powerful probe to examine the validity of the plume model. U-Th and U-Pa fractionation produced during melting is a function of the melting rate. In turn, this parameter should scale with mantle upwelling velocities. Simply stated, a larger melting rate (larger mantle upwelling velocity) yields smaller Th and Pa excess relative to their parent nuclides. A number of observations supports this approach: (1) there is a negative correlation between $^{230}$Th excess and buoyancy fluxes (2) based on new measurements of $^{231}$Pa in the Azores, Iceland and the Galapagos and literature data, we show here that there is also a well defined correlation between $^{231}$Pa excess and buoyancy flux (3) For Hawaii, Iceland and the Azores, $^{230}$Th excess (or $^{231}$Pa excess) increases as a function of the distance from the centre of the `hotspot'. These observations suggests that `hotspot' buoyancy fluxes are associated with a greater melt production per unit of time and that the centre of `hotspot' corresponds to a faster mantle upwelling velocity than its periphery. This is therefore in strong support of a model where ocean islands are associated with faster upwelling at depth. However, there is in fact not a simple relationship between melt productivity and upwelling velocities. Notably, the presence of volatiles, of mafic lithologies or of variably enriched peridotitic source could all affect melting rate and hence U-Th-Pa fractionation. We have considered these issues in great detail using a large data base for the Azores islands. While there are clear variations in mantle source composition, they cannot explain the observations of increasing $^{231}$Pa/$^{235}$U ratio with distance from the centre of the Azores hotspot . If we take into account the effect of water in the source of the Azores, it clearly affects the scaling between U-series fractionation and upwelling velocity but not the overall conclusions. Back

V44B-07

Re, Os, and Pt Fractionation by Melt Segregation

* Ballhaus, C (chrisb@uni-muenster.de) , University of Muenster, Corrensstrasse 24, Muenster, 48149 Germany
Bockrath, C (bockrat@uni-muenster.de) , University of Muenster, Corrensstrasse 24, Muenster, 48149 Germany

If zero-age basalts are enriched with respect to 187Os/188Os relative to present-day primitive mantle, one may assume that they tap reservoirs in the mantle that are superchondritic with respect to Re/Os and/or 187Os/188Os. Most interesting are coupled enrichments in 186Os/188Os and 187Os/188Os; if these signatures could only be derived from the outer core, they would testify that some mantle plumes indeed originate at the core-mantle boundary. We report experiments with (Fe,Ni,Cu)1-xS monosulfide in silicate mantle matrix that quantify noble metal fractionation during partial silicate melting. Our model permits the derivation of isotopically enriched melts from primitive mantle sources with time-integrated chondritic Os-isotope ratios. Melting experiments of (Fe,Ni,Cu)1-xS monosulfide in fertile mantle matrix to 1400°C and 3.5 GPa show that two sulfide phases are stable at the dry silicate solidus, a crystalline FeS-rich monosulfide and a Cu2S-enriched sulfide melt. The noble metals fractionate between the sulfide phases: Os, Ir and Ru into crystalline monosulfide, and Re, Rh, Pt, and Pt into the sulfide melt. During silicate melt segregation, crystalline monosulfide remains with silicate minerals in the mantle, concentrating Os, Ir, and Ru in the residue. The molten sulfide fraction is entrained in the silicate melt as immiscible droplets and is drained from the mantle along with the silicate melt, defining the noble metal inventory of the basaltic component. Physical processes are more important in fractionating the noble metals than chemical partitioning laws. The noble metal contribution to a basaltic melt by sulfide-silicate partitioning is small. Principally, it is possible to produce basalts with superchondritic Os-isotope ratios from chondritic mantle sources, as long as there is compositional heterogeneity among the mantle sulfides. Partial melting preferentially mobilizes and selectively entrains in the silicate melt sulfide compositions with low melting points, i.e., compositions rich in Cu, Re, Pt, and Pd. If sulfide heterogeneity is an ancient signature of the mantle, old enough to allow sufficient ingrowth of radiogenic 186Os and 187Os, the entrained sulfide component will on average be more radiogenic than bulk mantle, and so will be the basalt. There is indeed evidence for grain-scale heterogeneity among mantle sulfides both with respect to major elements, noble metals, and Os-isotopes. The key question is whether such heterogeneities may survive partial melting and may be passed on to a segregating silicate melt. Back

V53C-01

The Role of Recycled Oceanic Crust in Mantle Plumes -Revisited

* Sobolev, A V (asobolev@mpch-mainz.mpg.de), Max-Planck-Institut für Chemie, Postfach 3060, Mainz, 55020 Germany
* Sobolev, A V (asobolev@mpch-mainz.mpg.de , Vernadsky Institute of Geochemistry, Russian Academy of Sciences, Kosygin str.19, Moscow, 117975 Russian Federation
Hofmann, A W (hofmann@mpch-mainz.mpg.de) Max-Planck-Institut für Chemie, Postfach 3060, Mainz, 55020 Germany
Sobolev, S V (stephan@gfz-potsdam.de), GeoForschungsZentrum,, Telegrafenberg E, Potsdam, 14473 Germany
Sobolev, S V (stephan@gfz-potsdam.de) , Schmidt Institute of the Earth Physics, Russian Academy of Sciences, B. Gruzinskaya 10, Moscow, 123810 Russian Federation
Nikogosian, I K (niki@geo.vu.nl) , Faculty of Geosciences, Department of Petrology, Utrecht University, Budapestlaan 4, Utrecht, 3584 CD Netherlands
Nikogosian, I K (niki@geo.vu.nl) , Faculty of Earth and Life Sciences Department of Petrology, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081 HV Netherlands
Kuzmin, D V , Max-Planck-Institut für Chemie, Postfach 3060, Mainz, 55020 Germany
Gurenko, A A , Max-Planck-Institut für Chemie, Postfach 3060, Mainz, 55020 Germany
Kamenetsky, V S , Max-Planck-Institut für Chemie, Postfach 3060, Mainz, 55020 Germany
Krivolutskaya, N A , Vernadsky Institute of Geochemistry, Russian Academy of Sciences, Kosygin str.19, Moscow, 117975 Russian Federation

The role of recycled material in mantle plumes is difficult to quantify on the basis of incompatible trace elements and isotopes because of the great variability of subducted material. Another approach is to use major elements and compatible trace elements because these are more uniform in the mantle and are strongly controlled by the phase petrology of melting. Subducted crustal lithologies invariably differ from mantle peridotite, and this introduces olivine-free lithologies such as pyroxenites and eclogites into the mantle. Our massive study of olivine phenocrysts and trapped melt inclusions shows unusually high Ni and Si contents in many recent primary Hawaiian magmas. Similar compositions are found in the Canary Islands, W. Greenland, and the Siberian flood basalts. These magmas are not in equilibrium with an olivine bearing source under thick lithosphere (more than 100 km) typical of these localities, because an olivine-pyroxene assemblage would buffer both Ni and Si at lower levels. In contrast, magmas from plumes located under thin lithosphere, such as Iceland or Azores show no significant Si and Ni excess, and they could be in equilibrium with a shallow, olivine-bearing source. High-Si magmas can be produced by melting of eclogite, but this does not yield high Ni contents. Therefore, the eclogite-derived melt must acquire high Ni by converting surrounding peridotite to a solid pyroxenite, which ultimately melts a shallower level. Because unreacted peridotite may also begin to melt at shallow depths, this results in mixed melts derived from (secondary) pyroxenite and peridotite. In settings of thick lithosphere, the amount of peridotite-derived melt will be relatively small. Therefore, the recycled component represented by pyroxenite-derived melt may dominate. In settings of shallow melting, the peridotite will melt more extensively, and the signal from the recycled component will be diluted. Quantitative modeling shows that over half of the Hawaiian magma volume formed during the last 1 Myr came from secondary pyroxenite representing the recycled oceanic crust. The results are consistent with a plume with potential temperature of 1600 deg.C containing about 20 percent of recycled oceanic crust in the central part. These results are also consistent with estimates of volcano volumes, magma volume flux, and seismological observations. In the context of this model, the recent increase in Hawaiian magma flux is produced by an unusually high proportion of recycled crustal material in this part of the plume. Back

V53C-02

The Metasomatic Alternative for the Origin of OIB: a Model which Reconciles Experimental Petrology and Geochemistry

* Pilet, S (Sebastien.Pilet@img.unil.ch) , Institute of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, 1015 Switzerland
Hernandez, J (jean.hernandez@img.unil.ch) , Institute of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, 1015 Switzerland
Sylvester, P J (pauls@esd.mun.ca) , Dept. of Earth Science Memorial University of Newfoundland, Alexander Murray Building, St John's, NL A1B 3X5 Canada

Variation of trace element and isotopic ratios in OIB is commonly ascribed to the recycling of ancient oceanic crust associated with crustal or pelagic sediment assimilation. However this model based on geochemical arguments is in opposition with experimental petrology data. Partial melts of oceanic-crust lithologies produce silica saturated liquids whereas many oceanic island rocks are characterized by silica undersaturated compositions. Experimental data indicate that only partial melting of peridotite in presence of carbonate [1] or of pyroxenite [2] produce liquids which are close to the nephelinite or basanite major element compositions observed in oceanic islands. In this way, recycling of subducted oceanic basal lithosphere enriched by metasomatic veins seems to represent a convincing alternative for the source of OIB [3]. However, this hypothesis does not explain the formation of isotopic heterogeneity observed in OIB. Chemical variations observed in Cantal basalt (France), interpreted as the result of a lithospheric metasomatic mechanism [4], allow us to constrain the chemical evolution of a metasomatic agent within basal lithosphere. These data demonstrate that - 1) fractionation of trace element ratios (U/Pb, Th/Pb, Rb/Sr, Sm/Nd, Nb/La .) necessary to generate, after subduction and isolation, the EM and HIMU components can be explained by metasomatic process and - 2) partial melting of veins-plus-enclosing lithospheric mantle produce basalt with composition perfectly similar to major and trace elements composition observe in OIB. This suggest that isotopic and trace element variations observed in basalts from individual oceanic islands may more likely be the result of melting heterogeneous, metasomatised, subducted oceanic lithosphere rather than a mixture of chemically distinct mantle reservoirs. End-member isotopic compositions would correspond to the extreme trace element fractionation generated by metasomatic process within the lithosphere. The new interpretation of OIB sources proposed here requires a re-evaluation of the processes that control chemical evolution of Earth's mantle reservoirs and plumes geochemical tracers. Recycled metasomatised lherzolite may be the major mantle component sampled by OIBs; recycled oceanic crust and sediment may be less common in OIB sources than is commonly assumed. The metasomatic hypothesis is entirely consistent with isotopic heterogeneity observed in the source of oceanic basalts and experimental data which indicate that the most plausible source of OIB material is pyroxenites. [1] K. Hirose (1997) Geophys. Res. Lett. 24, 2837 [2] M.M. Hirschmann et al. (2003) Geology, 31, 481. [3] Y. Nui, M. O'Hara. (2003) J. Geophys. Res.,108, B4, 2209. [4] S. Pilet et al. (2004) Geology, 32, 2, 113. Back

V53C-03

A Fossil Mantle Plume under the Emeishan Flood Basalts: Integration of Geology, Geophysics and Geochemistry

* Xu, Y (yigangxu@gig.ac.cn) , Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640 China
He, B (hebin@gig.ac.cn) , Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640 China
Chung, S (sunlin@ntu.edu.tw) , Department of Geosciences, National Taiwan University, Choushan Road, Taipei, 10699 Taiwan

The plume hypothesis is now challenged because some fundamental aspects predicted by the modeling of plumes are found to be lacking in classic regions like Iceland and Yellowstone. Instead of invoking a bottom-up process, some researchers favor a top-down hypothesis for the formation of large igneous provinces (LIPs), in which shallow lithospheric processes may fuel melt production. Seismic investigations and tomographic models help trace mantle plumes in modern, active hotspots, but are of limited benefit in identifying ancient plumes, mainly because geophysics provides us with a snapshot of the present-day Earth's structure. Consequently the geological footprint associated with thermal anomalies are the clues to tracing ancient plumes. According to some theoretical models, pre-volcanic lithospheric uplift is the most important criteria used to identify the presence of plumes. The lack of such evidence, on the other hand, is an argument against the involvement of plumes in the formation of LIPs. Recent examination of the middle-late Permian sedimentology in southwest China reveals kilometer-scale lithospheric doming prior to the Emeishan flood volcanism (He et al., 2003). This, and correlations between diverse, independent parameters involving crustal doming, paleo-geography, sea level change, mantle melting mechanism and crust-mantle structure, provide evidence for a fossil mantle plume under the Emeishan LIP. Specifically, the consequences of plume-lithosphere interaction include: (a) pre-volcanic uplift including thinning of marine carbonates, a marine to sub-aerial transition, local provenance of clastic sediments, and a marked erosional unconformity, evident as palaeokarstic surfaces on the marine carbonates; (b) a domal structure (700 km radius); (c) variations in the thickness of volcanic rocks across the domal structure; (d) variations in flood basalt geochemistry from the center to the edge of the domal structure that are interpreted as high temperature melts in the center and lower temperature melts at the edge; (e) gradual decrease in crustal thickness from the center to the margin of the dome; and (f) the presence of high velocity lower crust (20-30km) immediately beneath the domal structure which is consistent with significant melt production and possible underplating/intrusion into the lower crust. Back

V53C-04

The Mantle Plume Hypothesis Pro and Con: Evidence from Earth's Most Voluminous Large Igneous Provinces

* Ingle, S (single@geo.titech.ac.jp) , Earth and Planetary Sciences, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
Coffin, M F (mcoffin@ori.u-tokyo.ac.jp) , Ocean Research Institute, University of Tokyo 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan

Mantle plumes are upwellings of large volumes of mantle material in focused conduits, the leading ends of which are referred to as plume heads. Large igneous provinces (LIPs) are suspected to form from magmatism resulting from plume head decompression melting, but, evidence for this theory for the origins of LIPs is mixed. We have now reached the point of having either to modify the theory to fit characteristics of individual LIPs or to abandon the theory and search for a more unifying explanation. A case study of the two biggest LIPs on Earth - the Ontong Java Plateau (OJP) in the western equatorial Pacific Ocean, and the Kerguelen Plateau / Broken Ridge (KPBR) in the southern Indian Ocean - allows us to examine key predictions of mantle plume theory, including: (1) subaerial eruption of large portions of oceanic LIPs (2) large extents of partial melting in the plume head, resulting in tholeiitic basalt-type magmas, coupled with (3) rapid formation of the LIP, (4) post-formation subsidence comparable to normal oceanic lithosphere, and (5) the presence of a hotspot track and/or an active hotspot. The KPBR formed largely above sea level over a protracted time period ($\sim$120 Ma - present) in the growing Indian Ocean basin. Early Cretaceous melts were derived from a heterogeneous source, complicated by subsequent local assimilation of continental crust. Most lavas recovered from the plateau are tholeiitic, but alkalic and evolved volcanics occur in several, widespread locations. Subsidence of the plateau has followed predictions for normal oceanic lithosphere. A prominent hotspot track, the Ninetyeast Ridge, connects Broken Ridge with Early Cretaceous continental basalts on the eastern margin of India. The Kerguelen hotspot is still active today, creating Heard and MacDonald Islands on the central plateau. The OJP was constructed well below sea level on existing Pacific lithosphere. Nearly the entire volume of magma is believed to have been created instantaneously, at $\sim$120 Ma, from large degrees of partial melting ($\sim$30%) of a homogeneous source. Melting extents were high, so either a large temperature anomaly, or a major decompression event is required because volatile contents were low. It has subsided relatively little, or erratically, since its formation. No known hotspot track is associated with the OJP, nor is any active hotspot. Some of these observations agree with expectations of a mantle plume head origin, but several are contrary to predictions. Alternative mechanisms for the formation of LIPs, including extraterrestrial (i.e. bolide impact) or tectonic causes, are also problematic. A few of the obvious concerns are (1) large bolide impact events might not occur often enough to account for the number of known LIPs, (2) tectonic origins for LIPs ignore the presence of LIPs on planetary bodies where evidence for plate tectonics is nil or scant. A common mechanism for the formation of LIPs is highly desirable, yet, at present, all existing hypotheses appear in some way deficient. Back

V53C-05

High Volatile Content and Shallow Melting at the end of the Siberian Flood Basalts: Experimental Results

Draper, D S (dave@draper.name) , Institute of Meteoritics, 1 University of New Mexico, Albuquerque, NM 87131 United States
* Elkins-Tanton, L T (Lindy@brown.edu) , Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
Jewell, J D (jessica.jewell@gmail.com) , Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
Thorpe, A (akthorpe@hotmail.com) , Brown University, Geological Sciences 324 Brook St., Providence, RI 02912 United States
Agee, C B (agee@unm.edu) , Institute of Meteoritics, 1 University of New Mexico, Albuquerque, NM 87131 United States

Constraints on the depth and temperature of melting for flood basalt lavas are critical for evaluating melting models. Obtaining primary melting conditions through straightforward experimental petrology is unfeasible for most flood basalts because of significant secondary processing. The final lavas in the Siberian flood basalts (SFB), however, are candidates to be nearly primary magmas, having experienced only olivine addition or subtraction following mantle melting. These ultramafic lavas, predominantly meimechites, form a 1400 m stack at the top of the SFB section in the Maymecha region. One-atmosphere, piston-cylinder, and multi-anvil experiments have been performed on a synthetic analog of a meimechite olivine melt inclusion to determine its pressure and temperature of original mantle melting. Although the major element compositions of meimechite melt inclusions have the same trends as do the bulk rocks, the alkali contents of the melt inclusions are systematically higher, suggesting that the lavas have lost alkalis in a post-eruption serpentinization event. Meimechites are enriched in incompatible elements, particularly the LREE (Basu et al., 1995; Arndt, 2003), and are hydrous, evidenced by groundmass phlogopite. Analysis of meimechite major element trends indicates that compositions with approximately 25 wt% MgO represent liquid compositions. The experimental composition has 25.5 wt% MgO, 8.3 wt% CaO, and Mg\# 77. The experimental composition with 2% water is multiply saturated on its liquidus at 3.0 GPa and about 1600\deg C with olivine, sub-calcic augite, garnet, and Ti spinel. This multiple saturation is at an anomalously high temperature, interpreted to imply an even larger volatile concentration in the source region, which would likely lower the temperature of melting by as much as 100 to 150\deg without largely changing pressure. Pre-eruptive water and carbon dioxide content estimates are being obtained in current research analyzing melt inclusions. The multiple saturation point is interpreted as the conditions of batch melting in the mantle, or the mid-point of melting during an adiabatic ascent. The experimental meimechite therefore is inferred to have originated from mantle melting at about 100 km depth, surprisingly shallow for melting in a plume under an ancient continental lithosphere. Alternative models for melting are also required to explain the strong geologic evidence for subsidence during the first kilometer of eruption (Federenko and Czamanske, 1997). We suggest that the lower lithosphere delaminated, pulling topography down and allowing shallow melting. As it sank and heated, the lithosphere would have dewatered, providing volatiles for the meimechite source. The volatile input from the lithosphere can also explain the unusual meimechite trace element compositions. We also note that orthopyroxene instability in the source region, implied by its absence at multiple saturation, is further evidence for hydrous metasomatism of the mantle. This model is not contradictory to a plume model, but requires an upwelling weak enough (with relatively little buoyancy) to allow the delaminating material to cause subsidence in the lithosphere. Back

V53C-06

Hundreds of Rimmed Circular Structures on Venus Formed From Impacts Before 3.9 Ga, Not From Young Plumes

* Hamilton, W B (whamilto@mines.edu) , Dept. of Geophysics, Colorado School of Mines, Golden, CO 80401 United States

Venus displays hundreds of circular structures, with topographic rims 10-2,000 km in diameter, that have the morphology, cookie-cutter superposition, and log frequency/log size distribution required of, and unique to, impact craters and basins. They nevertheless are assumed to be endogenic by specialists. Many have interior central or ring uplifts or broad, low volcanic constructs. Many are multiring. Old uplands are saturated with the structures, which there are variably eroded, whereas lowland structures are partly to entirely buried. The largest (Artemis, Heng-O, and Quetzalpetlatl, rim diameters 2000, 900, and 800 km) are among the youngest. Analogy with dated large, and similarly relatively late, Imbrium impact basin on the Moon requires ages greater than 3.85 or 3.90 Ga. Venus preserves much of its surface of late-stage main planetary accretion. Early investigators of Venusian radar imagery accepted the possible impact origin and great age of the circular structures, but in the late 1980s impact was replaced, almost without analysis, by plume conjectures. Almost all specialists now assume that Venus has a thermal structure and heat loss comparable to that of Earth, and that its only impact structures are mostly-pristine small to midsize (maximum rim diameter, 270 km) craters with an assumed age of less than 0.5 or 1.0 Ga. (Ages as old as 3.9 Ga are advocated here for these young craters.) The older circular structures are rationalized as produced by mantle plumes and upwellings that deformed crust and upper mantle from beneath, with or without extrusion of subordinate lava, and that magmatically and tectonically resurfaced Venus in a brief period before the late impacts. Extrapolation of plume conjecture to Venus from Earth has little merit. Terrestrial plume speculation is based on assumptions whose predictions have been consistently falsified. Not only do plumes probably not exist on Earth, but even the least-constrained attributions of geologic and tectonic features to them do not include circular structures that in any way resemble those of Venus. Conversely, Venusian conjectures neither address nor account for circularity and superpositions. The hot-Venus assumption behind young-surface speculation also is dubious. The lack of a magnetic field on Venus (its core is likely solid), the positive correlation of its topography and geoid (outer Venus is much stiffer than Earth), and its origin close to the Sun (less potassium, so much less early radiogenic heat), and other factors indicate Venus to be much colder internally than Earth below the depth of influence of greenhouse atmosphere. The most eroded and breached, or buried, of the quasi-pristine craters are discriminated only arbitrarily from the best-preserved of the ancient, and mostly larger, circular structures. From those in turn, there are all gradations back to the deepest-eroded, or the most-buried, structures of the old family. Broad, low volcanic constructs (unlike any terrestrial volcanoes) inside impact basins likely are products of cogenetic impact melts. Other large, low volcanoes also are circular, are isolated, and may be of impact melts that buried their basins. Broad tessera-surfaced plateaus are of layered rocks, display deformation and topography indicative of outward gravitational spreading, and may have formed from ancient impact-melt lakes. Venusian lowlands are floored not by young lava plains but by ancient sediments, probably including deposits in a transient ocean, derived from uplands. The plains are speckled with mud volcanoes (not lava cones) that, like minor deformation of the sediments, are due to top-down heating by the evolving atmosphere. Back

V53C-07

Large topographic rises, coronae, large flow fields and large volcanoes on Venus: Evidence for mantle plumes?

* Smrekar, S (ssmrekar@jpl.nasa.gov) , JPL, 4800 Oak Grove Dr., Pasadena, CA 91024 United States
Stofan, E (ellen@proxemy.com) , Proxemy Research, 20528 Farcroft Lane, Laytonsville, MD 20882 United States

Voluminous volcanic deposits at topographic rises, coronae, large flow fields and large volcanoes have led these features to be linked to mantle plumes. Topographic rises have broad, swell-like topography (typically $\sim$1800 km across, 1.5 km high), large positive gravity anomalies, and associated volcanism. Coronae are circular to irregular features (typically $\sim$300 km across, 1.0 km high), defined by their fracture annulus and associated with uplift and volcanism, followed by subsidence. Large volcanic flow fields ($>$30,000 km2) and large volcanoes ($>$100 km in diameter) are sites of voluminous outpourings of lava. We interpret the variations in styles of volcanism, surface deformation, topography and gravity signatures to indicate differences in the nature of the underlying thermal upwellings that formed these features. Most topographic rises are likely to be formed by primary or deep-seated plumes, while coronae and probably most volcanoes result from shallower upwellings or secondary plumes. Extension clearly plays a critical role in the formation of large flow fields, coronae and some large volcanoes. We do not interpret large flow fields to be related to plumes. There are a similar number of primary plumes on Earth and Venus, but Venus has a much larger number of secondary plumes. The increase number of secondary plumes on Venus may result from the lack of slab cooling at the core-mantle boundary, its lack of a low viscosity zone or its stronger lithosphere. Back