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                            |  | Why is the 
                              uplift and subsidence history of the Ontong Java 
                              Plateau anomalous compared to other large igneous 
                              provinces? |  Julie 
                          Roberge1 and Paul 
                          J. Wallace2 1Laboratorio 
                          Universitario de Petrología, Instituto de Geofisica, 
                          Universidad Nacional Autónoma de México 
                          (UNAM), Coyoacan, 04510, Mexico, D.F. roberge@geologia.unam.mx2Department of Geological Sciences, University 
                          of Oregon, Eugene, OR  97403-1272, pwallace@uoregon.edu
 
                           
                            |  
                                This webpage is based on: 
                                
                                  Roberge, 
                                    J., Wallace, P. J., White, R. V., and Coffin, 
                                    M. F., 2005, Anomalous uplift and subsidence 
                                    of the Ontong Java Plateau inferred from CO2 
                                    contents of submarine basaltic glasses. Geology, 
                                    33, 501-504. 
                                  Roberge, 
                                    J., White, R. V. and Wallace, P. J. , Volatiles 
                                    in submarine basaltic glasses from the Ontong 
                                    Java Plateau (ODP Leg 192): implications for 
                                    magmatic processes and source region compositions, 
                                    in: Origin 
                                    and Evolution of the Ontong Java Plateau, 
                                    Fitton J.G., Mahoney J.J.,  Wallace P.J. 
                                    and Saunders A.D., Eds., Geological Society 
                                    (London) Special Publication, 229, 
                                    239-257, 2004. |  Overview  The Ontong Java Plateau 
                          (OJP) in the western Pacific (Figure 1) is the largest 
                          volcanic oceanic plateau and may represent the largest 
                          magmatic event on Earth in the last 200 Ma. Relevant 
                          statistics are: 
                          Age = Cretaceous  (122 Ma)Volume = 40-45 million km3Area > 1.6 million km2The crest of the plateau is ~1700 m 
                            below sea level, and elsewhere it is ~2-3 km deep. 
 Figure 1: Location 
                          of the Ontong Java Plateau in the western Pacific and 
                          ETOPO5 bathymetric map of the Ontong Java Plateau showing 
                          locations of Leg 192 drill sites (stars).  Locations 
                          of previous ODP and DSDP drill sites that reached basement 
                          (small solid circles) are also shown.  Depth contours 
                          are in meters below sea level. Click here 
                          or on figure for enlargement.  The OJP is anomalous 
                          compared to other oceanic large igneous provinces such 
                          as the Kerguelen plateau in that it never formed a subaerial 
                          landmass and did not cause extinction (Figure 2) [Ed: 
                          see also other pages on 
                          Ontong Java]. 
 Figure 2: Comparison 
                          of OJP with other large igneous provinces. Click here 
                          or on figure for enlargement.  Submarine basaltic glasses 
                          preserve information on magmatic volatile contents (Figure 
                          3).  H2O and CO2 concentrations 
                          can then be used to estimate paleo-eruption depths and 
                          thus constrain better the uplift and subsidence history 
                          of the OJP. 
 Figure 3: Example of 
                          unaltered basaltic glass from pillow rims at sites 1183, 
                          1185, 1186, 1187 (left), and glass in non-vesicular 
                          glass shards in volcaniclastic rocks from site 1184 
                          (right). Water 
                          in the mantle source region for Ontong Java Plateau 
                          basaltic magmas? During ODP Leg 192, five 
                          widely spaced sites were drilled (Figure 1; Mahoney 
                          et al., 2001).  Unaltered glass from pillow 
                          basalt rims at four locations (ODP Sites 1183, 1185, 
                          1186, and 1187) and from non-vesicular glass shards 
                          in volcaniclastic rocks at Site 1184 were analyzed for: 
                          H2O and CO2 using 
                            Fourier Transform Infrared (FTIR) spectroscopy (Table 
                            1), andfor major elements using a Cameca SX-50 
                            Electron Microprobe at University of Oregon.  
                           Our results complement 
                          previously published data for glasses recovered from 
                          ODP Leg 130 Sites 803 and 807 (Michael, 
                          1999).  For consistency, we reanalyzed the 
                          glasses from Sites 803 and 807 because we used a different 
                          data reduction procedure for our CO2 analyses.  
                          Our new analyses and data reduction procedure result 
                          in CO2 values that are mostly 15 ppm (at 
                          lower concentration) to 30 ppm (at higher concentration) 
                          lower than those of Michael 
                          (1999). Table 1: Average dissolved 
                          H2O and CO2 contents of submarine 
                          basaltic glasses from the Ontong Java Plateau (OJP).   Note: H2O and 
                          CO2 were analyzed by FTIR spectroscopy using 
                          band assignments and absorption coefficients as described 
                          in Roberge 
                          et al. 
                          (2004).  Peak height measurements for CO2 
                          were calculated using a peak fitting program (S. Newman, 
                          unpublished).  This method yields values for experimental 
                          glasses (Dixon et al., 1995) that are comparable 
                          to the reference-glass subtraction and hand-drawn background 
                          method upon which the CO2 solubility relations 
                          have been established (J. Dixon, written comm.)  
                          The H2O and CO2 values reported 
                          for each site are averages (± 2σ in parentheses) 
                          of multiple glass chips (see Roberge 
                          et al., 
                          2004, for complete data and analytical uncertainties). 
                          Vapor saturation pressures were calculated using VolatileCalc 
                          1.1 (Newman & Lowenstern, 2002). Uncertainties 
                          (in parentheses) for saturation pressures, eruption 
                          depths, and subsidence values are based on propagation 
                          of 2σ uncertainties in the average H2O 
                          and CO2 values. The subsidence uncertainties 
                          also include uncertainties in depths of the samples 
                          in the drill hole. *Based on interpretation 
                          that Site 1184 volcaniclastic rocks were erupted in 
                          a shallow marine environment but deposited subaerially 
                          (Thordarson, 2004).
 An exciting discovery of Leg 192 was that basement at 
                          Site 1187 and the upper group of flows at Site 1185 
                          are composed of high-MgO, incompatible-element-poor 
                          basalt that are unlike basalts found elsewhere on the 
                          OJP (Figure 4).  Relatively low K2O, 
                          Na2O, and P2O5 in all 
                          glasses suggest that OJP basaltic magmas formed by large 
                          extents of melting.  Low-MgO basalts can be derived 
                          by fractionation at low to moderate pressure from parental 
                          magmas similar to the high-MgO Site 1187 basalts.
 
 Figure 4: Major element 
                          compositions of Ontong Java Plateau basaltic glasses. 
                          Data from Sites 803 and 807 and the island of Malaita 
                          are from Michael 
                          (1999). Lines show fractional crystallization paths 
                          for a parental magma with 17.6 wt% MgO calculated as 
                          described in the text. Crystallization of this parental 
                          composition at pressures of 1 bar to 2 kbar can largely 
                          reproduce the observed range of major element compositions. 
                          Under these conditions, the crystallization sequence 
                          is olivine, followed by olivine + plagioclase, followed 
                          by olivine + plagioclase + clinopyroxene. Click here 
                          or on figure for enlargement.
 However, H2O 
                          concentrations are similar in the two basalt types despite 
                          the lower K2O and TiO2 of the 
                          high-MgO glasses.  To understand H2O 
                          in mantle source regions it is useful to compare H2O/Ce 
                          ratios because these elements have a similar incompatibility 
                          to one another during mantle melting and fractional 
                          crystallization (Michael, 1995). H2O/Ce 
                          values for OJP basalt are 355-370 for high-MgO glasses 
                          and 270 for low-MgO glasses (Figure 5).  These 
                          values are higher than most depleted and enriched MORB 
                          (Michael, 1995).  However, the H2O/Ce 
                          values of all glasses may be elevated because of assimilation. 
                          If uncontaminated OJP magmas have low Cl/K like other 
                          mantle-derived magmas, then primary H2O/Ce 
                          values may be as low as 125-140. 
 Figure 5: H2O/Ce 
                          versus K/Ti for OJP basaltic glasses (left). Shown for 
                          comparison are H2O/Ce ranges for MORB glasses 
                          from various regions (MAR = Mid-Atlantic Ridge). Horizontal 
                          black bar shows the H2O/Ce ratio estimated 
                          as described in Roberge 
                          et al. (2004) for uncontaminated OJP magmas. Click 
                          here or on 
                          figure for enlargement.  In addition, trace element 
                          abundances demonstrate that OJP basaltic magmas formed 
                          by large degrees of melting (Figure 6).  However, 
                          as previously stated, OJP basalts have low H2O, 
                          similar to depleted MORB.  Therefore the large 
                          degrees of melting must have been caused by high melting 
                          temperature (>1550°C). 
 Figure 6: Graph of 
                          Nb vs. Zr showing the extent of melting required to 
                          produce OJP basalts (from Fitton & Goddard, 2004). Estimating 
                          paleo-eruption depths Vapor saturation pressures 
                          were calculated for all sites and then converted into 
                          eruption depths (1 bar = 10 m water depth) assuming 
                          equilibrium solubility of H2O and CO2 
                          at the time of quenching (Figures 7 and 8; Table 1).  
                          As expected, glass shards from the volcaniclastic deposits 
                          at Site 1184 have low vapor saturation pressures, indicating 
                          an average quenching depth of 540 ± 210 m. Site 
                          1183 glasses, which come from the shallowest water site 
                          on the central high plateau, also have relatively low 
                          vapor saturation pressures of 107 bars (1070 ± 
                          90 m), whereas Sites 1185, 1186 and 1187 have saturation 
                          pressures of 215 to 245 bars, yielding estimated eruption 
                          depths of 2150 to 2450 (±100) m.  Samples 
                          from Site 803 yield an average saturation pressure of 
                          290 bars (2900 ± 90 m).  
 Figure 7: H2O 
                          versus CO2 for Ontong Java Plateau basaltic 
                          glasses.  Symbols correspond to Ocean Drilling 
                          Program (ODP) site numbers.  Vertical lines represent 
                          degassing paths for basaltic melts with initial CO2 
                          contents of 200 ppm (solid line) and 2000 ppm (dashed 
                          line).  Also shown are vapor saturation curves 
                          for basaltic melts at pressures of 10-35 MPa.  
                          All calculations were made using VolatileCalc 1.1 (Newman 
                          & Lowenstern, 2002). 
 Figure 8: Eruption 
                          depth estimates (in mbsl) for Ocean Drilling Program 
                          (ODP) sites on Ontong Java Plateau. For all sites, present-day 
                          depth of top of igneous basement has been corrected 
                          for sediment loading.  Corrected basement depth 
                          (Dc) is obtained from the equation of Crough 
                          (1983): Dc = dw + ts(rs– 
                          rm)/( rw– rm), 
                          where dw is water depth (in m), ts 
                          is sediment thickness (in m), rs is average 
                          sediment density (1.9 g/cm3), rm 
                          is upper-mantle density (3.3 g/cm3), and 
                          rw is seawater density (1.03 g/cm3).  The estimated eruption 
                          depths for all sites should be viewed with caution for 
                          several reasons (Figure 9). Submarine basaltic pillow 
                          rims, particularly MORB samples, are commonly supersaturated 
                          with CO2 (Dixon & Stolper, 1995), 
                          so measured CO2 contents in pillow rims could 
                          potentially overestimate true eruption depths. However, 
                          submarine OJP lava flows are likely to have much larger 
                          volumes and longer flow distances than MORB flows. Geochemical 
                          data suggest that some OJP flows may have traveled 100s 
                          of km (P.J. Michael, written comm.).  This would 
                          allow time for dissolved CO2 to reach equilibrium 
                          values at the appropriate seafloor depth before final 
                          quenching.  In fact, such long downslope flow distances 
                          could have caused lavas to be vapor saturated near their 
                          eruption (vent) depth, which would be shallower than 
                          the final depth of emplacement (Michael, 
                          1999).  Thus we argue that our “eruption” 
                          depths calculated from CO2 data are minimum 
                          values because true emplacement depths could have been 
                          deeper.  This line of reasoning provides a plausible 
                          explanation for the large differences in CO2 
                          contents and inferred eruption depths of glasses from 
                          Site 807 Unit A and Units C-G (Table 1).  The low 
                          CO2 contents of Unit A glasses suggest that 
                          this may have been part of a very long lava flow that 
                          had an original vent in much shallower water.  
                          In contrast, Units C-G represent multiple flows, all 
                          of which have much higher CO2, and their 
                          CO2 contents probably more closely represent 
                          their original emplacement depth (3041 ± 240 
                          m). 
 Figure 9: Illustration 
                          of the potential problems of using CO2 content 
                          to infer paleo-eruption depth. Uplift 
                          and subsidence of the OJP Mesozoic marine magnetic 
                          anomalies in the Nauru Basin (adjacent to OJP) suggest 
                          that the OJP formed within ~130 to ~155 Ma oceanic crust. 
                          The depth of 10 to 35 Ma oceanic crust lies between 
                          3600 and 4700 m according to global age-depth curves 
                          (Ingle 
                          & Coffin, 
                          2004). Using the high Plateau (Site 1183) eruption 
                          depth of 1170 m and the Eastern Salient (Site 1184) 
                          eruption depth of 540 m we calculate that the maximum 
                          uplift was 2400-3500 m for the high Plateau and 3100-4200 
                          m for the Eastern Salient. Dynamic Uplift  The arrival of a hot 
                          and buoyant plume at the base of the lithosphere, combined 
                          with crustal thickening due to eruption and intrusion 
                          of a large volume of basaltic magma, should produce 
                          substantial surface uplift (Olson & Nam, 
                          1986).  Dynamic uplift is the thermal doming produced 
                          by viscous normal stresses imposed on the lithosphere 
                          by the rising of a plume head and has been estimated 
                          through experimental and theoretical studies (e.g. 
                          Farnetani 
                          & Richards, 
                          1994; Hill, 1991; Griffith 
                          et al., 
                          1989; Olson & Nam, 1986).  The 
                          results show that at first, before the diapir reaches 
                          the base of the lithosphere, the surface topography 
                          is determined by the diameter, density anomaly and depth 
                          of the diapir (plume head).  When the top of the 
                          diapir reaches approximately one diapir radius from 
                          the surface, an asymmetric surface swell appears increasing 
                          in height and decreasing in width (Figure 10A).  
                          When the upper edge of the diapir reaches 0.2 diapir 
                          diameters below the crust surface, the height of the 
                          surface swell attains a maximum, with a minimum width 
                          (Figure 10B).  From this point on, the swell will 
                          subside and increase in width as the diapir spreads 
                          laterally beneath the surface.  To estimate OJP’s 
                          uplift we need to assume a certain plume head volume, 
                          which can be estimated from the erupted volume of the 
                          plateau and the degree of partial melting needed to 
                          produce it.  Here, a plateau volume of 4.5 x 107 
                          km3, crustal and mantle densities of 2.9 
                          and 3.3 g/cm3, respectively, a plume temperature 
                          of ~1550°C, and 30% partial melting (Fitton 
                          & Godard, 2004) are used.  With these 
                          parameters, dynamic uplift models applied to the OJP 
                          predict an uplift of ~1000 to ~3000 m above the surrounding 
                          seafloor depending on the shape and diameter of the 
                          plume head (Neal 
                          et al., 
                          1997; Farnetani 
                          & Richards, 
                          1994; Hill, 1991; Griffith 
                          et al., 
                          1989; Olson & Nam, 1986). 
 Figure 10: Illustration 
                          of theoretical and laboratory dynamic models.  
                          All models reach similar conclusions: Maximum dynamic 
                          uplift for a high temperature (~1500°C) plume and 
                          25-30% partial melting is 1000 to 3000 m.  Isostatic Uplift  Models that explain hotspot 
                          uplift by isostatic compensation of thermally expanded 
                          mantle rather than the dynamic effects of a rising plume 
                          yield similar results (Ito 
                          & Clift, 
                          1998).  The isostatic effect of crustal thickening 
                          has also been modeled, suggesting an additional isostatic 
                          uplift of 2 to 4 km above the adjacent seafloor (Neal 
                          et al., 
                          1997).  Gladczenko 
                          et al. 
                          (1997) calculated the average OJP crustal density 
                          to be 2.86 g/cm3 on the basis of combined 
                          seismic velocity analyses and gravity modeling. However, 
                          given uncertainties in velocities and the non-unique 
                          nature of gravity modeling, it is appropriate to calculate 
                          isostatic uplift for a range of densities (2.8 to 3.0 
                          g/cm3).  Below sea level, water-corrected 
                          isostasy was calculated using  (1)
  where Δh is amount 
                          of uplift above seafloor, hOJP is Ontong 
                          Java Plateau crustal thickness, hOC is thickness 
                          of normal oceanic crust (7 km), ρw is 
                          water density (1.03 g/cm3), ρm 
                          is mantle density (3.3 g/cm3), and ρOJP 
                          is Ontong Java Plateau crustal density (2.8 to 3.0 g/cm3). 
                          For calculations above sea level, water- and air-corrected 
                          isostasy was calculated using  (2)
  where hw is 
                          water depth for normal 10-35 Ma oceanic crust (4.1 km). 
                          Using the high plateau (Site 1183) crustal thickness 
                          of ~30 km (Gladczenko 
                          et al., 
                          1997), we estimate isostatic uplift ranging from 
                          2400 m (ρOJP = 3.0 g/cm3) 
                          to 4700 m (ρOJP = 2.8 g/cm3) 
                          above the surrounding seafloor due to the effects of 
                          crustal thickening (Figure 11).  Adding the initial dynamic 
                          uplift (2000 ± 1000 m), and taking into consideration 
                          how this changes the seafloor water depth (which in 
                          turn influences the isostatic effect of crustal thickening), 
                          OJP maximum total uplift would range from 4300 to 6100 
                          (±1000) m above the surrounding seafloor (Figure 
                          11).  These estimates are larger than the estimated 
                          maximum uplift (2500 to 3600 m) based on H2O 
                          and CO2 data for the basaltic glasses from 
                          Site 1183, though there is a slight overlap when the 
                          uncertainties in dynamic uplift are considered. 
 Figure 11: Comparison 
                          of predicted vs. observed uplift of Ontong Java Plateau 
                          (OJP).  Diagram A shows estimated isostatic uplift 
                          due to crustal thickening.  Diagram B shows both 
                          isostatic and an average of 2000 m of dynamic uplift 
                          (dashed lines) based on plateau-specific models (Neal 
                          et al., 1997; Ito 
                          & Clift, 1998).  Diagonally ruled area 
                          shows maximum plateau uplift inferred from paleo-eruption 
                          depths based on CO2 data. Click here 
                          or on figure for enlargement.   Subsidence 
                          of the OJP  The cooling of the oceanic 
                          lithosphere causes the density of lithospheric rocks 
                          to increase.  Older lithosphere is more dense and 
                          cold than younger lithosphere and this causes the lithosphere 
                          to subside as it ages (Figure 12A).  Therefore, 
                          after their emplacement, oceanic plateaus subside as 
                          a result of cooling and contraction of the lithosphere 
                          (Figure 12B; Detrick & Crough, 1978; Coffin, 
                          1992).  Arrival of a hot mantle plume affects this 
                          subsidence by creating the initial dynamic uplift discussed 
                          previously, therefore reducing the subsidence for a 
                          certain amount of time (Figure 12C).  Subsidence 
                          curves for normal oceanic lithosphere and hotspot-affected 
                          lithosphere suggest that the 122 Ma OJP should have 
                          subsided ~2700 to 4100 m since its formation (Figure 
                          13). After correcting the present-day depth to the top 
                          of the igneous basement for sediment loading, we calculate 
                          the total subsidence of OJP by subtracting the corrected 
                          present-day basement depth from the original eruption 
                          depth estimated from basaltic glass H2O and 
                          CO2 data.  The subsidence estimates 
                          vary from 900 m (Site 803) to 1900 m (Sites 1184 and 
                          1185) with an average of 1500 ± 400 m over much 
                          of the plateau (Figure 14, Table 1). We have excluded 
                          Site 807 from our subsidence average because of the 
                          large differences in CO2 content between 
                          Units A and C-G glasses, but our preferred eruption 
                          depth based on the C-G glasses as described above suggests 
                          600 ± 250 m of subsidence at this site.  
                          Our estimated average subsidence for the OJP is lower 
                          than previous estimates based on microfossils (Figure 
                          1-16; Ito 
                          & Clift, 
                          1998) and CO2 in glasses from Site 807 
                          Unit A (Michael, 
                          1999). 
                           
                            |  | Figure 
                                12: A) Illustration of the principle of isostasy, 
                                which requires the oceanic crust to subside with 
                                age to offset the thickening and cooling of the 
                                lithosphere. B) Variations of subsidence 
                                with time for normal oceanic lithosphere (Parsons 
                                & Sclater, 1977; Stein & Stein, 1992).  
                                C) Variations of subsidence with time for a hot-spot-affected 
                                oceanic lithosphere (Ito 
                            & Clift, 1998). |  
 Figure 13: Subsidence 
                          estimates versus age for ODP sites on Ontong Java Plateau.  
                          Subsidence estimates based on microfossils are from 
                          Ingle 
                          & Coffin (2004). Subsidence estimates for other 
                          large igneous provinces (Detrick et al., 1977; Coffin, 
                          1992) are minimum values and assume that these features 
                          originally formed at sea level; true subsidence for 
                          these could be 1000-2000 m greater than the values plotted.  
                          Subsidence of hotspot-affected lithosphere (Ito 
                          & Clift, 1998) is calculated for plume excess 
                          temperatures (ΔT) ranging from 200°C (minimum 
                          subsidence) to 350°C (maximum subsidence). Symbols 
                          correspond to Ocean Drilling Program (ODP) site numbers. 
 Figure 14: Illustration 
                          of the calculation of subsidence.  Using the pressure-dependent 
                          solubilities of H2O and CO2, our 
                          data suggest original eruption depths (at 122 Ma) varying 
                          from ~1100 m below sea level (mbsl) on the central part 
                          of the plateau to 2200-3000 mbsl on the eastern edge.  
                          The glass shards from Site 1184 suggest a quenching 
                          depth of 500 mbsl. Click here 
                          or on figure for enlargement. Possible 
                          explanations for small initial uplift and subsidence
 1. Dense garnet granulite in lower OJP crust
 One possible explanation 
                          is that uplift was tempered by the presence of dense 
                          garnet granulite and possibly eclogite in the lower 
                          OJP crust that formed from cumulates and intruded and 
                          underplated gabbros (Neal 
                          et al., 
                          1997). 
                           
                            | In 
                                Favour Direct evidence 
                                for garnet granulite in the lower crust comes 
                                from xenoliths in 34 Ma alnöites on the island 
                                of Malaita (Neal 
                                et al., 
                                1997). | Against Seismic velocities, 
                                gravity data, phase equilibria, and crustal thickness 
                                estimates based on geophysical data do not support 
                                the widespread presence of eclogite (Gladczenko 
                                et al., 
                                1997; Richardson 
                                et al., 
                                2000). |  These data do not eliminate 
                          the possibility of high-density hidden cumulates in 
                          the lower crust, but they indicate that the contribution 
                          of such rocks to the average crustal density of OJP 
                          is significantly less than Neal 
                          et al. (1997) 
                          estimated. The upper limit that we used for average 
                          OJP crustal density (3.0 g/cm3) in our uplift 
                          modeling (Figure 14) allows for the presence of significant 
                          dense garnet granulite in the lower crust.  Using 
                          a density of 2.9 g/cm3 for the first 10 km 
                          of the column and 3.0 g/cm3 for the remaining 
                          23 km, and adding as much as 2 km of eclogite (density 
                          of 3.6 g/cm3) at the base of the column, 
                          the calculated initial isostatic uplift is 3 km, which 
                          is equivalent to the isostatic uplift of the entire 
                          column at a density of 3.0 g/cm3. Adding 
                          the dynamic uplift of 1-2 km, our model still predicts 
                          more initial uplift than is observed (Figure 11). 2. Underlying mantle 
                          cause? If crustal characteristics 
                          of the OJP are not responsible for the anomalous uplift 
                          and subsidence behavior, then the cause may be in the 
                          underlying mantle.  The production of an OJP-scale 
                          volume of basaltic crust would produce an enormous melt-depleted 
                          residuum in the upper mantle consisting of refractory 
                          harzburgite with relatively Fe-poor olivine (Neal 
                          et al., 
                          1997; Fitton & Godard, 2004) which 
                          would be buoyant relative to fertile mantle (Robinson, 
                          1988). 
                           
                            | In 
                                Favour Seismic tomography 
                                shows the presence of a rheologically strong and 
                                seismically slow upper-mantle “root” 
                                extending to ~300 km beneath the OJP, and the 
                                seismic characteristics of this root suggest it 
                                is chemical or mineralogical rather than thermal 
                                in origin (Richardson 
                                et al., 
                                2000; Klosko 
                                et al., 
                                2001; Gomer & Okal, 2003). | Against The volume of the 
                                root is much larger than can be explained by the 
                                volume of mantle remaining from melt extraction 
                                needed to form OJP basalts (Neal 
                                et al., 
                                1997). Given the enigmatic nature of the low-velocity 
                                root beneath the OJP, its role in causing the 
                                anomalous uplift and subsidence behavior of the 
                                plateau is unclear. |  3. Other mantle processes  Other mantle processes 
                          that might affect subsidence include slow buoyancy flattening 
                          of a plume (e.g., Phipps Morgan et al., 1995) 
                          and slow cooling of the lithosphere resulting from the 
                          thickness of the plateau, but why these would affect 
                          the OJP but not other oceanic LIPs is unclear.  
                          Another possibility is that large-scale magmatic underplating 
                          of basaltic magma for ~30 Ma after formation of the 
                          plateau provided a continued heat source, and thus reduced 
                          subsidence (Ito 
                          & Clift, 
                          1998). While there is evidence of some younger volcanic 
                          events to support this, the lack of voluminous volcanism 
                          post-122 Ma seems inconsistent with this hypothesis.  4. Large bolide impact  As an alternative to 
                          the mantle plume hypothesis, the OJP may have formed 
                          as the result of a large bolide impact (Glikson, 
                          1999) [Ed: see also OJ 
                          Impact page]. It has been proposed that this could 
                          explain the anomalous uplift and subsidence of the OJP 
                          because the impact hypothesis does not require a mantle 
                          temperature anomaly to generate large degrees of melting 
                          (Ingle 
                          & Coffin, 
                          2004). The model simulates a ~20 km diameter bolide 
                          of chondritic composition impacting a preexisting lithosphere 
                          of ~50 km thickness at a velocity ~20 km/s (see Ingle 
                          & Coffin, 
                          2004, Figure 4). Vertical impact and instantaneous 
                          vaporization of the ~4 km deep water column are assumed. 
                          The penetration depth would be about 60 km with an initial 
                          crater diameter of ~ 200 km (Ingle 
                          & Coffin, 2004).  Massive decompression 
                          melting will take place in the upper mantle, to a minimum 
                          depth of 300 km, assuming 100% partial melting resulting 
                          from the removal of the lithospheric overburden. This 
                          model also explains the low shear-wave velocities observed 
                          by Richardson 
                          et al. 
                          (2000) by catastrophic decrease in pressure of the 
                          solid asthenospheric mantle, moving laterally inward 
                          and upward from below to replace the extracted mantle 
                          during its emplacement beneath the OJP.   However, Tejada et 
                          al. (2004) have argued that the impact hypothesis 
                          is not consistent with geochemical and other geophysical 
                          data for the OJP, or with the Early Cretaceous paleoenvironmental 
                          record (see counter arguments in Ingle 
                          & Coffin, 
                          2004).  Furthermore, it remains controversial 
                          whether the thermal effects of a bolide impact would 
                          indeed create surface uplift and subsidence comparable 
                          to that of a hot mantle plume.  Korenaga 
                          (2005) argues that excavation-induced melting is 
                          essentially the same as melting of hotter-than-normal 
                          mantle and that the instantaneous depressurization by 
                          the formation of the crater is equivalent to increasing 
                          the potential temperature of the underlying mantle [Ed: 
                          see also OJ Puzzle page]. Summary 
                          and Conclusion Models of multiphase fractionation 
                          show that the high MgO Site 1187 samples could be parental 
                          to the low MgO groups. There is no evidence for 
                          high magmatic H2O contents that might have 
                          increased extents of mantle melting beneath the OJP.  
                          Instead, large extents of melting must have been caused 
                          by a relatively high mantle temperature.  Both the initial uplift 
                          and post-eruption subsidence of the Ontong Java Plateau 
                          are significantly less than predictions from thermal 
                          models of oceanic lithosphere and are less than what 
                          is observed for other oceanic large igneous provinces.  
                          A few hypothesis are: 
                           
                            The uplift was tempered 
                              by the presence of dense garnet granulite and possibly 
                              eclogite in the plateau’s lower crust that 
                              formed from cumulates and intruded and underplated 
                              gabbros (Neal 
                              et al., 
                              1997 ).  However, seismic velocities, gravity 
                              data, phase equilibria, and crustal thickness estimates 
                              based on geophysical data suggest that the contribution 
                              of a dense lower crust is significantly less than 
                              Neal 
                              et al. 
                              (1997)  estimated (Gladczenko 
                              et al., 
                              1997 ; Richardson 
                              et al., 
                              2000 ). 
                            Subsidence was tempered 
                              by the production of an enormous volume of melt-depleted, 
                              relatively buoyant residuum in the upper mantle 
                              that a plateau-scale volume of basaltic magma would 
                              produce (Neal 
                              et al., 
                              1997 ; Fitton & Godard , 2004).  
                              However, the volume of the root is much larger than 
                              can be explained by melt extraction needed to form 
                              the plateau (Neal 
                              et al., 
                              1997 ). 
                            The plateau 
                              may have been formed by a large bolide impact (Rogers, 
                              1982; Ingle 
                              & Coffin, 
                              2004), since it does not require an anomalously 
                              high mantle temperature and it would neither buoy 
                              the lithosphere nor lead to subsequent lithospheric 
                              cooling and contraction. However, whether the thermal 
                              effects of a bolide impact are different or the 
                              same as those of a mantle plume is not very well 
                              defined (J. Korenaga, written communication) and 
                              the geochemical aspect of the bolide impact hypothesis 
                              has yet to be proven (Tejada et al., 2004).  
                          More work is clearly needed to determine whether 
                            this, the world’s largest large igneous province, 
                            was formed by a mantle plume, bolide impact, or some 
                            other process.  If the OJP was formed by a plume, 
                            then there remains a major gap in our understanding 
                            of how large plumes interact with the Earth’s 
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                          last updated 31st December, 2006 |