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Rajdeep Dasgupta Department of Earth Science, Rice University

Geophysical (Seismic) View. Geochemical (Isotopic) View. Mantle Petrology finding a middle ground between geochemical and geophysical view of the Earth’s interior. Rajdeep Dasgupta Department of Earth Science, Rice University.

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Rajdeep Dasgupta Department of Earth Science, Rice University

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  1. Geophysical (Seismic) View Geochemical (Isotopic) View Mantle Petrology finding a middle ground between geochemical and geophysical view of the Earth’s interior Rajdeep Dasgupta Department of Earth Science, Rice University

  2. Lithologic characters of the isotopic mantle end members? Where are they located? • Are the isotopic end members lithologically or mineralogically distinct? • Can the major element characters of end members be reconciled with different crustal components that subduct? An Eye Opener View of the Earth’s Interior http://garnero.asu.edu/research_images/

  3. Petrology and geochemistry of ocean island basalts (OIBs) 1. OIB lavas with 8≤MgO≤16 wt.% (or 10-16 wt.%) 2. Samples have Sr-Pb isotopes and majors measured on the same sample 3. 659 samples (8≤MgO≤16 wt.%). Also examine less evolved subset of samples (10≤MgO≤16 wt.%) 4. Correct lavas for olivine fractionation by adding/subtracting equilibrium olivine so that lavas are in equilibrium with Fo90 olivine.

  4. Global OIB Dataset (GEOROC) SiO2 vs 206Pb/204Pb CaO/Al2O3 vs 206Pb/204Pb Jackson and Dasgupta (2008)

  5. Cont. crust? Cont. crust? 87Sr/86Sr (and εNd) correlate with incompatible elements (and element ratios) (Lavas in this slide [8-16 wt.% MgO] are not olivine fractionation corrected) Jackson and Dasgupta (2008)

  6. Major Element Signatures for the Mantle End Members Major elements as “surrogates” for isotopes… Willbold and Stracke (2006) (Only olivine fractionation corrected lavas with high initial MgO contents [10–16 wt.%] plotted) Jackson and Dasgupta (2008)

  7. was motivated by discussions during CIDER 2006 workshop • - one follow-up meeting and lots of e-mail exchange and phone calls

  8. Fe-Si Trend of OIBs Globally Dasgupta (in prep)

  9. Global Fe-Si Trend of OIBs Dasgupta (in prep)

  10. Dasgupta (in prep)

  11. Comments on the (Isotopic) End Member Compositions… Mantle Lherzolite + HIMU – Enrichment in FeO*, TiO2; SiO2 depletion CO2± Silica-deficient gt pyroxenite/eclogite EM1 – Enrichment in TiO2, K2O, K2O/TiO2 Continental sediments? EM2 – Enrichment in TiO2, K2O, K2O/TiO2 Continental sediments? Hawaiian tholeiites – Enrichment in FeO*, SiO2 MORB-like silica-excess pyroxenite/eclogite

  12. SiO2 vs 206Pb/204Pb Correlation little-affected by fractionation • Crystal fractionation is not the primary factor controlling major element variation of OIBs when looking at a narrow window of high and near primary MgO Jackson and Dasgupta (2008)

  13. TiO2 vs 206Pb/204Pb Jackson and Dasgupta (2008)

  14. Major element – isotope correlations in the global OIB dataset? Calculating Average Compositions • Calculating average major element compositions of different island groups • Define major element composition of the mantle end-members: Average the major element composition of isotopically-extreme lavas • HIMU – Cook-Australs (10 samples with >21 206Pb/204Pb) • EM1 – Pitcairn (6 samples with strongest EM1 flavor: low 206Pb/204Pb and high 87Sr/86Sr) • EM2 – Samoa (7 samples with 87Sr/86Sr>0.706 in global dataset)

  15. CaO/Al2O3 vs 206Pb/204Pb Correlation little-affected by fractionation Increasing filtering and olivine correction….. Again, fractionation correction does little to improve correlations. Jackson and Dasgupta (2008)

  16. Melt-rock reaction? • Plausible melt-rock reactions likely produce a trend that is oblique to the observed trend in global OIB dataset Jackson and Dasgupta (2008)

  17. Thermal divide at pressures ≥2 GPa Major element characters of End Member Source Rocks? Use of CMAS Projection of O’Hara Forsterite (Mg2SiO4) – Ca-Tschermaks (CaTs: CaAl2SiO6) – Quartz (SiO2)projected from Diopside CaMgSi2O6).

  18. Mantle End Member lavas in O’Hara Projection (Only olivine fractionation corrected lavas with high initial MgO contents [10–16 wt.%] plotted.) Jackson and Dasgupta (2008)

  19. Experimental Partial Melts in O’Hara Projection Jackson and Dasgupta (2008)

  20. Recycling…. -What is melting beneath hotspots? -High TiO2 in lavas from most hotspots indicates that eclogite contributed to the mantle source (Prytulak and Elliott [2008]). -Is eclogite a distinct lithology during melting? -Or homogenized, making a Ti-rich (fertile) peridotite? Hofmann and White (1982) Silica-deficient eclogite? Hot subduction? Is HIMU… CO2+peridotite? OR CO2 + SiO2-deficient eclogite? Silica-rich eclogite?Cool subduction?

  21. Genesis Ocean Island Basalts Petrology Geophysics Geochemistry

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