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

2. An Eye Opener View of the Earth’s Interior
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?

3. Petrology and geochemistry of ocean island basalts (OIBs)
1. OIB lavas with 8≤MgO≤16 wt.% (or wt.%)
2. Samples have Sr-Pb isotopes and majors measured on the same sample
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. SiO2 vs 206Pb/204Pb CaO/Al2O3 vs 206Pb/204Pb
Global OIB Dataset
(GEOROC)
SiO2 vs 206Pb/204Pb CaO/Al2O3 vs 206Pb/204Pb
Lavas with extreme major element compositions also have extreme isotopic compositions!
Jackson and Dasgupta (2008)

5. 87Sr/86Sr (and εNd) correlate with incompatible elements (and element ratios)
Cont. crust?
Fans of recycling of subducted sediments will love this! High K and K/Ti in continental-derived seds!
(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)
Majors as “surrogates” for isotopes
(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 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

13. Correlation little-affected by fractionation
SiO2 vs 206Pb/204Pb
Correlation little-affected by fractionation
This is because the variability of Al2O3, TiO2, and SiO2 concentrations between island groups is larger than what can be produced by fractionation along an olivine control line over the narrow range of MgO considered (Dasgupta et al., 2007; Supplementary Fig. 2).
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)

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

15. 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 Pb/204Pb and high 87Sr/86Sr)
EM2 – Samoa (7 samples with 87Sr/86Sr>0.706 in global dataset)

16. Correlation little-affected by fractionation
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)

17. 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)

18. 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).
Thermal divide at pressures ≥2 GPa

19. Mantle End Member lavas in O’Hara Projection
HIMU needs CO2….. Alberto Saal has observed HIMU melt inclusions with carbonatite!
(Only olivine fractionation corrected lavas with high initial MgO contents [10–16 wt.%] plotted.)
Jackson and Dasgupta (2008)

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

21. 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?
The most remarkable aspect of all this is that we still can’t be sure if we’re melting eclogite or peridotite!
Silica-rich eclogite? Cool subduction?

22. Genesis Ocean Island Basalts
Petrology
Geophysics
Geochemistry