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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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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


  • 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/


Petrology and geochemistry of ocean island basalts (OIBs) Where are they located?

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.


Sio 2 vs 206 pb 204 pb cao al 2 o 3 vs 206 pb 204 pb

Global OIB Dataset Where are they located?

(GEOROC)

SiO2 vs 206Pb/204Pb CaO/Al2O3 vs 206Pb/204Pb

Jackson and Dasgupta (2008)


87 sr 86 sr and nd correlate with incompatible elements and element ratios

Cont. crust? Where are they located?

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)


Major element signatures for the mantle end members
Major Element Signatures for the Mantle End Members Where are they located?

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)



Fe-Si Trend of OIBs Globally Where are they located?

Dasgupta (in prep)


Global Fe-Si Trend of OIBs Where are they located?

Dasgupta (in prep)


Dasgupta (in prep) Where are they located?


Comments on the (Isotopic) End Member Compositions… Where are they located?

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


Correlation little affected by fractionation

SiO Where are they located?2 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)


TiO Where are they located?2 vs 206Pb/204Pb

Jackson and Dasgupta (2008)


Major element – isotope correlations Where are they located?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)


CaO/Al Where are they located?2O3 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)


Melt rock reaction
Melt-rock reaction? Where are they located?

  • Plausible melt-rock reactions likely produce a trend that is oblique to the observed trend in global OIB dataset

Jackson and Dasgupta (2008)


Major element characters of end member source rocks

Thermal divide at pressures ≥2 GPa Where are they located?

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).


Mantle End Member lavas in O’Hara Projection Where are they located?

(Only olivine fractionation corrected lavas with high initial MgO contents [10–16 wt.%] plotted.)

Jackson and Dasgupta (2008)


Experimental Partial Melts in O’Hara Projection Where are they located?

Jackson and Dasgupta (2008)


Recycling
Recycling…. Where are they located?

-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?


Genesis Ocean Island Basalts Where are they located?

Petrology

Geophysics

Geochemistry


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