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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.

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slide1

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

slide2

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/

slide3

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.

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

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)

major element signatures for the mantle end members
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)

slide7

was motivated by discussions during CIDER 2006 workshop

  • - one follow-up meeting and lots of e-mail exchange and phone calls
slide9

Global Fe-Si Trend of OIBs

Dasgupta (in prep)

slide11

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

correlation little affected by fractionation

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)

slide14

TiO2 vs 206Pb/204Pb

Jackson and Dasgupta (2008)

slide15

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)
slide16

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)

melt rock reaction
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)

major element characters of end member source rocks

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).

slide19

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)

recycling
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?

slide22

Genesis Ocean Island Basalts

Petrology

Geophysics

Geochemistry

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