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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
An Eye Opener View of the Earth’s Interior
http://garnero.asu.edu/research_images/
• 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?
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.
Global OIB Dataset
(GEOROC)
SiO2 vs 206Pb/204Pb
CaO/Al2O3 vs 206Pb/204Pb
Jackson and Dasgupta (2008)
87Sr/86Sr
(and εNd) correlate with
incompatible elements (and element ratios)
Cont. crust?
Cont. crust?
(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 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)
- was motivated by discussions during CIDER 2006 workshop
- one follow-up meeting and lots of e-mail exchange and phone calls
Fe-Si Trend of OIBs Globally
Dasgupta (in prep)
Global Fe-Si Trend of OIBs
Dasgupta (in prep)
Dasgupta (in prep)
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
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)
TiO2 vs
206Pb/204Pb
Jackson and Dasgupta (2008)
Major element – isotope correlations
in the global OIB dataset?
Calculating Average Compositions
1. Calculating average major element compositions of different island groups
2. 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/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?
• 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?
Use of CMAS Projection of O’Hara
Forsterite (Mg2SiO4) – CaTschermaks (CaTs:
CaAl2SiO6) – Quartz (SiO2)
projected from Diopside
CaMgSi2O6).
Thermal divide at pressures ≥2 GPa
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)
Experimental Partial Melts in O’Hara Projection
Jackson and Dasgupta (2008)
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]).
Hofmann
and White
(1982)
-Is eclogite a distinct lithology during melting?
-Or homogenized, making a Ti-rich (fertile)
peridotite?
Silica-deficient
eclogite?
Is HIMU…
Hot subduction?
CO2+peridotite?
OR
CO2 + SiO2-deficient eclogite?
Silica-rich
eclogite?
Cool
subduction?
Genesis Ocean Island Basalts
Petrology
Geophysics
Geochemistry