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Silicon isotopes in the inner Solar System: Implications for core formation, solar nebular processes and partial melting
We report Si isotopic data on a suite of terrestrial mantle-derived samples, meteorites and a lunar sample. Our data on co-existing mantle minerals, peridotites and basalts demonstrate lack of any resolvable high temperature fractionation during igneous processes. We show that the δ 30Si of the bulk...
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Published in: | Geochimica et cosmochimica acta 2010-12, Vol.74 (23), p.6921-6933 |
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Main Authors: | , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | We report Si isotopic data on a suite of terrestrial mantle-derived samples, meteorites and a lunar sample. Our data on co-existing mantle minerals, peridotites and basalts demonstrate lack of any resolvable high temperature fractionation during igneous processes. We show that the δ
30Si of the bulk silicate Earth (BSE) is identical, within analytical uncertainties, to carbonaceous and ordinary chondrites (CHUR). Based on our data the difference between δ
30Si
BSE and δ
30Si
CHUR is 0.035
±
0.035. Whole-rock differentiated meteorites from different parent bodies (Mars, Vesta) and a lunar breccia sample also show similar δ
30Si suggesting broad-scale Si isotope homogeneity in the inner Solar System with an average δ
29Si
=
−0.20
±
0.01 and δ
30Si
=
−0.39
±
0.02 relative to the NBS28 Si isotope standard.
A difference between δ
30Si
BSE and δ
30Si
CHUR of 0.035, as observed in our study, translates to less than 1.67
wt.% Si in the core considering a continuous accretion model whereas estimates using a batch model are even lower. Within uncertainties (±0.035‰) in the δ
30Si difference between the BSE and CHUR, a maximum of 3.84
wt.% Si could be present in the Earth’s core whereas at δ
30Si
BSE–δ
30Si
CHUR
=
0, there is no requirement of Si in the Earth’s core. Such low Si in the core necessitates the presence of other light elements in the core to explain its density deficit. Our data also places constraints on the oxidation state of the Earth’s mantle during core segregation. The uncertainties in estimating the concentration of oxidized Fe in the mantle during the first 90% of accretion arise from uncertainties in the estimates of the equilibrium partition coefficient of silicon between metal and silicate at conditions relevant to core formation. For δ
30Si
BSE–δ
30Si
CHUR
=
0.035
±
0.035, the concentration of oxidized Fe in the mantle during the first 90% of accretion could be as low as ∼1%. However, at δ
30Si
BSE–δ
30Si
CHUR
=
0, the Si isotope data do not require any change in the mantle concentration of oxidized Fe during accretion from the present day value of 6.26%. |
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ISSN: | 0016-7037 1872-9533 |
DOI: | 10.1016/j.gca.2010.08.034 |