Silicon isotopes in the inner Solar System: Implications for core formation, solar nebular processes and partial melting

Abstract

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%.