Silicon isotope variations in the inner solar system: Implications for planetary formation, differentiation and composition

Abstract

Accurate and precise Si isotope measurements were obtained using magnesium doping and high-resolution plasma source mass spectrometry for samples representative of the Earth, as well as lunar samples, meteorites from Mars (SNC), eucrites, a howardite, carbonaceous chondrites (CC), ordinary chondrites (OC) and enstatite chondrites (EC). Our data confirm that significant Si isotope fractionations exist among the inner solar system planetary bodies. They show that the Earth and the Moon share the same Si isotopic composition, which is heavier than all other measured bodies, in agreement with most of previous studies. At the other end of the spectrum, enstatite chondrites have the lightest Si isotope compositions. In order to precisely estimate the amount of Si that may have entered the Earth’s core, we developed a refined model of Si partitioning based on continuous planetary accretion that takes into account the likely variations in T, P and fO2 during the Earth’s accretion, as well as isotopic constraints involving metal–silicate partitioning derived from both experimental and natural sample data sets.Assuming that the difference between the isotopic signature of the bulk silicate Earth (BSE) and chondrites solely results from Si isotope fractionation during core formation, our model implies that at least ∼12wt% Si has entered the Earth’s core, which is greater than most of the estimates based on physical constraints on core density or geochemical mass balance calculations.This result leads us to propose two hypotheses to explain this apparent contradiction: (1) At least part of the Earth’s building blocks had a Si isotope composition heavier than that observed in chondrites (i.e., δ30Si>−0.39‰). (2) If on the contrary the Earth accreted only from material having chondritic δ30Si, then an additional process besides mantle–core differentiation is required to generate a stronger isotope fractionation and lead to the observed heavy isotope composition of the bulk silicate Earth. It may be the loss of light Si isotopes during partial planetary vaporization in the aftermath of the Moon-forming giant impact. This process, which may have affected metallic cores, required a thorough isotopic re-equilibration between core and silicate to explain the similar heavy isotope composition of the silicate portions of the Earth and the Moon.