Si isotope systematics of meteorites and terrestrial peridotites: implications for Mg/Si fractionation in the solar nebula and for Si in the Earth's core

Abstract

High precision Si isotope ratios have been measured for a series of meteorites and terrestrial samples using high-resolution multi-collector ICP-MS. Our results differ from those reported in an earlier study [Georg et al., 2007. Si in the Earth's core. Nature 447, 1102-1106] in two important aspects. First, our data set reveals systematic differences in δ 30Si between different chondrite groups that are correlated with their Mg/Si elemental ratio. Second, in agreement with the previous study, δ 30Si for the terrestrial samples are higher than values for chondrites, but the difference between the Bulk Silicate Earth (BSE) and the carbonaceous chondrites (Δ 30Si BSE − c arbonaceous chondrites = 0.08 ± 0.04‰ (1 standard deviation)) is about a factor of 2 smaller than previously reported. The δ 30Si versus Mg/Si trend defined by the chondrite groups can be explained by reaction of olivine with a SiO-rich vapor to form enstatite, starting from a carbonaceous chondrite composition. In contrast, the difference between the BSE and carbonaceous chondrites must reflect a different process, and can be explained by incorporation of Si into the Earth's core during metal–silicate equilibration in a deep magma ocean. The observed Si isotope fractionation is consistent with the temperatures and pressures of metal–silicate equilibration derived from siderophile element abundances in the Earth's mantle.