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

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 δ30Si between different chondrite groups that are correlated with their Mg/Si elemental ratio. Second, in agreement with the previous study, δ30Si for the terrestrial samples are higher than values for chondrites, but the difference between the Bulk Silicate Earth (BSE) and the carbonaceous chondrites (Δ30SiBSE − carbonaceous chondrites = 0.08 ± 0.04‰ (1 standard deviation)) is about a factor of 2 smaller than previously reported. The δ30Si 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.