Metal-silicate silicon isotopic fractionation and the composition of the bulk Earth

Abstract

The difference in the Si isotopic composition between the Earth and primitive meteorites had been used to constrain the amount of Si in the Earth's core. However, there is presently a debate on the magnitude of the isotopic fractionation between metal and silicates as function of temperature based on experimental data. Here, we use a natural sample, an enstatite meteorite, Itqyi, as a natural experiment to determine an independent Si isotopic fractionation factor between metal and silicate. We determined the temperature of equilibrium between metal and silicate as well as the Si isotopic composition between the phases. We find that the dependence of Si isotopes with temperature to be:Δ30Sisilicate-metal=7.6(±0.7)×106(1SD)T2 Using this dependence of the δ30Si with temperature we estimate the bulk Earth δ30Si as a function of Si content of the core for different plausible conditions. Even when using the most extreme parameters, we show that the bulk Earth must be isotopically heavier than any chondrites groups. We therefore confirm that core formation alone cannot account for the isotopic difference between the Earth and primitive meteorites. We show that there is no correlation between δ30Si and the Mg/Si ratio suggesting that forsterite fractionation in the solar nebula may have had only a limited effect, if any. Our new results therefore confirm that volatility should have had a fundamental effect in shaping terrestrial planets chemical composition.