Nebular fractionation of silicon isotopes and implications for silicon in Earth’s core

Abstract:

The silicate Earth has a heavy silicon isotopic composition relative to all chondrite groups, which was taken as evidence for the presence of sufficient amounts of silicon in Earth's core to explain its density deficit. A difficulty remains, however, in that chondrites themselves show variable silicon isotopic composition. For example, enstatite chondrites, the only group of meteorites that matches the Earth’s composition for 17O, 48Ca, 50Ti, 54Cr, and 92Mo, have very light silicon isotopic compositions that would require unrealistic amounts of silicon in Earth’s core if they were its main constituents. We have measured the silicon isotopic composition of several achondrites that had not been measured before. In particular, we have found that angrites have a heavy silicon isotopic composition, similar to the Earth and the Moon. These meteorites formed under relatively oxidizing conditions (~IW+1) and core formation in their parent-body occurred at relatively low pressure (<0.1 GPa), so core-mantle silicon isotopic fractionation is excluded as a cause for their Earth-like silicon isotopic composition. Angrites are among the most volatile-depleted meteorites and their heavy silicon isotope signature most likely reflects isotopic fractionation by nebular processes. All chondrite groups and the bulk silicate Earth form a trend in silicon isotopic composition vs. Mg/Si ratio, which we will show quantitatively can be explained by a simple nebular process. This can also explain the similarity in silicon isotopic composition between lunar and terrestrial rocks. Therefore, silicon isotopes in terrestrial rocks provide no constraints on the amount of Si in the core and are consistent with the presence of other light elements such as oxygen.