Metal–silicate silicon isotope fractionation in enstatite meteorites and constraints on Earth's core formation

Silicon has long been considered a possible light element in Earth's core. If differences in 30Si/28Si ratios between metal (core) and silicate (mantle and crust) can be quantified, silicon isotopes may be used to constrain the amount of this element in the core, and in so doing elucidate the conditions that attended Earth's differentiation. We investigate Si-isotope fractionation between metal and silicate in metal-rich enstatite meteorites as an analogue for Earth's differentiation. We report here a 5 to 6‰ difference in the 30Si/28Si ratio between Si in metal and Si in silicate in the aubrites (enstatite achondrites) Mount Egerton and Norton County. The meteorites are believed to have derived from enstatite chondrites by melting and thermal metamorphism with final equilibration at 1200 and 1130 ± 80 K, respectively. Using the measured silicate–metal Si-isotope fractionation in these rocks we obtain a temperature dependence for fractionation of Δ30Sisilicate-metal=7.64×106±0.471σT2, in agreement with independent experimental and theoretical determinations. The measured silicate–metal fractionation suggests a ∼ 0.8‰ difference in the 30Si/28Si ratio between Earth's core and mantle at P/T conditions relevant to core formation. Our results, based on thermodynamic calculations for Si solubility in iron-rich metal and the measured Si-isotopic silicate–metal fractionation, imply at least ∼ 6 wt.% Si in the core (depending on the exact Δ30SiBSE-chondrite value). The Si-isotope data also require that oxygen fugacity in the lower mantle increased during or after the process of core segregation by 1 to 2 log units.