Silicon Isotope Fractionation between Silicate and Metal from an Enstatite Meteorite: Implications for Earth’s Core Formation

Introduction: The presence and amount of Si in Earth's core has important implications for models of the processes and chemistry of core segregation. A metal core containing 5 to 7 wt. % Si can be produced when assuming continuous accretion and equili-bration along the silicate liquidus terminating at ~35 GPa and ~3,000 K, and with a concomitant increase in fO 2 from 4 to 2 log units below IW [1]. Metal segregation at lower pressures and temperatures, such as in smaller planetesimals, would require even more reducing conditions (e.g., ΔIW = −6) for similar metal concentrations. We have investigated the isotope fractionation between Si in metal and Si in silicate in an enstatite (E) meteorite in order to (i) test the assertion that there is a strong Si isotope fractionation between core and mantle during planet formation [2], and (ii) to assess the significance of comparing formation conditions of E chondrites to those of Earth. Results: Mt. Egerton (USNM 3272) is an enstatitic meteorite formed from E chondrites. It consists mainly of coarse-grained enstatite and metal, the latter with 2.06 wt.% Si. Our MC-ICPMS acid digestion analyses show that the δ 30 Si NBS-28 value of Si in the metal is 5.2 ‰ lower than that of Si in the coexisting enstatite. This fractionation from a natural system confirms results of recent experiments [3], which show a large 30 Si/ 28 Si fractionation between Si in silicate and Si in metal at high temperatures (2078 K, 1 GPa). The natural data exhibit a larger fractionation than that observed in the laboratory, likely due to the lower formation temperature of Mt. Egerton (see below). Discussion: A slow rate of Si tracer diffusion in silicate, despite a more rapid tracer diffusion rate in metal, rules out post-crystallization diffusive resetting of Si isotope ratios. Therefore, the measured Δ 30 Si (silicate-metal) is reflective of the crystallization temperature of the meteorite. Theoretical Si isotope fractionation calculations, combined with experimentally obtained fractiona-tions [3], predict a temperature of 1281 K (1008°C) for a Δ 30 Si (silicate-metal) of 5.2 ‰. This temperature is in good agreement with the ~1200 K obtained from the Si concentration of the metal [4]. We conclude that Δ 30 Si (silicate-metal) has potential as a thermometer. Using 2990 K for multi-stage Earth core formation [1], the negligible effects of pressure suggested by theory, and the Δ 30 Si silicate-metal vs. T …