Isotopic Fractionation during Multi-Stage Core Formation

Abstract:

Here, we use a coupled model of astrophysical N-body accretion and core-mantle differentiation to investigate the effects of core formation on bulk planetary isotopic composition. During the process of planetary differentiation and core formation, the elemental and isotopic composition of the bulk silicate Earth (BSE) is established from the initial composition of the protoplanetary building blocks and subsequent metal-silicate fractionating reactions (Rubie and Jacobson, 2016; Bourdon et al., 2018). Metal-silicate equilibration between molten silicate and core-forming liquids during planetary differentiation is responsible for the depletion of moderately siderophile elements (e.g. Fe, Ni, Si, and S) in the BSE. Theoretically core formation should also isotopically fractionate these same elements, but whether it does is debated in the literature (e.g. Shahar et al., 2016; Liu et al., 2017). For example, the Fe isotopic composition of terrestrial rocks are heavy with respect to their likely chondritic progenitors (e.g. Poitrasson et al., 2004; Weyer et al., 2005), and the fractionation of Fe isotopes varies with pressure and temperature (Shahar et al., 2016; Liu et al., 2019) as well as the composition of the Fe metal alloy (Elardo et al., 2019), but whether these fractionation effects are significant enough to explain the terrestrial Fe isotopic anomaly is unclear. Prior work relies upon limited experimental laboratory work, especially beyond Fe, and simple single-stage core formation models, which struggle to reproduce the abundances of the moderately siderophile elements and are inconsistent with astrophysical models of planet formation. We present simulated isotopic fractionation during the process of planet growth, using sophisticated models of N-body accretion coupled with a planetary differentiation model capable of simulating multi-stage core formation including the ability to track isotopic fractionation. Repeated core formation events may cumulatively increase the fractionation effect of metal-silicate equilibration on the resultant isotopic signature of the BSE. Preliminary results of our model show that, in addition to its established compatibility with terrestrial bulk chemistry, a multi-stage core formation scenario is a promising explanation for BSE isotopic anomalies.