Experimental Results on Fractionation of the Highly Siderophile Elements (HSE) at Variable Pressures and Temperatures during Planetary and Magmatic Differentiation

Abstract

The platinum-group elements (PGEs; Os, Ir, Ru, Rh, Pt, Pd), along with rhenium and gold, are grouped together as the highly siderophile elements (HSEs), defined by their extreme partitioning into the metallic, relative to the oxide phase (> 104). The HSEs are highly refractory, as gauged by their high melting and condensation temperatures, and were therefore relatively concentrated in the feedstock for the terrestrial planets, as defined by the composition of chondritic meteorites (e.g., Anders and Ebihara 1982; Horan et al. 2003; Fischer-Godde et al. 2010). However, the planetary formation and differentiation process has since acted on this chemical group to produce a rich variety of absolute and relative inter-element fractionations. For example, analysis of iron meteorites suggests a significant decoupling of the HSE in the cores of planetesimals, and likely Earth’s core, with Os, Ir, Ru (IPGE-group) and Re concentrated in the metal phase, and Pt, Rh, Pd (PPGE-group) plus Au usually concentrated in the residual liquid (Goldstein et al. 2009). In terms of the silicate Earth, analysis of mantle rocks reveals very low levels of the HSE, but relative abundances similar to chondrites (see review by Day et al. 2016, this volume), in part reflecting HSE segregation into core-forming iron (Ringwood 1966; Ganapathy et al. 1970). This is in contrast to mantle-derived melts, whose HSE abundances are highly fractionated, with relative depletions in the IPGE-group compared to PPGE-group, as well as Re and Au (Barnes et al. 1985). Resulting Re/Os and Pt/Os fractionation also influence the long-term evolution of the 187Re to 187Os and 190Pt to 186Os decay systems, and, hence, the development of distinctive Os-isotope reservoirs (Walker et al. 1997; Shirey and Walker 1998; Day 2013). The emplacement of mantle-derived magmas into Earth’s crust results in …