Metalsilicate Equilibria And Core Formation
Authors: Arculus R J , Holmes R D , Powell R , Righter K 
Published: November 2023

The segregation of metal from silicate was of major physical and chemical importance in the early development of the Earth, but many of the details of this process are obscured by subsequent events. Important clues can be gathered from the nature of metal-silicate equilibria preserved in the disrupted fragments of differentiated meteorites, but under the higher pressure conditions prevailing in the Earth, the operation of different metal partitioning and separation processes are manifested in a set of unique geochemical characteristics. With respect to differentiated meteorites, clear evidence from thermodynamic calculations and experiments shows that the compositions of coexisting metal and silicate in the various pallasite groups are relict from equilibrium siderophile element partitioning events. However, the inconsistency of MgO/(MgO + FeO) (Mg#, a function in part of the distribution of Fe between silicate and metal components) and siderophile element abundances in silicate components of the postulated Eucrite Parent Body (EPB) [eucrites, diogenites, main group pallasites (MGP), and Group IIJAB irons] precludes straightforward analysis of the geochemistry and physics of metal formation and core separation. Nevertheless, metal separation in the absence of a large gravitational field clearly occurred The Mg# of the Earth’s upper mantle is unlike any major meteorite group or the Mg# of material drawn from the same O isotope reservoir (Moon and enstatite chondrites), but shows some overlap with, for example, chondritic silicate inclusions in the IAB irons, the olivine of the MGP, and some other rare chondrites. It is not obvious, however, that the terrestrial Mg# necessarily reflects the redox equilibria prevailing in the solar accretion disk, given the evidence for chemical fractionation of the Earth’s upper mantle with respect to chondrites of various lithophile elements ( e.g., Mg/Si, Ca/Al, and Nb/U). Furthermore, the102-dependence of Fe, Ni, and Co partitioning between coexisting silicate and metal results in dramatic variations in Ni/Co ratios over small ranges in 102• The preservation of chondritic Ni/Co ratios in the Earth’s upper mantle is not readily explained by segregation of small amounts of highly siderophile elements during later accretionary stages of relatively oxidized materials. Separation of liquid metal from solid silicate at low pressures is unlikely due to the high surlitce tension of the metal. Increased solubility of O in liquid Fe at high pressures (&gt;6 GPa) results in a reduction of surlitce tension by a factor of 5, sufficient to allow complete wetting and permeation of a solid silicate matrix. In the case of the low pressure ( &lt;&lt;6 GPa) EPB and Moon, metal separation possibly required the transient involvement of an extensively molten silicate state. Metal production and destruction near the surlitce of the Earth may also have involved molten silicate, but additional separation of liquid metal from and equilibrium with solid silicate ( as a porous medium) at high pressures in the mantle is probable.