Reconciling metal\u2013silicate partitioning and late accretion in the Earth

Terry-Ann Suer,James M D Day,Beatrice Doisneau,Stephan Borensztajn,Julien Siebert,Laurent Remusat,Guillaume Fiquet

doi:10.1038/s41467-021-23137-5

Abstract

Highly siderophile elements (HSE), including platinum, provide powerful geochemical tools for studying planet formation. Late accretion of chondritic components to Earth after core formation has been invoked as the main source of mantle HSE. However, core formation could also have contributed to the mantle’s HSE content. Here we present measurements of platinum metal-silicate partitioning coefficients, obtained from laser-heated diamond anvil cell experiments, which demonstrate that platinum partitioning into metal is lower at high pressures and temperatures. Consequently, the mantle was likely enriched in platinum immediately following core-mantle differentiation. Core formation models that incorporate these results and simultaneously account for collateral geochemical constraints, lead to excess platinum in the mantle. A subsequent process such as iron exsolution or sulfide segregation is therefore required to remove excess platinum and to explain the mantle’s modern HSE signature. A vestige of this platinum-enriched mantle can potentially account for 186Os-enriched ocean island basalt lavas.

Highlights

Siderophile elements (HSE), including platinum, provide powerful geochemical tools for studying planet formation
The major elemental chemical compositions of the two regions were measured by SEM-Energy Dispersive X-rays (EDX) and the platinum concentration in the silicate was quantified by NanoSIMS
Similar quench features in the silicate melt were previously observed in metal–silicate laser-heated diamond anvil cell (LHDAC) experiments[21,23] and cannot be attributed only to the presence of Highly siderophile elements (HSE)

Summary

Introduction

Siderophile elements (HSE), including platinum, provide powerful geochemical tools for studying planet formation. Archean samples show non-chondritic platinum stable isotope compositions, likely reflecting the imprint of core-forming processes[13] These observations are consistent with an alternative hypothesis of core-mantle equilibration at high pressures and high temperatures (HP–HT) as a way of producing the excess of HSE in the mantle[14]. We add constraints to the origin of the mantle’s platinum content by providing the first measurements of metal–silicate partition coefficients of platinum carried out in the laser-heated diamond anvil cell (LHDAC) (Supplementary Table 1) This is the only static compression device capable of producing the HP–HT conditions relevant to a deep magma ocean (e.g.,22). We used transmission electron microscopy (TEM) to assess the presence of nanoparticles/nuggets and to evaluate how they formed in the LHDAC samples

Methods

Results

Conclusion