C1 - Partitioning of siderophile volatile elements during core formation

Research areas: Petrology, geochemistry, cosmochemistry

Summary:

A crucial event in Earth’s history, which likely gave rise to the atmosphere and oceans and eventually allowed life to flourish, was the arrival of volatile elements on Earth. However, geoscientists argue about how and when volatiles were delivered to Earth, and this part of Earth’s history remains unclear. Most recent models favour heterogeneous accretion, such that the composition of the building blocks of Earth and the other terrestrial planets changed from exhibiting reduced, volatile-poor conditions in their early stages to more oxidized, volatile-rich conditions towards the end of accretion, while Earth’s core formation was still ongoing.

However, these heterogeneous accretion models are challenged by claims that Earth accreted largely dry and that most (if not all) volatiles were delivered after the main stages of accretion and core formation were complete, after metal segregation on Earth was no longer active. If volatiles were added during both accretionary phases both before and after core formation then we must find out which volatile fractions were delivered when in order to better understand how Earth formed. To investigate this question, we study the systematics of the siderophile volatile elements (SVEs), because abundances of many of these elements in the bulk silicate Earth are now well-constrained and their partitioning between metal, sulphide, and silicate melts provides first order constraints on the timing of volatile delivery to Earth.

Thus, the aim of this subproject is to produce coherent sets of metal–silicate and sulphide–silicate partition coefficients for S, Se, Te, Tl, Ag, As, Au, Cd, Bi, Pb, Sn, Cu, Ge, Zn, In and Ga. In this project, we will disentangle the respective effects of temperature, pressure, and fO2 from element partitioning processes using a systematic series of isothermal and isobaric experiments with varying fO2. In addition, to investigate the effects of varying melt compositions, which may reflect the late stages of core formation on Earth, the Moon and potentially also on Mars, we will determine partition coefficients at different C and S concentrations and Ca/Mg ratios. We will additionally constrain how SVEs partition between sulphide melts and silicate melts/solid silicates during cooling of a terrestrial and lunar magma ocean. Overall, our experimental results will provide direct information about when volatile elements accreted and they will help us determine how much volatile-rich material was accreted during and after core formation.