22-EPN3-015: Trace Element Partitioning Between (Mg,Ca)S and Highly Reduced Magmas – Implications for the Volatile Budget and Thermal Evolution of Mercury

Visit by Laurie Llado and Yishen Zhang of the University of Liege (Belgium) to TA2 Facility 21 – OU NanoSIMS 50L (UK).
Dates of visit: 31 July – 18 August 2022

Report Summary: Mercury’s magma ocean is thought to have contained abundant dissolved sulfur, which would have formed sulfides once the magma ocean reached sulfide saturation. Due to the low concentrations of Fe²⁺ in the silicate portion of Mercury and S speciation at low fO₂, it is likely that crystallisation of the magma ocean produced minerals of the MgS-CaS solid solution. In this study, we aim at calculating the U, Th, and K trace element partitioning between (Mg,Ca)S sulfides and their equilibrium silicate melt to better understand the thermal evolution of Mercury.

To this end, the U, Th, and K trace element compositions of ultra-small (Mg,Ca)S crystals (3-15 μm) produced in experiments performed under highly reduced conditions were measured using the Open University NanoSIMS (UK). Preliminary results shows that U and Th partition coefficients of MgCaS crystals are lower than U and Th partition coefficients in CaS and FeS. Further investigation is required to confirm these preliminary results and to determine more accurately the U, Th and K partition coefficients. Nevertheless, the acquired U/Mg and Th/Mg ratios and theirs related partition coefficients will be used to provide comprehensive knowledge about the thermal evolution of Mercury.

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