Martian core composition from experimental high-pressure metal-silicate phase equilibria
H. Gendre, James Badro, N. Wehr, S. Borensztajn
Abstract
Current Martian core composition models suggest an iron-rich core alloyed with 10 to 20 wt. % of sulfur. Although Mars is more oxidised than Earth, oxygen is usually discarded as a potential light element candidate, since its dissolution into iron is negligible at the pressures and temperatures prevailing during Mars's primitive differentiation. However, it has recently been shown that oxygen interacts with the sulfur in the metal, which dramatically increases its solubility. Here, we investigated this novel process by carrying out metal-silicate equilibration experiments between 2 and 12 GPa, and 1673 and 2473 K, using piston-cylinder and multi-anvil presses. The experimental results show that oxygen was systematically incorporated in the metallic phase alongside sulfur, and a thermodynamic model was developed to parametrise this interaction. The oxygen-sulfur interaction parameter arising from those thermodynamic equations was fitted and used in a multi-stage core modelling simulation. We found that a Martian core containing 14 to 19 wt. % S (maximum permissible concentration according to cosmochemical constraints) will also contain between 1.3 and 3.5 wt. % O. This would help to match the Martian core density estimate while being cosmochemically consistent.