Thermal State and Solidification Regime of the Martian Core: Insights from the Fe-Ni-S Liquidus at 20 GPa

Abstract

A series of multi-anvil experiments have been conducted to define the iron-rich liquidus of the ironnickel- sulfur (Fe-Ni-S) system at 20 GPa, the estimated pressure of the Martian core-mantle boundary (CMB), across its entire temperature range. Due to the fineness of quenched liquid features at high temperatures and low S concentrations, spatial analysis techniques, in additional to electron microprobe analysis, were used to determine the compositions of some quenched liquid phases. The maximum concentration of S in the Fe(Ni) solid is only 0.6 wt.%, comparable to the Fe-S binary system, but far lower than prior results for the Fe-Ni-S system at 20 GPa. We attribute this to the tendency of FeNi alloys to form body-centered cubic (BCC) structures when at high Ni contents, in contrast with face-centeredcubic (FCC) structures when Ni contents are low. Our liquidus curve is nearly ideal, featuring a maximum temperature depression of 80 C from 10 to 16 wt. % S, the likely range of Martian core composition. The slight negative departure from ideal behavior contrasts with the positive departure of liquidii previously applied to the Martian core without sufficient experimental constraints. Unlike existing liquidii at 23 GPa, which predict a fully molten core for a narrow range of sulfur content between 14 and 15 wt. % S, our result is consistent with a molten state for all proposed core compositions, and establishes a new minimum CMB temperature of 1200 C. Extrapolating our liquidus to high pressures and comparing it to calculated areotherms, we find that three core crystallization regimes are possible. For a Martian core with 10 to 12 wt. % S, crystallization takes the form of iron snow, while for cores with 13 to 16 wt. % S, solidification occurs near the center of the planet in the form of Fe3S crystallization. At 13 wt. % S, Fe3S would crystallize everywhere in the core, introducing a new scenario with uncertain and potentially interesting consequences for the Martian dynamo.