Solidification of lunar core from melting experiments on the Fe–Ni–S system

Abstract

Paleomagnetic records revealed that the early Moon had an Earth-like core dynamo, which was likely driven by thermochemical buoyancy force associated with core solidification. The cause for the cessation of the ancient lunar dynamo at about 3.56 Ga ago remains controversial, partly because the composition and temperature of the Moon are not well constrained and the solidification process of its core remains poorly understood. Here we report experimental data at 5.1 GPa showing that the liquidus temperatures of the Fe–Ni–S system are ∼50–150 K lower than that of the Fe–S system, implying that a Ni-bearing core could remain molten to lower temperatures. Calculating the liquidus temperature gradient using previous data at 3 GPa and the new results at 5.1 GPa, we find that an Fe-S core containing less than ∼4.0 wt.% S would freeze from the center of the Moon. At higher S contents, the core would precipitate solid Fe near the core-mantle boundary. Based on the prevailing lunar core models, a change in core solidification from the bottom-up regime to the top-down regime during the lunar history is possible only if its bulk S content falls between about 2.0 and 4.0 wt.%.