Electrical Investigation of the Fe-S-O-Mg and Fe-S-O-Si Systems up to 10 GPa and Application to Terrestrial Cores

Significant amounts of light elements (S, O, Mg, Si) may compose the metallic cores of terrestrial planets and moons. For instance, Mercury's reducing conditions are consistent with a silicon-bearing core, while the more oxidizing conditions on Mars are compatible with the presence of S and O in the core. On Earth, a combination of light elements has been suggested to explain the density drop between the solid and liquid cores. Light elements are thought to significantly affect the transport properties, specifically electrical and thermal conductivity, which will impact dynamics of core convection and hence the generation of global magnetic fields by the dynamo process. However, the effect of light elements on the crystallization and physical properties of the core is not well understood at present.

We present an experimental study of the electrical properties of core analogues in the Fe-S-O-Mg and Fe-S-O-Si systems under pressure (P) and temperature (T) conditions relevant to core crystallization in small bodies. Experiments were performed up to 10 GPa and up to ~1600°C on Fe-S-O-Mg and Fe-S-O-Si samples using the multi-anvil apparatus and the 4-electrode technique. The starting materials were homogeneous high-purity powder mixtures and sample compositions contained 5 wt.% S, up to 3 wt.% O, up to 2 wt.% Mg, and up to 1 wt.% Si. For both systems, we observe that electrical resistivity changes nonlinearly with increasing T and its P-dependence depends on the amount and chemistry of light elements added to Fe. We identify the alpha-gamma (α →ϒ) magnetic transition in solid iron as well as the onset of melting at higher T. At defined T, Si-bearing samples are more resistive than Mg-bearing samples. The amount of light element affects the resistivity increase observed during melting; for example, the presence of 0.1 wt.% Si increases resistivity by a few % upon melting whereas 1wt.% Si increases resistivity by a factor of about 2.4.

We will discuss the effect of S, O, Mg, and Si on the electrical resistivity of metallic cores. Estimates of thermal resistivity for both systems will be presented and applied to different core cooling scenarios. Bottom-up and top-down crystallization regimes will be considered.