Electrical resistivity of the Fe–Si–S ternary system: implications for timing of thermal convection shutdown in the lunar core

Abstract

The composition of the lunar core has been suggested to be Fe-rich with varying amounts of lighter elements, such as Si and S. Presence of Si and S affects electrical and thermal transport properties and thus influences core thermal processes and evolution. Paleomagnetic observations constrain a high intensity magnetic field that ceases shortly after formation of the moon (~ 3.5–4.2 Ga year ago), and thermal convection in the core may contribute to generation of this field. In this study, the electrical resistivity of Fe-14 wt% Si-3 wt% S was measured in both solid and molten states at pressures up to 5 GPa and thermal conductivity was calculated via the Wiedemann–Franz Law from the electrical measurements. The results were used to estimate the adiabatic conductive heat flux of a molten Fe-14 wt% Si-3 wt% S lunar core and compared to a Fe-2-17 wt% Si lunar core, which showed that thermal convection of either core composition shuts down within the duration of the high intensity magnetic field: (1) 3.17–3.72 Ga year ago for a Fe-14 wt% Si-3 wt% S core; and (ii) 3.38–3.86 Ga years ago for a Fe-2-17 wt% Si core. Results favouring compatibility of these core compositions with paleomagnetic observations are strongly dependent on the temperature of the core-mantle boundary and time-dependent mantle-side heat flux.

The Electrical Resistivity of Liquid Fe-16wt%S-2wt%Si at High Pressures and the Effect of S and Si on the Dynamo in the Ancient Vestan Core

“…The electrical resistivity of Fe-16S-2Si from this study is compared to previously measured values near 5 GPa and the melt of pure Fe (~1800 K) in Figure 8. [31][32][33][34][36][37][38]54].…”

“…Experimental measurements of the resistivity of Fe-S-Si alloys have been made at high temperatures and high pressures in previous studies [30][31][32][33][34]. Several studies have been performed on the binary systems Fe-S and Fe-Si.…”

“…Expected composition of the core of Vesta indicated by a dotted rectangle [19,21]. Citations given in diagram [31][32][33].…”

“…In the ternary system, three experimental resistivity studies at high pressures and high temperatures have been published. These are Pommier et al [31] with Fe-3wt%S-8wt%Si, Littleton et al [32] with Fe-3wt%S-14wt%Si, and Lenhart et al [33] with Fe-8wt%S-4.5wt%Si. Suehiro et al [34] measured the resistivity of Fe-3wt%S-3wt%Si at high pressures but only at ambient temperature.…”

Impurity Resistivity of the Earth's Inner Core

“…However, the resistivity of multi‐component Fe alloys in the Earth's core is difficult to estimate from the binary alloys because of the breakdown of the Matthiesen's rule (Gomi et al., 2016; Gomi & Yoshino, 2018; Inoue et al., 2020; Zhang et al., 2022). Furthermore, experiments on ternary systems are still limited (Berrada et al., 2022; Lenhart et al., 2023; Littleton et al., 2022; Pommier, 2020; Suehiro et al., 2017). Theoretical works are also demonstrated by using the super‐cell method (e.g., Pozzo et al., 2014) or the coherent potential approximation (CPA) (e.g., Gomi & Yoshino, 2018; Zidane et al., 2020).…”