Electrical resistivity of nickel, iron and iron-silicon alloy melts at high pressure with implications for the thermal conductivity of the Earth’s core

The Earth’s liquid outer core (OC) is composed of Fe alloyed with up to 10% Ni and a small fraction of light elements. However, the effect of light elements such as Si on the transport properties of liquid Fe-alloy in Earth’s OC is not clear. Thermal conductivity (κ) and related electrical resistivity (ρ) are the least constrained parameters in OC. Therefore, the characterization of transport properties of Ni, Fe and Fe-Si at high pressure has important geophysical implications for the Earth’s core. The ρ of solid and liquid Ni, Fe and Fe 4%Si was measured at pressure and temperature up to 12 GPa and 2100 K, respectively. All experiments were conducted in a large volume multi-anvil press and the measurements were carried out using the new adaptation of the 4-wire method. A standard COnsortium on Materials Properties Research in Earth Sciences (COMPRES) octahedron cell was used as the pressure medium, while the internal components were redesigned to permit the preservation of the liquid sample geometry, to contain the melt and minimize the effect of diffusive contamination. In the solid state, the ρ of solid Fe and Ni exhibits the familiar pressure-dependent decrease after the Curie temperature (Tc). The anomalous ρ of Fe-4.5wt%Si above Tc is strongly modulated by temperature and pressure, and it is attributed to the phase transitions and structural ordering in the alloy. The ρ of liquid Ni remains constant at the onset of melting at all pressures. While ρ of liquid Fe decreases up to 5 GPa, it remains invariant along the melting boundary after the δ-γ-liquid triple point. The ρ of liquid Fe-4.5wt%Si remains constant along the melting boundary and matches 120 μΩcm for pure liquid Fe within the experimental uncertainties. The results are interpreted in the context of pressure dependent icosahedral short range ordering (ISRO) in liquid 3d metals and alloys. Based on this, it is postulated that ρ of Fe-alloys along the melting boundary remains invariant up to Earth’s inner core boundary. The κ at the core-mantle boundary and inner core boundary were calculated using the Weidemann-Franz law