Thermal and compositional convection in Earth’s core are thought to be the main power sources driving geodynamo. The viability and strength of thermally and compositionally-driven convection over Earth’s history depend on the adiabatic heat flow across the core-mantle boundary (CMB) which is governed by the thermal conductivity of a constituent Fe-Ni-light element alloy at the pressure-temperature (P-T) conditions relevant to the core. Silicon is often proposed to be an abundant light element alloyed with Fe along with ∼5 wt% Ni, but the thermal transport properties of Fe-Ni-Si alloys at high P-T remain largely uncertain. Here we measured the electrical resistivities of Fe-10wt%Ni and Fe-1.8wt%Si alloys up to ∼142 GPa and ∼3400 K using four-probe van der Pauw method in laser-heated diamond anvil cell experiments. Our results show that the resistivities of hcp-Fe-1.8Si and Fe-10Ni display quasi-linear temperature dependence from ∼1500 to 3400 K at each given high pressure. Addition of ∼2 wt% Si in hcp-Fe significantly increases its resistivity by ∼25% at ∼138 GPa and 4000 K, but Fe-10wt%Ni has similar resistivity to pure hcp-Fe at near CMB P-T conditions. Using our measured values of electrical resistivities, we model thermal conductivities via the Wiedemann-Franz law, giving a nominal thermal conductivity of ∼50 W m−1 K−1 for liquid Fe-5Ni-8Si alloy at the topmost outer core, implying an adiabatic (conductive) core heat flow of ∼8.0 TW. The outer core has a much lower thermal conductivity than the inner core due to light-element differentiation across the solidifying inner-core boundary. Our studies imply that the adiabatic core heat flow is low enough to enable thermal convection to drive the geodynamo over most and possibly all of Earth’s history, while the strength of compositional convection increases with the inner-core growth and accounts for ∼83% of the buoyancy flux to the present-day geodynamo.

Youjun Zhang, Mingqiang Hou, Peter Driscoll, Nilesh P. Salke, Jin Liu, Eran Greenberge, Vitali B. Prakapenka, Jung-Fu Lin, “Transport properties of Fe-Ni-Si alloys at Earth’s core conditions: Insight into the viability of thermal and compositional convection,” Earth Planet. Sci. Lett. 553, 116614 (2021). DOI: 10.1016/j.epsl.2020.116614 abstract

Electrical resistivity measurements of Fe alloys in a laser-heated DAC and SEM microphotographs of recovered samples. (a) Representative image of an experimental assembly for the electrical resistivity measurements using the four-probe van der Pauw method in a LHDAC. The image shows an Fe-1.8Si alloy loaded in a sample chamber with cBN gasket insert, SiO2 insulator, and four Pt leads at ∼138 GPa and 300 K. (b) The cross heart area of Fe-1.8Si alloy was double-side heated to ∼1978 K at ∼138 GPa by continuous laser-heating with a laser spot size of ∼10 μm in diameter on both sides of the sample. Analysis of the measured thermal radiation spectra shows homogenous temperature distributions on both sides of the sample. (c) Recovered Fe-1.8Si alloy sample with a thickness of around 1.5 μm at ambient pressure (white dashed-line) after FIB cutting. (d) Recovered Fe-10Ni alloy sample with a thickness of around 1.3 μm at ambient pressure (white dashed line). The white dots in (c) (points 1-8) and (d) (points 9-17) show the locations of the SEM/EDS analyses of the samples and surrounding thermal insulator (silica). The analysis results are listed in Supplementary Material Tables S1 and S2.