Abstract Earth’s presently‐active magnetic field is thought to be generated by geodynamo via convection of liquid outer core, primarily composed of Fe alloyed with Ni and light elements, for example, Si. Core thermal conductivity critically controls its thermal evolution, dynamics, and available energy (thermal vs. compositional) powering the geodynamo over Earth’s history. Here we precisely measured thermal conductivity of solid Fe0.85Ni0.06Si0.09 (Fe–7wt%Ni–5wt%Si) to 148 GPa/2950 K and Fe to 155 GPa/3246 K, respectively. We show that a liquid outer core with Fe0.85Ni0.06Si0.09 has a moderate thermal conductivity of ∼73 W m−1 K−1, in the moderate zone over the previously reported broad range (∼20–250 W m−1 K−1) that includes various core compositions. Such a modest isentropic, conductive heat flow at core‐mantle boundary (CMB) (∼11 TW) is smaller than present‐day CMB heat flow (∼15 TW), suggesting a presently‐active thermally‐driven geodynamo and a maximum inner core age of ∼0.8–1.3 billion‐years.

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