Low thermal conductivity of iron-silicon alloys at Earth core conditions with implications for the geodynamo

TL;DR

This study measures the thermal conductivity of iron-silicon alloys under Earth core conditions, revealing lower conductivities that imply a reduced heat flow and older inner core age, impacting geodynamo models.

Contribution

It provides the first direct measurements of Fe-Si alloy conductivities at core pressures and temperatures, highlighting silicon's significant impact on thermal properties.

Findings

Silicon alloying reduces iron's thermal conductivity by about half at core conditions.

Lower conductivity implies a decreased heat flow of around 3 TW across the core-mantle boundary.

Results suggest the Earth's inner core could be older than two billion years.

Abstract

Earth core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth history. Here we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m-1 K-1, lower than pure Fe at similar pressure-temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.