Baptiste Le Bellego, Celia Dalou, Béatrice Luais, Pierre Condamine, Vincent Motto-Ros, Laurent Tissandier
Geochimica et Cosmochimica Acta 422: 397-411
Voir en ligne : https://doi.org/10.1016/j.gca.2025.11.038
The segregation of metallic cores from silicate mantles during early planetary differentiation is a key process shaping the chemical evolution of terrestrial bodies. A critical factor controlling metal-silicate equilibration during this stage is the diffusive behavior of moderately siderophile elements, which governs chemical exchange timescales. As a moderately siderophile and moderately volatile element, Ge is particularly sensitive to redox conditions, pressure, temperature, and the presence of light elements in the metal phase, making it an ideal tracer
of core formation processes. However, experimental constraints on Ge diffusion under relevant high-pressure, high-temperature, and low oxygen fugacity conditions are lacking.
Here, we present new experimental measurements of Ge diffusion coefficients in Fe-Ni metal and silicate (CMAS) melt, analogous to planetary cores and mantles, under high-pressure (0.5 – 1 GPa), high-temperature (1350 ◦C) conditions and low oxygen fugacities (IW-5.4 to IW-1.5). Ge diffusion in liquid silicate and liquid metal was found to be significantly faster (~10 11 m2/s) than in solid metal (~10 13 m2/s), with transport further influenced by oxygen fugacity and Si content. Under highly reducing conditions, high Si concentrations inhibit Ge diffusion in solid metal by reducing vacancy availability and inducing partial melting, forming immiscible metal droplets that act as localized Ge sinks. Diffusion timescale calculations indicate that, for Earth-like planets,
even at high temperatures (1800 ◦C), estimated equilibration times are too long for large metal fragments (>10 m) to fully equilibrate before descending to the core. Thus, additional processes such as turbulent convection or percolation are required for efficient metal–silicate exchange. In contrast, on Mars-like bodies with long-lived magma oceans, solely diffusion, even at low temperature (1350 ◦C), could be sufficient to equilibrate large metal fragments.