The geochemical mechanisms driving planetary differentiation and core formation are key to understand the evolution of planetesimals, and moons. The partitioning of siderophile elements between metal and silicate reservoirs provides crucial constraints on the thermodynamic conditions governing core-mantle segregation. Among these elements, germanium is a valuable tracer of metal-silicate differentiation, as its moderately siderophile behavior is strongly influenced by oxygen fugacity and temperature, with a lesser dependence on pressure. While previous studies have established metal-silicate partition coefficients for Ge under equilibrium conditions (Kegler+,2011; Righter+,2011; Righter+,2017; Siebert+,2011), diffusional processes remain poorly constrained due to the scarcity of data for planetary materials.

In this study, we address the effect of oxygen fugacity and metal state – solid (IW-2.5 to IW-1.5) vs. liquid (IW-5.4) – on Ge diffusion coefficient in a metallic and a silicate phase. Diffusion experiments were conducted at 1350°C, 1GPa using a piston-cylinder (CRPG-Nancy) with CMAS silicate glass (≈3200ppm Ge) and Fe₉₀Ni₁₀ alloy capsules for 20min to 140h. Diffusion profiles were analyzed using electron microprobe (LMV-Clermont-Ferrand) and µLIBS imaging (ILM-Lyon) using a semi-quantitative model (Le Bellego+,2024).

Our experiments provide a mean germanium diffusion coefficient of D(Ge)solid-metal=3.09E-13 ± 1.57E-14m²/s in solid metal, whereas Ge diffuses two orders of magnitude faster in liquid metal (D(Ge)liquid-metal=1.35E-11 ± 1.47E-12m²/s). The liquid metal phase occurs only at low fO₂, as Si enrichment lowers the FeNi melting point. In silicate melts, where Ge exists primarily as GeO₄, diffusion is even faster than in molten metal (D(Ge)liquid-silicate=2.64E-11 to 9.04E-11m²/s, 1.7% mean RSD). The diffusion of Ge occurs with change in valence state, from GeO₄ in silicate to Ge in the FeNi phase. Hence, we highlight that the magnitude of Ge diffusion from silicate to metal is strongly dependent on fO₂, offering new insights into Ge behavior during metal-silicate segregation and planetesimal core formation.