Seminar by Raúl Fonseca (Bochum University) – « The case for metal saturation in the lunar mantle »

Lunar basalts, unlike their terrestrial counterparts, are highly reduced—often containing native metallic iron. This suggests that the Moon’s mantle may also be reduced, possibly even saturated with metal. However, the question remains: Do the measured oxygen fugacities (fO₂) of lunar samples truly reflect the mantle’s original conditions? Indeed, some processes, such as hydrogen implantation from the solar wind or degassing of carbon monoxide during volcanic eruptions, could have locally reduced the samples after their formation. If so, the fO₂ values we measure today might underestimate how oxidizing the lunar mantle actually is. Adding to the complexity, correlations between siderophile elements (e.g., tungsten, copper) and incompatible elements (e.g., uranium, ytterbium) in lunar basalts seem inconsistent with a metal-rich mantle source.

Yet, when considering the median fO₂ of the lunar interior, it appears that metallic iron could have remained stable in the mantle during melting. To explore this further, we review the evidence for and against the presence of residual metal in the lunar mantle—and what it means for our understanding of the Moon’s interior. For example, thermodynamic models suggest that at fO₂ levels around IW-1 or lower (where IW = iron-wüstite buffer), a metallic phase containing ~10 wt.% sulfur could coexist with silicate melts. To test this, we conducted high-pressure, high-temperature experiments (~1500°C, 1.5 GPa) on highly siderophile elements (Pt, Pd, Rh, Os, Ir, Re), as well as tungsten and molybdenum. We also ran an experiment at 4.5 GPa and 1900°C to simulate conditions at the lunar core-magma ocean boundary.

Our results show that the low abundances of siderophile elements in mare basalts align with a scenario where the lunar mantle was: 1) Saturated in a small amount (~0.1 wt.%) of Fe-Ni-S metallic melt during partial melting, and 2) Previously depleted in siderophile elements due to core formation. This suggests that the Moon’s mantle may have retained residual metal even after its early differentiation, with implications for how we interpret the geochemical signatures of lunar rocks.