Onset of magma ocean solidification on Mars inferred from Mn-Cr chronometry

Abstract The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. Here we applied the short-lived 53Mn-53Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic 53Cr variations, and instead have a uniform +20.3±1.4 (95% conf.) parts-per-million excess in 53Cr/52Cr relative to the terrestrial mantle. The investigated group of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of 53Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of 53Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within