Sulfide segregation during Martian differentiation: Constraints from copper isotopes

<p indent="0mm"> The segregation of sulfides during the Martian magma ocean stage is one of the key processes for understanding Martian differentiation, as it governs the global budget of chalcophile elements. A recent study by <xref rid="CITATION26" xml:base="bibr">Wang et al. (2025)</xref> provided constraint of the efficiency of this process through analyzing of the copper isotope composition (δ⁶⁵Cu) of Martian meteorites. By applying a geochemical screening strategy, the study identified eight samples from 15 meteorites that best represent the pristine Cu isotopic characteristics of the Martian mantle. Based on these representative samples, the δ⁶⁵Cu value of the bulk silicate Mars was determined to be -0.03‰±0.08‰. This value is heavier than the value derived from the bulk Martian composition model (-0.30‰±0.09‰) based on the accretion model. To explain this isotopic discrepancy, a quantitative differentiation model was developed. The model proposes that during the late-stage crystallization of the Martian magma ocean (about 89% crystallization), the sulfide melts become saturated, segregate, and migrate toward the Martian core. This process leads to significant fractionation of copper isotopes in the residual silicate melt. The model further reveals that efficient sulfide segregation ceases once the magma ocean crystallizes at depths shallower than <sc>102~105 km.</sc> Sulfides saturated at shallower depths cannot reach the core are eventually trapped in the Martian mantle. By integrating copper isotopic systematics with quantitative modeling, the study constrains the sulfur (400–443 µg/g) and copper (6–8 µg/g) abundances in the Martian mantle and provides new insights for investigating the differentiation histories of other terrestrial planetary bodies.