Volatile loss of martian magmas drives redox evolution and volatile transfer from interior to subsurface

The martian surface contains extensive evidence for volatile enrichment of hydrogen, sulfur, and carbon; however, shergottites, the most common martian meteorite, are comparatively volatile-depleted, making it difficult to reconcile the volatile budgets of the interior and surface. Additionally, shergottites exhibit large redox variations of 1–3 log units from initial crystallization to final cooling, which cannot be explained solely by closed-system processes such as fractional crystallization. Here, we model how carbon, hydrogen, and sulfur in shergottitic melts can degas and exsolve during ascent and cooling. We find that shergottitic melts release the majority of their initial carbon at crustal pressures during ascent, along with significant hydrogen and sulfur, and melts continue to degas during cooling; effectively transferring volatiles from the interior to the upper subsurface. Critically, our model results indicate that this volatile release drives the oxidation of shergottites by 0.9–2.7 log units of oxygen fugacity across various petrologic and geochemical groups, indicating that volatile loss is an important mechanism capable of altering redox state during shergottite petrogenesis.