Sulfur in apatite from the Nakhla meteorite record a late-stage oxidation event

Abstract

Estimates of the oxygen fugacity (fO2) recorded by the Martian nakhlite meteorites from direct observations of the main igneous phenocryst assemblages range from values similar to that recorded by the quartz-fayalite-magnetite oxygen buffer to ∼two orders of magnitude lower. Inferences of changes in fO2 during the late stages of crystallization, volcanic degassing, and emplacement of the nakhlite cumulate pile have been made based on variable sulfide and apatite chemistry. We present S-XANES measurements of the oxidation state of sulfur in apatite and associated mesostasis glass in Nakhla to place direct constraints on the magnitude of changes in fO2 experienced by the Nakhla portion of the nakhlite cumulate pile during apatite crystallization. Nakhla apatites range from containing dominantly S2− to containing dominantly S6+. This, together with correlations between S2−, Cl, and FeO in the mesostasis glass near these apatites, suggest that our measurements capture directly the oxidation of the interstitial late-stage Nakhla magmas as the result of Cl-saturation and degassing. As the result of this degassing, at least part of the nakhlite cumulate pile experienced an increase in fO2 of ∼1.5–2.5 orders of magnitude during apatite crystallization and final mesostasis cooling. Based on these measurements, the sulfur oxidation states of apatites in the other nakhlite meteorites are predicted to range from exclusively S2−-bearing to exclusively S6+-bearing.