New temperature and oxygen fugacity data of Martian nakhlite from Northwest Africa (NWA) 5790 and implications for shallow sulphur degassing

Abstract

Newly analysed titanomagnetite–ilmenite (Tim–Ilm) intergrowths from Martian nakhlite meteorite Northwest Africa (NWA) 5790 yielded crystallisation temperature up to 1032 °C and oxygen fugacity (fO2) up to ΔQFM + 1.6, notably higher than previous estimates for nakhlite magmas (temperature < 950 °C, fO2 = ΔQFM − 0.5 to ΔQFM + 1). To interpret how the magma was reduced from ΔQFM − 0.5 to ΔQFM + 1.6, we used D-Compress to model the sulphur degassing process within a single thick lava pile. For fO2 to significantly decrease in this extended range, a sulphur-rich (S content 4000–7000 ppm) Martian lava flow had to degas all the sulphur species at a certain final degassing pressure, which was 2–4 bar for NWA 988 and Lafayette and < 0.7 bar for Y-000593 and Nakhla. These final degassing pressure data are in good agreement with the Martian nakhlite burial depth estimated by other petrological and geochemical methods. These estimates are also comparable with the excavation depth of ~ 40 m based on the small (6.5 km in diameter) impact crater over the Elysium lava plain. The fO2-controlled sulphur degassing pressure may constitute a method for estimating the burial depth of sulphur-rich lava flows on Mars.