Water in the martian interior: Evidence for terrestrial MORB mantle-like volatile contents from hydroxyl-rich apatite in olivine–phyric shergottite NWA 6234

Abstract

Understanding the distribution and behavior of volatiles in martian magmas and the corresponding mantle provides crucial constraints on planetary properties such as mantle melting temperatures and melt production, as well as the availability of magmatic fluids for any potential occurrences of life. Current evaluations of the pre-eruptive volatile concentrations of martian magmas are based on analyses of hydrous minerals in martian meteorites such as apatite, because they record the volatile content (OH, F, Cl) and the halogen and water fugacities of their parental magmas. Estimates for the pre-eruptive volatile contents of martian basaltic magmas range from nearly anhydrous to 2wt% H2O. However, these magmas may have potentially degassed upon eruption and/or emplacement losing their magmatic water while retaining some halogens. Here we report apatite compositional data from the martian meteorite NWA 6234, an intermediate olivine–phyric shergottite, and calculate F2, Cl2, H2O fugacity ratios for its parental magma. Apatites in NWA 6234 contain the highest observed OH–F content among martian meteorites and importantly have volatile fugacity ratios similar to terrestrial basalts. This suggests that the actual content of fluorine, chlorine and water of the martian mantle, parental to SNC meteorites, are higher than previously thought and are similar to the volatile content of the terrestrial mid-ocean ridge basalt source region.