ACIDIC, HIGH D/H MAGMATIC FLUIDS IN THE DEEP MARTIAN INTERIOR: EVIDENCE FROM MARTIAN AMPHIBOLE AND GLASS COMPOSITIONS

There is a large debate about the pre-eruptive water concentrations of Martian basalts. Some groups call for magmas with ~2 wt% H₂O, wetter than many basalts on Earth, while others conclude that Martian magmas were nearly anhydrous, with only tens of ppm H₂O. The relative proportions of water and halogens in Martian magmas are debated, with lines of evidence pointing to a far more important role for chlorine on Mars compared to Earth. Further, the D/H ratio of the Martian interior is poorly constrained, with models for deep Martian D/H ranging from near 0‰ (terrestrial) to +1000‰ and even +3000‰, which would necessitate changes in our understanding of Mars' early history.

Kaersutite amphiboles, apatites, and micas in igneous Martian meteorites provide valuable information regarding the pre-eruptive volatile content of Martian magmas. The chassignites, Chassigny and NWA 2737, are olivine cumulates whose olivine and pyroxenes indicate bulk formation at great depth; these meteorites have melt inclusions containing hydrous minerals whose compositions should be good probes of volatile ratios in the Martian interior.

For the chassignite NWA 2737, we report: abundances of H₂O (0.15±0.01% weight), F, and Cl contents and D/H (+3700-4100‰) of kaersutite; and F and Cl contents of melt inclusion glasses. From that data, we calculate the H₂O/Cl ratios of the melts that crystallized kaersutites in both chassignites, using crystal chemical models for OH and Cl partitioning between melt and amphibole. We calculate H₂O/Cl of 0.5-1 in the melts in equilibrium with the amphiboles in both NWA 2737 and Chassigny, compared to H₂O/Cl of 4-100 in typical basalts and andesites on Earth. These results suggest that by mass, Cl ≈ H₂O within Mars, and that there are deep Martian reservoirs of high D/H ratio such that melt inclusions from deep within Mars can crystallize kaersutite and apatite with +3000-4000‰ D/H. Martian magmas degassed volatiles that were high in Cl—and consequently highly acidic—to the Martian surface, creating environments with few parallels on Earth. High Martian atmospheric D/H value may be more representative of its interior than previously thought, rather than the product of preferential loss of H from an Earthlike initial reservoir over the past 4.6 Ga.