T60 Geochemistry, Mineralogy, and Petrology of Mars

Sunday, 4 November 2012
1:30 PM - 5:30 PM
217A

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

Paper number: T60-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

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.