Hydrogen isotopic systematics of nominally anhydrous phases in martian meteorites

Hydrogen isotope compositions of the martian atmosphere and crustal materials can provide unique insights into the hydrological and geological evolution of Mars. While the present-day deuterium-to-hydrogen ratio (D/H) of the Mars atmosphere is well constrained (~6 times that of terrestrial ocean water), that of its deep silicate interior (specifically, the mantle) is less so. In fact, the hydrogen isotope composition of the primordial martian mantle is of great interest since it has implications for the origin and abundance of water on that planet. Martian meteorites could provide key constraints in this regard, since they crystallized from melts originating from the martian mantle and contain phases that potentially record the evolution of the H2O content and isotopic composition of the interior of the planet over time. Examined here are the hydrogen isotopic compositions of Nominally Anhydrous Phases (NAPs) in eight martian meteorites (five shergottites and three nakhlites) using Secondary Ion Mass Spectrometry (SIMS).

This study presents a total of 113 individual analyses of H2O contents and hydrogen isotopic compositions of NAPs in the shergottites Zagami, Los Angeles, QUE 94201, SaU 005, and Tissint, and the nakhlites Nakhla, Lafayette, and Yamato 000593. The hydrogen isotopic variation between and within meteorites may be due to one or more processes including: interaction with the martian atmosphere, magmatic degassing, subsolidus alteration (including shock), and/or terrestrial contamination. Taking into consideration the effects of these processes, the hydrogen isotope composition of the martian mantle may be similar to that of the Earth. Additionally, this study calculated upper limits on the H2O contents of the shergottite and nakhlite parent melts based on the measured minimum H2O abundances in their maskelynites and pyroxenes, respectively. These calculations, along with some petrogenetic assumptions based on previous studies, were subsequently used to infer the H2O contents of the mantle source reservoirs of the depleted shergottites (200-700 ppm) and the nakhlites (10-100 ppm). This suggests that mantle source of the nakhlites is systematically drier than that of the depleted shergottites, and the upper mantle of Mars may have preserved significant heterogeneity in its H2O content. Additionally, this range of H2O contents is not dissimilar to the range observed for the Earth’s upper mantle.