Volatile elements in Martian apatite: Insights from new meteorites NWA 7034 and NWA 8159

Measurements of volatile abundances at the martian surface (e.g. H2O, Cl, CO2, CH4) are a priority for the latest generation of martian rovers (Curiosity and ExoMars) due to the astrobiological importance of these compounds. Mars is a prime candidate to find water as recent Mars orbiter and lander missions have revealed evidence for the presence of liquid water early in Mars’s history (e.g., Nochian ~3.9 to 4.5 Ga). Geomorphological features (e.g., fluvial valleys), water rich sediments and minerals (e.g. clays) and evaporates (e.g. gypsum) all indicate hydrological activity has occurred on Mars. However, rovers are limited to surface exploration and therefore are unable to provide insight to the volatile content of the martian interior. Martian meteorites identified on Earth consist largely of volcanic basaltic material. Therefore, the volatile contents of primary igneous minerals within these martian basalts can indicate the volatile content of the source regions at depth in the martian interior. The discovery of Northwest Africa (NWA) 7034 (and pairings) and NWA 8159 has expanded the diversity of the martian meteorites from the common Shergottite, Nakhlite, Chassignite (SNC) types observed. NWA 7034 is a polymict basaltic breccia, whereas NWA 8159 is an augite basalt. This study investigates the apatite volatile content of these two unique martian meteorites. The apatite volatile content in NWA 8159 relates to the volatile abundances of the parental melt, hence providing insight into a previously unstudied martian volcanic province. This study showed these apatites are F-rich, indicating an unusually F-rich parental melt for martian meteorites, which typically have Cl-rich apatite compositions. Results from this study confirm the presence of more Mars typical Cl-rich apatites within NWA 7034 and reveal a significant OH component within individual apatite grains. The volatile content of NWA 7034 is in fact uncharacteristically uniform for a polymict breccia (i.e., no groupings of apatite can be seen in terms of their volatile abundances (Cl: F: OH)), indicating post-crystallisation processes- possibly a thermal event have overprinted the original Cl, F and OH contents of these apatite grains. Three distinct groupings of apatites can be noted on the basis of hydrogen isotope composition, water content and texture of NWA 7034 apatite. This implies that there are at least two different apatite histories that have been recorded. Group 1 is characterised by high δD values paired with low water contents, and inferred to be the result of devolatilisation during shock and/or thermal heating. Group 2 consists of intermediate δD values and high water contents, which could be controlled by hydrothermal alteration. Group 3 is characterised by low δD values and high water contents, probably caused by terrestrial contamination, but possibly representative of a martian mantle signature