Deuterium and 37Chlorine Rich Fluids on the Surface of Mars: Evidence from the Enriched Basaltic Shergottite Northwest Africa 8657

Abstract Apatite, a major hydrous mineral in martian meteorites, has the potential to reveal history of volatiles on Mars. Here we report the H and Cl isotopic systematics of apatite from the enriched basaltic shergottite NWA 8657 to better understand the processes influencing volatiles at or near the martian (sub)surface. The apatite in NWA 8657 has the highest reported δD value (up to 6509 ‰) of any phosphates in martian meteorites. The positive correlation between the H2O contents (72‐2251 ppm) and δD values (3965‐6509 ‰) as well as between the Cl contents (1.28‐6.17 wt%) and δ37Cl values (‐1.7‐4.0 ‰) in NWA 8657 apatite are consistent with mixing between volatiles derived from the martian mantle and meteoric water/fluid. The D‐ and 37Cl‐enrichment of apatite in NWA 8657 implies isotopic exchange with sub‐surficial fluids during post‐crystallization hydrothermal event(s). Plain Language Summary Mars was probably a wet and warm world during its earliest geological history, supported by many lines of evidence. Present‐day Mars is cold and arid although recent remote sensing data support the existence of subsurface glaciers and seasonal fluid activities at some locations. In this study, our aim was to use ground truth about the potential water‐rock interaction on or near‐surface environments on recent Mars via measuring the H and Cl isotopic compositions of apatite from a geologically young meteorite from Mars; the enriched basaltic shergottite NWA 8657. The petrography and mineral chemistry of apatite and other associated phases, e.g., merrillite, maskelynite, mesostasis, and pyroxene, were documented prior to in situ H and Cl isotopes analysis. The water contents of apatite, merrillite, maskelynite, mesostasis, and pyroxene are positively correlated with the δD values. All of the datasets can be reconciled in terms of mixing between two endmembers, a D‐poor (~0 ‰) mantle reservoir and a D‐rich (5920 ± 500 ‰) near‐surface/underground water reservoir. Moreover, apatite in NWA 8657 displays 37Cl‐enriched (~1 ± 1 ‰ in average) characteristics. These signatures are consistent with D‐ and 37Cl‐rich fluid‐assisted isotopic exchange in recent near‐surface environment on Mars.