FTIR constraints on Martian mantle water from clinopyroxene in nakhlite and shergottite meteorites

Water on Mars has traditionally been estimated from hydrogen contents in melt inclusions and apatite, yet these may not account for magma degassing or post-crystallization dehydration. Pyroxenes offer an alternative approach, as they incorporate trace amounts of water during crystallisation via charge-balancing structural defects, that are retained after dehydration enabling experimentally reversing water loss and constraining magmatic water contents. Here we report the first hydrothermal rehydration experiments on pyroxene from both nakhlites and shergottites. The treated nakhlite augites contain 140‒185 ppm H 2 O, consistent with previous values for Nakhla (130 ± 26 ppm) and within the range of terrestrial basaltic pyroxene. Using clinopyroxene-melt partition coefficients, this corresponds to 1.59 ± 0.03‒1.83 ± 0.10 wt% average H 2 O in the nakhlite magma, slightly higher than previous estimates (0.69‒1.42 wt% H 2 O). The complex hydrothermal history of nakhlites, including evidence for magmatic degassing and interaction with H 2 O-poor, Cl-rich fluids, suggests these estimates may represent minimum values of the nakhlite magmatic water content. Assuming a low degree of partial melting, the nakhlite mantle source is expected to contain 80‒91 ppm H 2 O, overlapping previous estimates (59‒184 ppm), and comparable to Earth’s MORB mantle (54‒330 ppm). This is consistent with a common magmatic source for all nakhlites, and broadly consistent with water estimates for the chassignite mantle source (39‒252 ppm), suggesting that any exogenous fluid assimilation had a negligible effect on the net water budget. In contrast, shergottite pigeonites show no detectable water by Fourier Transform Infrared Spectroscopy, consistent with expected water contents of ∼ 2‒11 ppm H 2 O derived from published source compositions for depleted and enriched shergottites. Together, these results refine our understanding of Martian magmatic water content, consistent with a common, more water-rich mantle source for nakhlites, distinct from the more heterogeneous and generally drier shergottite mantle reservoirs