Inventory of H2O in the ancient Martian regolith from Northwest Africa 7034: The important role of Fe oxides

Water‐rich Martian regolith breccia Northwest Africa (NWA) 7034 was analyzed by Fourier transform infrared spectroscopy and transmission electron microscopy to determine the inventory and phase distribution of H2O (used herein to refer to both molecular H2O and OH− structural components in hydrous minerals). Hydrous Fe oxide phases (hydromaghemite and an unidentified nanocrystalline Fe‐bearing oxide phase observed with hydromaghemite) and phyllosilicates (saponite) were identified as the primary mineralogic hosts for H2O with a minor contribution from Cl‐rich apatite. Based on mass balance calculations and modal abundances of minerals constrained by powder X‐ray diffraction and petrography, we can account for the entire 6000 ppm H2O measured in bulk rock analyses of NWA 7034. This H2O is distributed evenly between hydrous Fe‐rich oxides and phyllosilicates, indicating that Fe oxides could be as important as phyllosilicates for H2O storage in Martian surface material. Key Points H2O in NWA 7034 is hosted by Fe oxyhydroxides, phyllosilicates, and phosphatesH2O is evenly distributed between hydrous Fe oxides and phyllosilicatesFe oxide/hydroxides could be important hosts for water on the Martian surface