Hydroxychloride‐bearing Fluid Inclusions in Ultramafic Rocks from New Caledonia: Implications for Serpentinization in Saline Environments on Earth and beyond

Serpentinization produces H-2 and synthesizes CH4 that can support nonphotosynthetic habitability on Earth and possibly beyond. Although fluids on Earth and beyond that may participate in serpentinization are extensively saline with elevated levels of chlorine, chloride minerals are generally not considered to be a crucial component of serpentinization. In this study, we report the formation of kuliginite, a recently discovered hydroxychloride mineral with the ideal formula of Fe3Mg(OH)(6)Cl-2, during internal serpentinization within olivine-hosted fluid inclusions in ultramafic rocks from New Caledonia, southwest Pacific. It is inferred that kuliginite may form during serpentinization by Cl-rich saline fluids under certain conditions on celestial bodies, which is largely unrevealed due to our insufficient knowledge about kuliginite. As a mineral containing similar to 57 wt.% FeO that is mainly ferrous, kuliginite, even in minor amounts, may play an important role in iron partitioning during serpentinization, with a great influence on the production of H-2 and synthesis of CH4. In a word, stabilization of kuliginite retards iron oxidation, whereas further hydration of kuliginite supplements canonical pathways of iron oxidation. In addition, as a hydroxychloride, kuliginite may have a significant weight in the chlorine budget of serpentinite. Therefore, kuliginite in the serpentinized fluid inclusions presented in this study may have general implications for serpentinization in certain saline environments on Earth and beyond, which needs to be further explored in the future.