Micron-scale crystals in halite are proxies for Permian acid brines

Abstract Permian bedded halite of the Nippewalla Group of Kansas and the Opeche Shale of North Dakota, USA, formed in extremely acid saline lakes. However, a full understanding about the compositions of those brines is not known. Micron-scale crystals within primary fluid inclusions in halite give clues about the geochemistry of past lake waters. Here, we use petrographic, microthermometric, and Raman spectral observations of crystals in chevron and cumulate halite crystals from two subsurface cores to make interpretations about past Pangean lake water chemistry. Our results show that (1) entrapped crystals are situated within primary fluid inclusions and as solid inclusions along growth bands in host depositional halite; (2) the presence of entrapped crystals is heterogeneous amongst fluid inclusions of single growth bands; (3) crystals in fluid inclusions do not change size upon heating or cooling; (4) at least nine different sulfate minerals were identified, including Ca-sulfates, Sr-sulfate, Mg-sulfates, Fe-sulfates, and Al-sulfates; and (5) rare carbonate grains partially coated with hematite were observed in some inclusions. We conclude that these crystals precipitated, alongside halite, in complex sulfate-rich, metal-rich acid saline lake waters. Rare hematite-coated carbonate grains likely were blown into the lakes by winds. We propose that these petrographic and spectroscopic methods can be used to refine past water compositional data from other bedded evaporites. In addition, any salt minerals returned in samples from Mars should be evaluated for accidental daughter crystals to reach a better understanding of past Martian water compositions and habitability.