Gypsum in modern Kamchatka volcanic hot springs and the Lower Cambrian black shale: Applied to the microbial-mediated precipitation of sulfates on Mars

摘要

Gypsum is a mineral that commonly precipitates in hydrothermal environments. This study reports the electron microscopic analyses of gypsum morphologies and crystal sizes found in hot springs on the Kamchatka Peninsula, Russia, and compares these analyses with gypsum morphologies of hydrothermal genesis found in Lower Cambrian black shale. In sediments of the Kamchatka hot springs, we observed prismatic, prismatic pseudo-hexagonal, fibrous, tubular, lenticular and twinned gypsum crystals, with crystal sizes ranging from <200 nm to >200 mm. The coexistence of diverse crystal habits of gypsum implies a constant interaction between hot spring geochemistry and the metabolisms of the microbial community. The crystallization of Ca- and Ba-sulfates in the black shale of the Lower Cambrian, which shows similar but less varied morphology, was influenced by post-depositional hydrothermal fluids. The partial replacement of pyrite by sulfates in a situation coexisting with rich biomass deposits and animal fossils indicates limited modification of the sedimentary records by biological materials. If the gypsum precipitated on Mars underwent similar interactions between microbial communities and their geochemical environments, the resulting crystal habits could be preserved even better than those on Earth due to the weak geodynamics prevailing on Mars throughout its evolutionary history.