Controls on Mineral Formation in High pH Fluids From the Lost City Hydrothermal Field

Abstract Although the serpentinite‐hosted Lost City hydrothermal field (LCHF) was discovered more than 20 years ago, it remains unclear whether and how the presence of microbes affects the mineralogy and textures of the hydrothermal chimney structures. Most chimneys have flow textures comprised of mineral walls bounding paleo‐channels, which are preserved in inactive vent structures to a varying degree. Brucite lines the internal part of these channels, while aragonite dominates the exterior. Calcite is also present locally, mostly associated with brucite. Based on a combination of microscopic and geochemical analyses, we interpret brucite, calcite, and aragonite as primary minerals that precipitate abiotically from mixing seawater and hydrothermal fluids. We also observed local brucite precipitation on microbial filaments and, in some cases, microbial filaments may affect the growth direction of brucite crystals. Brucite is more fluorescent than carbonate minerals, possibly indicating the presence of organic compounds. Our results point to brucite as an important substrate for microbial life in alkaline hydrothermal systems.