Brucite chimney formation and carbonate alteration at the Shinkai Seep Field, a serpentinite-hosted vent system in the southern Mariana forearc

Brucite-carbonate chimneys were discovered from the deepest known (∼5700 m depth) serpentinite-hosted ecosystem—the Shinkai Seep Field (SSF) in the southern Mariana forearc. Textural observations and geochemical analysis reveal three types (I–III) of chimneys formed by the precipitation and dissolution of constitutive minerals. Type I chimneys are bright white to light yellow, have a spiky crystalline and wrinkled surface with microbial mat and contain more brucite; these formed as a result of rapid precipitation under high fluid discharge conditions. Type II chimneys exhibit white to dull brown coloration, tuberous textures like vascular bundles, and are covered with grayish microbial mats and dense colonies of Phyllochaetopterus. This type of chimney is characterized by inner brucite-rich and outer carbonate rich zones and is thought to have precipitated from lower fluid discharge conditions than type I chimneys. Type III chimneys are ivory colored, have surface depressions and lack living microbial mats or animals. This type of chimney mainly consists of carbonate, and is in a dissolution stage. Stable carbon isotope compositions of carbonates in the two types (I and II) of active chimneys are extremely ¹³C-enriched (up to +24.1‰), which may reflect biological ¹²C consumption under extremely low dissolved inorganic carbon concentrations in alkaline fluids. Type III chimneys have ¹³C compositions indicating re-equilibration with seawater. Our findings demonstrate for the first time that carbonate chimneys can form below the carbonate compensation depth and provide new insights about linked geologic, hydrologic, and biological processes of the global deep-sea serpentinite-hosted vent systems.