Mn-carbonate deposition in a seafloor hydrothermal system (CLAM field, Iheya Ridge, Okinawa Trough): Insights from mineralogy, geochemistry and isotope studies

A seafloor hydrothermal system located at the Iheya Ridge (Okinawa Trough), named CLAM, deposits Mn-carbonate chimneys that have no analogue found so far on the seafloor. The chimneys are composed of Mn-calcite and Ca-rhodochrosite. The crystallographic differences between these carbonates appear to control the rare earth elements (REE) partitioning between them that results in enrichment of the Ca-rhodochrosite in middle and heavy REE, and enrichment of the Mn-calcite in light REE. Chemistry of the CLAM hydrothermal fluids suggests: (1) low water/rock ratio of the hydrothermal system; (2) phase separation and dominance of low -chlorinity vapor phase; (3) sub-seafloor formation of Na-rich alteration minerals during fluid/rock reactions; (4) removal of some elements from the seawater to the host rocks during the seawater/rock interaction; (5) high Mn/ Ca ratio of the basement rocks is responsible for the high Mn concentration in the hydrothermal fluids. C-O -isotope compositions of the CLAM Mn-carbonates suggest they precipitated through binary mixing of end -member hydrothermal fluid and seawater accompanied by progressive degassing and cooling of the fluid. Mn-calcite precipitated from almost pure end-member hydrothermal fluid, whereas Ca-rhodochrosite precipitated from seawater-dominated vent fluid. Mg-isotope fractionation during Mn-carbonate precipitation is assumed to depend on carbonate growth conditions and resulting carbonate mineralogy. S-isotope composition of the CLAM Mn-carbonates suggests that the Ca-rhodochrosite precipitated in oxic conditions through rapid mixing of hy-drothermal fluid and seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) through slow mixing of hydrothermal fluid and seawater. Sr-isotope composition of the CLAM hydrothermal fluids is close to that of Okinawa Trough deep seawater. In contrast, Sr-isotopes in the CLAM Mn-carbonates are more variable, indicating that Sr was derived from seawater, local lavas and sediments. Nd-isotope composition of the Mn-carbonates indicates that Nd was derived from the local lavas and sediments. Pb in the majority of the CLAM Mn-carbonates is of sedimentary origin (Pb isotope data), but involvement of anthropogenic Pb in the hydrothermal system is inferred for some Mn-calcite samples. Stability phase diagram modeling coupled with C-O-S-Sr-isotope data suggest that in the CLAM vent fluid the rhodochrosite is stable in a narrow Eh-pH range (6 < pH < 10; Eh > 0) and in a wide range of [Mn] and [Ca] activities, whereas calcite precipitates from a close to the end-member hydrothermal fluid in reduced conditions (Eh < 0).