研究業績

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 hydrothermal 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).