Geochemical constraints on reaction temperature and pressure and heat- and mass-transfer efficiency at Rainbow Hydrothermal Field

Seafloor vent fluids hosted by oceanic core complexes (OCCs) are thought to represent circulation of seawater-derived hydrothermal fluid along deeply penetrating, low-angle detachment faults. However, estimation of the source temperatures and pressures of such fluids has been limited because geochemical methods typically require vent fluid silica concentrations to be buffered by quartz, a condition not often met at oceanic core complexes. Here, we extend the calculation of the Si-Cl geothermobarometer to enable predictions of fluid Si concentrations in equilibrium with any Si-buffering mineral assemblage, rather than being restricted to quartz. We apply this method to Rainbow Hydrothermal Field vent fluid compositions and find that they are consistent with buffering by a plagioclase+talc+chlorite+tremolite mineral assemblage at conditions ranging from (430 °C, 359 bar) to (470 °C, 468 bar), which correspond to depths of 1.3–2.4 km below the seafloor. These estimates agree well with the locations of seismically imaged magma chambers within the Rainbow Massif. Additionally, calculations of fluxibility and Fe solubility support this relatively shallow origin for Rainbow vent fluids and imply relatively efficient heat and Fe extraction from the seafloor. We estimate that only 24–30 % of the heat content and almost no Fe is lost during upflow of Rainbow vent fluids.

Compared to other OCC-hosted seafloor vents, the source region of Rainbow vent fluids is anomalously shallow, an observation consistent with geological interpretations of the Rainbow Massif. Vent fluids at Rainbow Hydrothermal Field have exhibited apparently stable and greater-than-seawater salinity for over two decades. We interpret these vent fluids as vapors derived from a higher-salinity source fluid that developed over multiple cycles of magma injection and phase-separation inherent to the formation of oceanic crust along slow-spreading, non-volcanic segments of oceanic spreading centers. Alternatively, higher-salinity source fluids could be derived from mineral hydration reactions associated with serpentinization of ultramafic rocks. The occurrence of greater-than-seawater salinity vent fluids is thus predicted to be a common feature of OCC-hosted vent fields, as indicated by several known examples, including the TAG, Kairei, and Edmond vent fields.