Experimental measurement of H_2(aq) solubility in hydrothermal fluids: Application to the Piccard hydrothermal field, Mid-Cayman Rise

In mid-ocean ridge hydrothermal systems the fluid redox state is controlled by oxidation of ferrous iron in the host rock and generation of aqueous hydrogen, H_2(aq). A quantitative interpretation of redox state requires conversion of H_2(aq) concentrations to H₂ fugacity, f H₂. Here we present the results of hydrothermal experiments that calibrate the f H₂-concentration relationship in saline hydrothermal fluids, Y_H2-Cl. H_2(aq) concentrations were measured between 400 and 500 °C, 210 and 510 bar, in both single-phase fluids and low-density vapors in the KCl-H₂O system. The assemblage hematite-magnetite was used to buffer f H₂. Values of Y_H2-Cl are well above unity and decrease as temperature increases. Along isotherms Y_H2-Cl decreases as the fluid density decreases. At 400 °C, 330 bar and 1000 mmol/kg KCl Y_H2-Cl is 316 bar/molal and decreases to 4 bar/molal at 500 °C, 400 bar and 33 mmol/kg KCl. In combination with previous determination of H₂S_(aq) solubility at hydrothermal conditions, the data presented here permit calculation of the redox conditions of natural vent systems. Redox conditions calculated for the basalt-hosted Piccard hydrothermal field suggest amphibolite facies mineral assemblages are present at depth, consistent with the extremely hot temperatures (>500 °C) inferred from analysis of other aspects of vent fluid chemistry (e.g., Cl and SiO₂). Piccard may be the first location of active venting that records, through its composition, evidence for an alteration assemblage that had been previously observed only in fossilized submarine hydrothermal systems.