Titanium reactors for redox-sensitive hydrothermal experiments: An assessment of dissolved salt on H2 activity-concentration relations

Abstract

Fixed-volume titanium alloy hydrothermal reactors permit research into fundamental aspects of mineralogy and aqueous geochemistry at the T-P conditions encountered on the frontiers of submarine and terrestrial geothermal system exploration. The impressive mechanical strength of new titanium alloys facilitate studies involving fluids and minerals at conditions up to 500 °C and 50 MPa. The corrosion resistance of titanium is a well-known attribute that also allows experiments to be performed in acidic, NaCl-bearing aqueous fluids. Titanium, however, can suffer irreversible surface reactions in aqueous fluids at high temperatures that may produce or consume appreciable concentrations of dissolved hydrogen (H2(aq)), complicating the interpretation of redox processes in homogenous and heterogeneous chemical systems. Here we report results of hydrothermal experiments designed to examine the direction and magnitude of changes in dissolved H2(aq) in deionized water and NaCl bearing aqueous fluid contained within a newly constructed Ti alloy (Ti-6242) reactor. The redox sensitivity of these and related experiments, however, required testing and assessing surface passivation involving pre-treatment with nitric acid at moderately high temperature. Results indicate that passivation procedures were required to render the Ti surface of the reactor unreactive in both low and high hydrogen-bearing fluid (H2O and NaCl-H2O systems). Once passivated, experiments were then conducted at 400 °C to assess the effect of dissolved NaCl on dissolved H2(aq) concentrations coexisting with the well-known HM (hematite-magnetite) redox buffer. These data indicate highly non-ideal activity-concentration relations for H2(aq) in the NaCl bearing fluids. Accordingly, the common assumption of unity for the activity coefficient of dissolved H2(aq) in hydrothermal fluids is not supported. The implications of this are important for accurate redox reconstruction for compositionally variable natural and experimental hydrothermal systems at elevated temperatures and pressures. The experimental strategy of monitoring the rate and magnitude of H2(aq) production and consumption in situ, during on-going experiments, provides quantitative insight on the effectiveness of the passivation process and the usefulness of redox buffers in hydrothermal geochemical applications.