Ultramafic-Hosted Hydrothermal Systems at Mid-Ocean Ridges: Chemical and Physical Controls on pH, Redox and Carbon Reduction Reactions

Abstract

Experimental, theoretical and field investigations of hydrothermal alteration processes in ultramafic systems at mid-ocean ridges, indicate that these systems have the capacity to buffer pH at surprisingly low values (pH T,P = 4.9-5.2), which profoundly affects fluid chemistry. Sluggish reaction kinetics of olivine at elevated temperatures and pressures, (e. g., 400°C, 500 bars), together with SiO 2 and Ca dissolution from coexisting pyroxene minerals, enhance the stability of tremolite and talc accounting for the observed acidity. Moreover, oxidation of ferrous silicate components in unstable minerals, especially pyroxenes, generates high H 2(aq) concentrations, which together with the relatively low pH, increase Fe solubility, consistent with the Fe-rich nature of vents fluids issuing from ultramafic-hosted hydrothermal systems at Rainbow and Logatchev at 36°N and 14°N, respectively, on the Mid-Atlantic Ridge. The high dissolved Cu and Ni concentrations, and low H 2 S (aq) of these vent fluids, indicate redox buffering by magnetite-bornite-chalcocite-heazelwoodite (Ni 2 S 3 )-fluid equilibria, as indicated by experimental and theoretical data. Data show that dissolved Cu is particularly sensitive to temperature change, while H 2 S (aq) and Fe are affected less by this, although Fe is highly sensitive to pH and dissolved chloride. Dissolved chloride concentrations observed for both the Rainbow and Logatchev hydrothermal systems depart significantly from seawater and suggest supercritical phase separation in subseafloor reaction zones. The relatively high temperatures required for this, together with the high rates of fluid flow at Rainbow, indicate a magmatic heat source. The most unusual feature of fluids issuing from the Rainbow and Logatchev hydrothermal systems, however, involves high dissolved concentrations of methane and other hydrocarbon species, and detectable carbon monoxide. Experimental data indicate that reducing conditions and mineral catalytic effects may account for this, although the reported CO (aq) at Rainbow is well below predicted levels, suggesting re-equilibration at lower temperatures.