Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: An experimental study at 4 00°C, 500 bars

Olivine (Fo₈₉), orthopyroxene (En₈₅), and clinopyroxene (Di₈₉) were reacted, individually and in combinations, with NaCl-MgCl₂ at 400°C, 500 bars to better assess alteration and mass transfer in ultramafic-hosted hydrothermal systems at mid-ocean ridges. Data indicate that temperature plays a key role in mineral solubility and kinetic processes, which influence the compositional evolution of the fluid. At the temperature and pressure of the experiments, the rate of olivine hydrolysis is sluggish as indicated by the limited extent of mass transfer between the fluid and mineral and absence of hydrous alteration phases. In contrast, reactions involving pyroxenes proceed rapidly, which result in significant increases in dissolved Ca, SiO₂, Fe and H₂, and formation of SiO₂-rich secondary minerals (talc and tremolite) and magnetite. SiO₂ release from pyroxene occurs in non-stoichiometric proportions and is a critical factor governing the stability of secondary minerals, with attendant effects on fluid chemistry. Magnetite and talc-fluid equailibria were used to calculate fluid pH at elevated temperatures and pressures. In general, pH is relatively low in the orthopyroxene- and clinopyroxene-bearing experiments due to constraints imposed by talc-fluid and talc-tremolite-fluid equilibria, respectively. Even in experiments where the olivine/pyroxene ratio is as great as 3, which is typical for abyssal peridotite, the low pH and high Fe concentrations are maintained. This is in sharp contrast to theoretical predictions assuming full equilibrium in the MgO-CaO-FeO-Fe₂O₃ -SiO₂-Na₂O-H₂O-HCl system at 400°C, 500 bars. Ultramafic-hosted hydrothermal systems, such as the recently discovered Rainbow system at 36°13.80′N, 33°54.12′W on the Mid-Atlantic Ridge, indicate reaction processes in keeping with results of the present experiments, as suggested by vent fluid chemistry and temperature. In particular, relatively high SiO₂, Ca, H₂, and Fe concentrations characterize the Rainbow vent fluids. Indeed, Fe concentrations are the highest of any vent system yet discovered and require a relatively low pH in the subseafloor reaction zone from which the fluids are derived. This, together with the SiO₂ concentrations of the vent fluids, strongly indicates fluid buffering by silica-rich phases produced during pyroxene dissolution, the likely abundant presence of olivine notwithstanding. Time-series observations at Rainbow are clearly needed to between constrain the temporal evolution of hydrothermal alteration processes of ultramafic rocks in subseafloor reaction zones. In the absence of events permitting fluid continuous access to fresh rock, pyroxene will ultimately be consumed and vent fluids may then reflect changes imposed by bulk compositional constraints characteristic of ultramafic bodies at depth, which would be in better agreement with theoretical phase relations for the fully equilibrated system.