Phase Equilibria Controls on the Chemistry of Vent Fluids from Hydrothermal Systems on Slow Spreading Ridges: Reactivity Of Plagioclase and Olivine Solid Solutions and the pH-Silica Connection

Theoretical reaction path models were combined with experimental data to determine fluid-mineral equilibria controls on the chemistry of vent fluids issuing from ultramafic-hosted hydrothermal systems (e.g., Rainbow and Logatchev I) on the slow spreading Mid-Atlantic Ridge. The model involves the incremental reaction of Ca-bearing (evolved seawater) with a calcic-plagioclase and olivinebearing mineral assemblage, and simulates aspects of an open geochemical system. Incipient reaction predicts Ca removal in exchange for Na, low silica, and high pH. With reaction progress, however, the Na for Ca exchange reaction reverses, while dissolved silica and pH increase and decrease, respectively. The combination of significant dissolved silica (8-9 mmol kg^-1), together with elevated dissolved Ca concentrations (60-70 mmol kg^-1) coexisting with secondary calc-silicate (tremolite) and Mg-chlorite mineralization, constitutes an important pH buffer. The inherently reducing nature (high H₂) of the model system enhances Fe solubility, while low dissolved H₂S contributes to elevated dissolved Cu, in keeping with compositional effects reported for Rainbow and Logatchev vent fluids. Data indicate that H2S is controlled at values more than 10 times lower than H₂ owing to bornite-chalcocitemagnetite- fluid equilibria. Olivine is not unreactive in the silica-bearing fluid at 400-425°C (500 bars) and is predicted to participate in the formation of calc-silicate, talc, and chlorite alteration phases. Model predictions, however, also indicate metastability of Fe-rich olivine. Experimental studies performed to examine olivine recrystallization reactions in silica bearing fluid indicate preferential dissolution of the forsterite component and precipitation of a more Fe-rich phase along with talc on the surface of the precursor olivine. Additional experimental data are required, however, to better determine the implications of this for the redox and pH evolution of vent fluids at slow spreading mid-ocean ridges.