Mechanisms of Serpentinization Utilizing Olivine–Plagioclase–H2O System under Hydrothermal Conditions

Silica activity influences the reaction pathways that may occur during serpentinization. At the crust–mantle boundary, silica activity and mass transfer are expected to increase. In this study, hydrothermal ‘tube-in-tube’ experiments utilizing the olivine–plagioclase–H2O system were conducted to assess silica-related mechanisms of hydrothermal alternation at the crust–mantle boundary. Experimental conditions were 230 ˚C and 2.80 MPa (i.e., vapor–saturated pressure) with run times up to 7,980 hours. Reaction products in the olivine zone varied with increasing distance from the olivine–plagioclase boundary, primarily changing from Al–serpentine to serpentine ± brucite + magnetite. No secondary minerals were apparent in the plagioclase zone. Total H2O content in the products was greatest at the boundary compared to the inner tube, where total H2O became homogeneous after the appearance of brucite at 3,184 h. During the experiments, olivine hydration and metasomatic reactions proceeded simultaneously. In contrast to talc-forming metasomatic reactions in olivine–quartz experiments, metasomatic reactions involving plagioclase–buffered fluids led to a decrease in fluid pressure. These reactions suggest that contrasting effects between quartz-buffered and plagioclase-buffered fluids on the chemical and mechanical behavior of the crust–mantle boundary.