Effect of Silica Transport on Serpentinization in the Ol-Opx- H2O System

Silica activity plays an important role in controlling reaction paths and rates of serpentinization in the oceanic lithosphere. In this study, we conducted hydrothermal experiments on serpentinization in the olivine (Ol)- orthopyroxene (Opx)-H2O system at 250 °C and vapor-saturated pressure (3.98 MPa). We used a tube-in-tube vessel with powders of Ol/Opx/Ol zones, and the extents of serpentinization and solution chemistry were analyzed in detail. The run time was up to 1512 hours. lica activity of the solutions decreased with time from 0.144 to 0.005 mmol/kg, whereas the Mg concentration and pH were nearly constant. The experimental products were serpentine (mainly lizardite) ± magnetite, and neither talc nor brucite was found. The extent of serpentinization, measured by thermogravimetry and SEM-images, revealed that serpentinization was most extensive along the boundary between the Ol and Opx zones, and decreased gradually away from the boundary. This extent of serpentinization pattern is explained by the coupled processes of silica diffusion and surface reactions in both zones (silica-consuming reaction in the Ol zone and silica- releasing reaction in the Opx zone).