Phase-Equilibrium Controls on SiO2Metasomatism by Aqueous Fluid in Subduction Zones: Reaction at Constant Pressure and Temperature

The mineral-fluid equilibria that govern silica redistribution by aqueous fluids in subduction zones were evaluated at constant pressure and temperature in the model system MgO-SiO2-H2O (MSH). At 100 moles fluid/cm3 rock; but at depth, flow of only several moles fluid/cm3 rock is sufficient to produce assemblages characteristic of silica metasomatism. Decreasing temperature with time in most subduction zones suggests that the potential for silica metasomatism is greatest at early stages of convergence. Subducted ultramafic rocks showing evidence of high-temperature silica metasomatism therefore may provide windows into early subduction processes. In general, temporal changes in subduction parameters favoring increasing temperatures will enhance the potential for silica metasomatism, whereas those leading to lower temperatures should be expected to decrease the capacity for silica transfer with time.