The Role of Silica Precipitation in the Permeable-Impermeable Boundary Within the Earth’s Crust

Abstract

Silica is one of the dominant constituents of the Earths crust, and dissolution and precipitation of silica minerals have a significant role on the hydrological properties of the Earths crust. However, the role of dissolution and precipitation of silica minerals on fracture permeability is still unclear. In this study, the hydrothermal flow-through experiments were conducted to reveal the mechanism and kinetics of silica precipitation in the crustal conditions. Large amount of silica precipitation occurred only in the supercritical conditions of water. The strong temperature dependence of silica precipitation can be explained by the homogeneous nucleation of quartz. With increasing Al and Na concentrations in the solution, the dominant precipitates changed systematically from amorphous silica to cristobalite to quartz, and precipitation rate increased. The Kakkonda geothermal field, Japan, has the deep drilling well that penetrated the boundary between the hydrothermal convection zone and the heat conduction zone. Calculation of quartz solubility revealed that the depth of a local minimum of quartz solubility corresponds to that of the permeable-impermeable boundary at the Kakkonda geothermal field. These experimental and calculation results suggest that the rapid quartz precipitation plays a significant role in forming and sustaining the permeable-impermeable boundary within the Earths crust, especially at geothermal fields.