Diffusion of dissolved SiO2 in H2O at 1 GPa, with implications for mass transport in the crust and upper mantle

The diffusivity (D) of dissolved SiO2 in quartz-saturated H2O was determined at 1 GPa, and similar to 530-870 degrees C using a custom-designed Ag diffusion cell consisting of two chambers - both containing quartz + H2O - connected by a narrow capillary. During a diffusion experiment, quartz saturation was maintained at different levels in the two chambers by placing the diffusion cell in the thermal gradient of a standard piston-cylinder assembly. The diffusivity was computed from the total mass of SiO2 transported from the "hot" to the "cold" chamber during the course of an experiment. Over the temperature range investigated, the results conform to an Arrhenius-type dependence of D-SiO2 (m(2)/s) upon T(K)(-1): D-SiO2 = 2.8(-2.0)(+4.7) x 10(-5) exp(-6300 +/- 1060/T) The significance of the constants in this equation (in particular, the similar to 52 kJ/mole apparent activation energy) is uncertain, because the SiO2 content of the fluid varies markedly with temperature, due to the strong temperature dependence of quartz solubility. Nevertheless, the above expression is probably a good representation of the temperature dependence of D-SiO2 in the crust, where aqueous fluids are likely to approach quartz saturation at all depths. One experimental result at 0.6 GPa suggests little dependence of D-SiO2 upon pressure at crustal conditions. At the low end of the temperature range investigated, the measured diffusivities are identical to values calculated from the Stokes-Einstein equation using high P-T viscosity estimates for H2O. Disagreement between measured and calculated diffusivities at higher temperatures (a factor of similar to 4 at 850 degrees C) may be due to one or more of the following factors: (1) inadequacy of the Stokes-Einstein relationship as a description of transport in supercritical H2O; (2) inaccuracy of viscosity estimates of H2O; or (3) concentration effects on diffusion over the temperature range investigated. Given the presence of interconnected porosity in deep-seated rocks, the diffusive transport distances for aqueous silica implied by the above equation are impressive even on a geologic scale, exceeding 0.5 km in 10(6) years at temperatures of 500 degrees C or higher. The combined effect of the high D-SiO2 with the high and strongly temperature-dependent solubility of quartz at crustal conditions raises the possibility of significant diffusive fluxes through a stationary fluid in a normal geothermal gradient.