Solute-sieving-induced calcite precipitation on pulverized quartz sand: Experimental results and implications for the membrane behavior of fault gouge

We report on a preliminary experiment with an analog for fault gouge composed of clay-sized quartz particles. An undersaturated calcium carbonate solution was forced through a layer of clay-sized quartz particles. Calcite crystals, identified by secondary electron imaging and energy dispersive X-ray analysis, formed on the gouge. Calcite precipitation was not due to changes in temperature or pH related to ion exchange. A rise in Ca and bicarbonate concentrations on the high pressure side of the membrane is the most likely cause of calcite precipitation. Solute-sieving by clay-rich sediments has been previously described. Our results suggest that the solute-sieving properties of fault gouge should be considered as a possible mechanism for the selective cementation of some faults even when the gouge has a low clay mineral content. The formation of calcite cements in one fault zone in central New Mexico may be explained by solute-sieving by fault gouge. This zone is strongly preferentially cemented by calcite relative to the surrounding materials, yet oriented concretions within the cemented zone suggest that the groundwater flow was nearly perpendicular to the fault at the time of cementation.