Peridotite dissolution and carbonation rates at fracture surfaces under conditions relevant for in situ mineralization of CO_2

摘要

Whereas the dissolution of pure single phases (e.g. olivine and other mafic minerals) has been the focus of many studies, no investigation has been reported on the progress of reactions at and within polymineralic, transgranular fracture surfaces cutting peridotites. We document experiments that address the evolution of dissolving peridotite surfaces, and the rates of dissolution and carbonation reactions that occur at these surfaces, under both open- and closed-system conditions relevant for in situ CO_2-sequestration. The results of experiments, conducted under quasi-open system conditions, on solid samples of peridotite whose surfaces were taken as an analogue of free fracture surfaces, demonstrate apparent rates of olivine dissolution at the free surfaces that are equivalent to or upwards of 100 times greater than those determined for pure olivine under similar conditions. This increase in apparent olivine dissolution rate is ascribed to fluid penetration along grain boundaries and veins, which resulted in increased accessibility of reactive olivine surface area. Apparent dissolution rates under closed system conditions, where serial olivine dissolution and magnesite precipitation take place, are ～1 order of magnitude slower owing to changes in fluid composition and pH. In both of these reaction environments, it is apparent that serpentinized veins and mineral grain boundaries within the peridotite allowed fluids to penetrate the rock and to promote the release of divalent metals and silica from within the sample or simulated fracture walls. These results indicate that the microstructure of free (fracture) surfaces exerts a dominant control on peridotite dissolution and carbonation rates, relative to the proportion of highly reactive minerals that compose such ultramafic rock. The rapid dissolution we observe under open system conditions implies that CO_2-mineralization may be viable, in open systems, if the composition of injected fluids can be maintained at or near the initial composition. Fracture clogging that results from precipitation and fluid phase pH changes in closed systems, however, can drastically limit peridotite carbonation.