Dissolution experiments on a polyphase basalt surface under conditions relevant to offshore CO2 storage.

The basaltic crust attracts increasing attention as a promising lithology for CO2 storage, due to its common occurrence, its vast storage capacity in pores, and its chemical composition rich in divalent cations - required to bind the dissolved CO2 in form of carbonate minerals. The availability of divalent cations for carbonate mineralization critically depends on the dissolution kinetics of the basaltic host rock. Numerous laboratory experiments have been conducted on a variety of rock forming minerals to investigate these dissolution kinetics and mechanisms under various experimental conditions. In the case of heterogeneous materials, such as polymineralic rocks, the identification and quantification of rate determining parameters is challenging and requires further investigations.In our experiments, we analyze the dissolution behavior of an intact, micro-crystalline basalt sample, typical for mid ocean ridge basalts, in contact with CO2-charged water under temperature and pressure conditions relevant to offshore CO2 storage. We combine flow-through dissolution experiments with Raman coupled Vertical Scanning Interferometry (RcVSI) in order to obtain spatially resolved images of both the topography and the chemical composition of the rock surface. The consecutive topography measurements by VSI allow us to quantify spatial differences in surface reactivity and examine their relation to chemical and structural properties provided by Raman spectroscopy. The data show significant differences in dissolution rates both between the different minerals and within single phenocrysts. Furthermore, the spatial heterogeneities in surface reactivity indicate an important influence of the rock texture as well.This combination of measurements provides the means to investigate the dissolution kinetics of polymineralic rocks and to determine differences in the dissolution kinetics of different minerals simultaneously. The results provide important information about rock internal parameters that contribute to the overall dissolution behavior of the rock. The results improve our understanding of dissolution processes that are critical to the efficiency of carbonate mineralization for long-term CO2 storage in submarine basaltic aquifers.