Dissolution and surface roughening of Columbia River flood basalt at geologic carbon sequestration conditions

Carbon storage in basalt reservoirs can mitigate CO2 emissions to the atmosphere that contribute to climate change. For basalt reservoirs, CO2 injection leads to Ca2 +, Fe2 +, and Mg2 +-rich solutions that may result in carbonate precipitation for long-term stable carbon sequestration. Examination of basalt surfaces as dissolution progresses can offer important information about which minerals are dissolving, the timing and sequence of dissolution, and the effects these processes have on surface roughness and morphology. We carried out two series of experiments using two polished Columbia River flood basalt samples in CO2-rich water at 150 °C and 100 bar to observe the physical and geochemical changes during dissolution. One series reacted a sample over short time increments, while the other series examined a separate sample over longer time increments. Scanning electron microscopy, 2D profilometer analysis, and 3D laser confocal microscopy were combined with ICP-MS to characterize dissolution. Dissolution resulted in pitting, dissolution along fractures and grain boundaries, and an increase in species concentrations in the bulk solution over incremented reaction times. Based on these observations, early dissolution of olivine grains contribute Mg2 + and Fe2 + to aqueous solution in initial stages (< 1 week at a pH < 4), while slower continuous dissolution of Ca-rich pyroxene contributes Mg2 +, Fe2 +, and Ca2 + to the bulk solution over a longer period of time. The complete dissolution of olivine grains resulted in pits up to 200 μm deep. Dissolution of plagioclase and matrix was slower and resulted in the formation of micro-sized textures (< 10 μm). Following 1-2 months of reaction, the surface roughness parameters (mean and root mean squared) increased by factors of 42 and 28, respectively, while surface area of the flood basalt increased 20% relative to the starting polished surface. The results of this study indicate 1) pyroxene is the sustaining contributor of divalent metal cations during dissolution of basalt and 2) the limited connectivity limits of olivine and pyroxene grains the exposure of new reactive surface areas.