The dissolution rates of naturally altered basalts at pH 3 and 120 °C: Implications for the in-situ mineralization of CO2 injected into the subsurface

Abstract

Due to their widespread distribution in the Earth's crust, it seems likely that altered basalts could be targeted for CO2 storage via subsurface carbon mineralization. To assess the potential efficiency of this approach, the steady state release rates of major elements from a suite of altered basalts have been measured at pH 3, 120 °C, and far from equilibrium conditions. The altered basalt samples have chemical compositions close to that of fresh basalt, but their mineralogy varies due to their alteration either at the Earth's surface or hydrothermal conditions at temperatures up to 250 °C. The studied altered basalts contain variable amounts of primary plagioclase and pyroxene, and substantial secondary phases including quartz, zeolites, epidote, chlorites and clay minerals. Despite their differing mineralogy, the steady-state element release rates of all the altered basalts are similar to each other when normalized to geometric surface area. These rates, however, are one to three orders of magnitude slower than corresponding release rates of basaltic glass and fresh crystalline basalt, depending on the element and on whether the rates are normalized to initial BET or geometric surface area. If present in small amounts in the altered basalts, calcite dissolves rapidly in the acidic reactive fluids, and does not contribute to the measured steady-state calcium release rates. Taken together, the results of this study indicate that altered basalt formations can provide sufficient divalent cations for subsurface carbon mineralization. As the element release rates of these altered basalts are lower than those of basaltic glass or fresh crystalline basalt, efforts to carbonate subsurface altered basalts may be best targeted at systems having temperatures in excess of 100 °C to compensate for the lower reactivity of these rocks.