Effect of transport limitations and fluid properties on reaction products in fractures of unaltered and serpentinized basalt exposed to high P_CO2 fluids

A series of high temperature and high pressure (45 °C and 100 °C; 10 MPa) experiments were conducted to examine geochemical alterations of fractured serpentinized and unaltered basalts exposed to high _PCO2 fluids. Net mineral dissolution was observed in flow-through experiments that examined reactions under advection-dominated conditions, whereas carbonate mineral precipitation occurred along basalt fractures after 6 weeks of reaction in static batch experiments where mass transport was diffusion limited. Consistent with prior work, increased temperature and salinity enhanced dissolution in flow-through experiments, and greater congruency in silicate dissolution was observed at higher temperature. Analysis of the reacted cores via X-ray computed tomography revealed regions of enhanced dissolution along the fracture pathway that correspond to contact with large grains of pyroxene and olivine. In the batch experiments, discrete Mg-bearing siderite crystals were observed along the entire fracture of the fine-grained unserpentinized basalt samples, while siderite formed in clusters along the fracture of the coarser-grained serpentinized basalt. Results of this study demonstrate how diffusive mass transport conditions and distribution of reactive mineral phases may be key factors in determining the extent of silicate dissolution and secondary mineral carbonation during geological carbon sequestration in fractured basalts.