Contributions of visible and invisible pores to reactive transport in dolomite

Abstract

Recent technical advances have demonstrated the importance of pore-scale geochemical processes for governing Earth's evolution. However, the contribution of pores at different scales to overall geochemical reactions remains poorly understood. Here, we integrate multiscale characterisation and reactive transport modelling to study the contribution of pore-scale geochemical processes to the hydrogeochemical evolution of dolomite rock samples during CO2-driven dissolution experiments. Our results demonstrate that approximately half of the total pore volume is invisible at the scale of commonly used imaging techniques. Comparison of pre- and postexperimental analyses demonstrate that porosity-increasing, CO2-driven dissolution processes preferentially occur in pores 600 nm-5 μm in size, but pores <600 nm in size show no change during experimental alteration. This latter observation, combined with the anomalously high rates of trace element release during the experiments, suggests that nanoscale pores are accessible to through-flowing fluids. A three dimensional simulation performed directly on one of the samples shows that steady state pore-scale trace element reaction rates must be ∼10× faster than that of dolomite in order to match measured effluent concentrations, consistent with the large surface area-to-volume ratio and high reactivity of these pores. Together, these results yield a new conceptual model of pore-scale processes, and urge caution when interpreting the trace element concentrations of ancient carbonate rocks.