Quantification de la réactivité des minéraux en réponse aux perturbations naturelles du CO2

Understanding complex reactions between CO2-rich water and rocks is fundamental for quantifying the carbon cycle. Earth’s CO2 degassing in hydrothermal fields integrates long-term consumption of the reactive surface area of the minerals involved in water rock interaction. We use the composition of CO2-rich geothermal waters to estimate the reacting surface area (RSA) of minerals in a granite, responsible for the gradient in pCO2 partial pressure observed in the field. The results show that the kinetic rates of mineral dissolution are not linear, as generally assumed. We found that the reactive surface areas of rock forming minerals in the granite are not constant over long-term interaction process, invalidating the assumptions intrinsic to predictive modelling of reactive CO2 transport. In particular, the proportion of biotite/albite reactive surface area varies by 4 orders of magnitude for the range of pCO2 gradient measured (6 orders of magnitude). The conservative estimation of the mineral’s reactive surface area offered here may explain the correlations known between gas discharge and tectonic regimes, by the production of pathways resulting from the preferential dissolution of minerals that, by producing permeability variations, may connect the crust to earth surface.