Séminaire
Understanding complex reactions between CO2-rich water and rocks is fundamental forquantifying the carbon cycle. Earth’s CO2degassing in hydrothermal fields integrateslong-termconsumption of the reactive surface area of the minerals involved in water rockinteraction.We use the composition of CO2-rich geothermal waters to estimate thereacting surface area (RSA) of mineralsin a granite, responsible for the gradient in pCO2partial pressure observed in the field. The results show that the kinetic rates of mineraldissolution are not linear, as generally assumed. We found that the reactive surface areasof rock forming minerals in the granite are not constant over long-term interactionprocess, invalidating the assumptions intrinsic to predictive modelling of reactive CO2transport. In particular, the proportion of biotite/albite reactive surface area varies by 4orders 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 mayexplain the correlations known between gas discharge and tectonic regimes, by theproduction of pathways resulting from the preferential dissolution of minerals that, byproducing permeability variations, may connect the crust to earth surface.
thomas.thiebault@upmc.fr