Frictional instabilities of basalts and calcite-built marbles in the presence of pressurized H2O- and CO2-rich fluids

Fluid-rock interactions control earthquake nucleation and the evolution of natural and man-induced seismic sequences. Experimental studies of fault frictional properties in the presence of pressurized fluids can provide unique insights into these interaction. We performed 14 friction experiments on cohesive silicate-built (basalts) and calcite-built rocks (Carrara marbles) in the presence of pressurized pure H2O, pure CO2 and H2O+CO2 fluids to investigate the triggering of frictional instabilities associated to CO2 storage in basalts (e.g. injection H2O+CO2 mixtures to fix CO2 to newly formed carbonate minerals) and, to a less extent, the processes driving to earthquake triggering in carbonate-built rocks. Experiments were performed at room temperature on 50/30 mm external/internal diameter hollow-shaped rock cylinders with the rotary shear apparatus SHIVA (INGV, Rome). Sample were inserted in a pressure vessel and the experiments performed under drained conditions. After imposing an initial normal stress of 15 MPa, an initial shear stress of 5 MPa and an initial pore fluid pressure of 2.5 MPa, the pore fluid pressure was increased in steps of 0.1 MPa every 100 s till the main frictional instability was triggered. The main instability was defined as the instant at which the sample accelerated to a slip rate of >0.3 m/s (seismic slip rate). Un-deformed and deformed samples, the slip surfaces, the slipping zones and the wall rocks were investigated with optical microscope, XRD, XRF and micro-Raman spectroscopy; H2O+CO2 and H2O fluids were recovered after the experiments to determine the enrichment of the chemical species (Ca++, Mg++, etc.). Carrara marble was more prone to slip in the presence of pressurized H2O+CO2 mixtures than in pure CO2 and H2O fluids; instead, in basalts, the injection of pressurized H2O+CO2 delayed the main frictional instability with respect to the experiments performed in pure H2O and anticipated with respect to pure CO2 fluids. Main instabilities were preceded by creep and slip burst events ("precursory events"): the number and frequency of slip burst events was larger in the experiments performed on basalts. Moreover, in basalts enriched in clay minerals (1) fault reactivation occurred at lower pore fluid pressures at a given normal stress and (2) the frequency of precursory events decreased, making the fault more “silent” and unstable than the fault made of less altered basalts. In the experiments, fluids may play both a chemical and mechanical role. Pure CO2 mainly contributes to pressurize the experimental faults in both basalts and Carrara marbles, with minimal chemical interaction with the host rock. Instead, H2O+CO2 mixtures resulted in formation of H+ ions which caused dissolution of the two rock types, as suggested by the enrichment in Ca2+ and Mg2+ cations measured in their respective aqueous solutions. Noteworthy, in the case of basalts, the high concentration of the Ca2+ and Mg2+ cations in solution and dissolved from glass, pyroxene and feldspars resulted in precipitation of calcite and dolomite (= mineral carbonation) in the experimental slipping zone. The rapid carbonation processes observed in our experiments, which last only 30-40 minutes, demonstrates the great effectiveness of the large scale CO2 storage projects in basaltic rocks as the CarbFix in Iceland.