Chemical alteration and mineral growth under high pCO2 conditions: Insights from the mineral chemistry of carbonate phases in the Caprese Reservoir (Northern Apennines, central Italy)

Carbon cycle is one of the most complex geochemical cycles of the Earth as it involves atmosphere, hydrosphere, biosphere and lithosphere. Among these different geochemical spheres, mechanisms and reaction rates governing migration and fixation of carbon (i.e. fluxes), such as the stability of the different carbonates minerals under specific conditions in crustal reservoirs, are still matter of detailed studies. In this work, SEM and XRD investigations and EMP analyses were combined to study the mineralogical and chemical compositions of carbonate phases occurring in strongly altered Triassic volcanic rocks, hosting a CO2 reservoir (namely the Caprese Reservoir), located in eastern Tuscany (central Italy). The Caprese crustal fluid reservoir, lying at 3300 m depth beneath the Quaternary San Cassiano basin, hosts a supercritical CO2-(N2)-rich phase and a Na-Cl-rich saline (up to 81.5 g/L) brine. The mineralogical assemblage consists of authigenic microcrystalline quartz, Ca-Fe-Mg carbonates, Na-K-clay minerals and Fe-oxides, the primary minerals being almost completely chemically altered by secondary processes. Scanning electron microscope images and cation (Ca-Fe-Mg) contents in carbonates highlighted that zoned ankerite was re-grown on calcite crystals due to interactions processes with CO2-rich fluids. The geochemical and mineralogical data provided useful insights for understanding both the processes that formed the C-bearing minerals and their growth in deep, saline reservoirs.