Dolostone pulverization induced by coseismic rapid decompression of CO2-rich gas in nature (Matese, Apennines, Italy)

Abstract

South Matese, Apennines, is a hydrothermally and seismically active extensional area characterized by CO2outgassing and Mw≤7.1 earthquakes. There, meters-sized pockets of incohesive pulverized dolostone are hosted within Mesozoic carbonates at the hanging wall of seismically active normal faults. The aim of this paper is to understand the pulverization process. The pulverized dolostone is finely comminuted (down to a few microns), but primary structures, mainly bedding, are preserved. The grain size distribution is similar to that of previously studied pulverized rocks associated with active faults and dissimilar to that of carbonate cataclasites and fault gouges. The pulverized pockets are surrounded by zones (halos), in which the loose grains are cemented, in their original position, by microcrystalline calcite, resulting in a cemented micro-mosaic breccia. Stable isotopes from the cement are compatible with calcite precipitation from rapidly CO2-degassing shallow waters. Comparing our observations with results of laboratory experiments on carbonate pulverization through rapid decompression of pore-hosted CO2, the best explanation for the pulverized dolostone may lie on local accumulations of pressurized CO2-rich gas, suddenly decompressed during earthquakes. The limited permeability of the gas-saturated dolostone must have prevented a prompt escape of the gas from the rock, which was therefore anhydrously pulverized by the rapid expansion of the trapped gas. The sudden decompression must have suctioned bicarbonate-rich groundwaters, from which microcrystalline calcite rapidly precipitated, fossilizing the freshly pulverized dolostone. Calcite precipitation formed an impermeable shield around the pulverized pockets, which, therefore, remained internally uncemented. This process may have occurred over multiple cycles at depths shallower than the CO2subcritical–supercritical boundary (ca. -800m). Although hypothetical, the proposed mechanism is for the first time suggested for an active tectonic environment. The gas rapid decompression could have been triggered by coseismic processes (e.g., dynamic unloading or transient tensile pulses) previously proposed for the formation of other pulverized rocks. The presented case may improve our knowledge of possible chemical-physical processes connected with the subsurface storage of CO2in seismically active areas.