Geochemical traces of CO2-rich fluid flow along normal faults in central Italy

Abstract

SUMMARY We studied the oxygen and carbon isotopic composition of fault rocks, as well as fractured and fragmented platform carbonates, along large basin-bounding normal fault zones in central Italy. The internal architecture of the principal fault segments found in these zones consists of faulted and brecciated Quaternary sediments in the hanging walls, and of four different carbonate structural domains (A‐D) in the footwalls. In the footwall damage zones, we identified fractured and fragmented platform carbonates (domain A) and pulverized carbonates (domain B). Within the fault cores, we recognized both matrix-supported (domain C) and cement-supported fault rocks (domain D). The latter structural domain is located primarily along the main slip surfaces of the fault cores. Analysing the geochemistry of powder samples of fault rocks, fractured and fragmented platform carbonates, Quaternary basinal sediments, and platform carbonate host rocks we observed two different trends in δ 18 O‐δ 13 C space. These two isotopic trends are interpreted as the signatures of two different fault fluids. The first, and most prominent, isotopic trend is related to a meteoric-derived fluid, the second one to a groundwater-derived fluid originated from the local South Marsica aquifer. Although we do not have any laboratory data on the T/P conditions of fluid mineralization, based on the isotopic difference between the fluid sources and the precipitated solutes, we predicted a mineralization temperature of ∼25 ◦ for the meteoric-derived fluid, and ∼100 ◦ for the groundwater-derived fluid. The stable isotope composition of the four structural domains shows the focussing and compartmentalization of the meteoric-derived fluid primarily at the hanging wall/footwall contacts. The groundwater-derived fluid also moved through the fragmented carbonates associated with small, intracarbonate normal faults to the east of the southernmost studied normal fault zone. Considering the high CO2 content of the South Marsica aquifer, we suggest that pockets of high pCO2 fluids may have formed within the fault cores during exhumation and ongoing normal faulting.