Ultramafic rocks carbonation during paleo-seismic events: evidence from exhumed faults (Voltri Massif, Italy)

Faults generally act as preferential pathways for fluids; the high fracture density within their damage zones can increase the host-rock permeability, enhancing the rock reactive surfaces and speeding-up fluid-rock interactions. Accordingly, fault zones, in the presence of an aqueous CO2-rich fluid, could drive and enhance the natural carbonation of ultramafic wall rocks (peridotite and serpentinite); as a consequence, the drastic changes in lithology and mineralogy could also cause important variations in permeability and strength of the damage zone during deformation, through cycles of fault sealing. Here we report of exhumed detachment zones occurring in peridotite and serpentinised peridotite (Voltri Massif, Ligurian Alps, Italy) that show evidences of paleoseismic activity combined with the cyclic precipitation of carbonates from CO2-rich hydrothermal fluids. The damage zone of the studied faults contains different types of fault rocks, namely cataclasite, carbonated fault gouge, hydraulic breccias at the footwall and carbonated peridotite in the hangingwall. The fault cores are characterised by formation of a ca. 70 cm-thick cataclastic dolomite rich layer, containing dolomitic spherulites resembling the texture of cave pearls or ooolites with a serpentine core relic, but linked to the damage zone evolution. Cataclasites, containing damaged spherulites, are further cross-cut by carbonate coated slip zones and by chalcedony shear veins, with mirror-like surfaces, filamentous silica slickenlines, and granular injection veins, all of which provide evidence for paleoseismic activity. We combined field data with microstructural (SEM-EDS, EBSD), mineralogical, geochemical (high- resolution elemental imaging by Laser Ablation-ICP Time of Flight-Mass Spectrometry, δ13C, δ18O analyses), and mass transfer calculations to characterise fully the mechanical behaviour of these faults, fluid-rock interactions during deformation and quantify the amount of CO2 stored within the structure as hydrothermally precipitated carbonates.