A new approach for constraining temperature, fractionation process, and fluid evolution in clay-rich fault zones: a case study from Lemnos Island (Greece)

Fluid circulation in the shallow crust is modulated by faults, which can act as barriers, conduits, or combined systems. In fault zones, fluids may vary in temperature and composition, originating from meteoric, connate, or magmatic/hydrothermal sources, and leading to the precipitation of various minerals in fault-related rocks and veins (e.g., carbonates, silicates, sulphates, oxides/hydroxides, clay minerals). In limestone, stable isotopes analyses (C, O), clumped isotopes, microthermometry of fluid inclusions, and U-Pb dating on carbonate mineralizations (e.g., slickenfibers, veins) are generally applied to determine the temperature and the source of fluids circulating within the fault zone during deformation. Discriminating temperature and fluid origin in clay-rich fault zones is more challenging, due to the coexistence of detrital minerals derived from the mechanical comminution of the host rocks and authigenic/synkinematic minerals precipitated during transient frictional heating or by prolonged fluid circulation. The compositional and temperature variation of fluids over time is recorded by authigenic minerals, that may reflect mixing with external sources or deformation at different depths and structural levels. The extent of fluid interaction with detrital minerals also contributes to their isotopic signature, and the evaluation of fluid sources can be very tricky due to the various mineral-water fractionation factors for every mineral. Indeed, H and O isotopes studies in clay-rich fault zones are generally applied as long as fault rock samples are nearly mono-mineralic, leading to very low number of data to develop a reliable dataset. To solve this issue, we applied a multi-method approach based on X-ray diffraction analyses of clay minerals, paleotemperature evaluation, and H, O isotope studies of different grain size fractions (from <0.1 to 10 µm) combined with a new calculation that allows to evaluate the fractionation processes of every single mineral (detrital vs. authigenic). In addition, K-Ar ages on syn-kinematic K-bearing minerals allowed to determine the age of faulting and eventually build an evolutionary model of fluid composition and temperature. In this contribution, we investigated two regional-scale fault zones on Lemnos Island (Greece), the Kornos-Aghios Ioannis extensional fault and the Partenomythos extensional fault, that are affected by Si-rich hydrothermal alteration. Our findings show that authigenic clay minerals (illite-smectite) from the <0.1 fractions are not in isotopic equilibrium with the host-rock, suggesting a meteoric-derived component infiltrated during faulting and recorded by clay minerals as a progressive change in fluid composition through time. These results represent an important step forward for fluid characterization in clay-rich fault zones, improving our understanding on how temperature and fluid source control the formation of authigenic minerals and fractionation processes in fault rocks.