Unmixing fluid histories in polymineralic fault rocks using isotopic evolution and the PolyFrac tool: a case study from Lemnos Island, Greece

This study investigates the evolution of the isotopic composition of fluids in equilibrium with polymineralic fault rocks from the Kornos-Aghios Ioannis and Partenomythos fault zones on Lemnos Island (North Aegean Sea, Greece). Using a multi-method approach by combining stable isotope analysis (δD, δ 18 O), K-Ar geochronology on fault rocks, and X-ray diffraction of distinct grain-size fractions (from <0.1 to 10 μm) separated from clay-rich gouges, this study examines grain-size dependent isotopic fluid signatures preserved in clay-rich gouges. Coarse fractions record equilibrium water compositions with δD w between −96.5‰ and − 61.6‰ and δ 18 O w from −2.5‰ to 4.0‰, whereas fine fractions yield increased δD w (from −54.6‰ to −45.4‰) and δ 18 O w (2.0‰ to 3.7‰) values, approaching modern meteoric water signatures. These trends reflect progressive fluid-rock interaction within the fault zones, initially involving hot hydrothermal fluids ( T > 130 °C) in Aquitanian-Burdigalian times followed by subsequent re-equilibration at shallower crustal levels with meteoric waters (~60–65 °C). Authigenic illite-smectite in the <0.1 μm fraction of one fault gouge yields a ~ 16 Ma K-Ar age, marking the latest episode of recorded faulting and meteoric fluid infiltration. To resolve the complex isotopic signals arising from polymineralic clay-rich fault rocks, PolyFrac was developed as a quantitative spreadsheet-based tool. It integrates hydrogen and oxygen isotope data, X-ray diffraction-derived detrital and authigenic mineral proportions, and fluid temperature estimates to reconstruct fluid histories in polymineralic systems. • Fluid isotopic signatures can be constrained from polymineralic fault rocks. • Minerals of different grain-sizes are not always in isotopic equilibrium. • Different grain-size fractions can record fluid mixing and multiple fluid sources. • PolyFrac allows modelling of fluid isotopic evolution.