Fingerprinting kinetic isotope effects and diagenetic exchange reactions using fluid inclusion and dual-clumped isotope analysis

Geochemical analyses of carbonate minerals yield multiple parameters which can be used to estimate the temperature and water composition at which they formed. Analysis of fluid trapped in minerals is a potentially powerful tool to reconstruct paleotemperatures as well as diagenetic and hydrothermal processes, as these could represent the parent fluid. Internal fluids play important roles during the alteration of carbonate fossils, lowering energetic barriers associated with resetting of clumped isotopes, as well as mediating the transport of elements during diagenesis. Here, we explore the behavior of the ∆₄₇–∆₄₈ “dual-clumped” isotope thermometer during fluid-carbonate interaction and demonstrate that it is highly sensitive to the water/carbonate ratio, behaving as a linear system in “rock buffered” alteration, and as a decoupled system in water-dominated systems due to non-linear mixing effects in ∆₄₈. Dry heating experiments show that the extrapolated “heated” end-member is indistinguishable from the predicted ∆₄₇ and ∆₄₈ value expected for the experimental temperature. Furthermore, we evaluate two common laboratory sampling methods for their ability to thermally alter samples. We find that the temperature of the commonly used crushing cells used to vapourize water for fluid inclusion δ¹⁸O analyses is insufficient to cause fluid-carbonate oxygen isotope exchange, demonstrating its suitability for analyses of fluid inclusions in carbonates. We also find that belemnites sampled with a hand-drill yield significantly warmer paleotemperatures than those sampled with mortar and pestle. We conclude that thermally-driven internal fluid-carbonate exchange occurs indistinguishably from isotopic equilibrium, limited by the extent to which internal water and carbonate can react.