Restricted internal oxygen isotope exchange in calcite veins: Constraints from fluid inclusion and clumped isotope-derived temperatures

Abstract

The distribution of oxygen isotopes between calcite and fluid inclusions has demonstrated utility for reconstructing near-surface calcite precipitation temperatures. For calcite that formed at depth, however, the resilience of this paleothermometer to diagenetic oxygen isotope alteration is poorly constrained. Clumped isotopes also document calcite precipitation temperatures and are similarly vulnerable to diagenetic alteration. Post-entrapment isotope exchange between calcite and fluid-inclusions could alter the calcite-fluid oxygen isotope distribution (αc-fi), as well as the clumped isotope composition (Δ47) of calcite, and therefore these two seemingly independent paleo-thermometers are potentially linked via the same alteration process. Using closed-system batch fractionation equations, we have modelled various scenarios of oxygen isotope exchange between water and host-rock during burial, as well as internal oxygen isotope exchange between calcite and fluid inclusions during exhumation. Assuming both paleo-thermometers record concordant temperatures at the time of vein formation, our models predict that if a fraction of calcite is available for isotopic interaction with fluid inclusions, the fluid inclusion and clumped isotope-derived paleothermometers yield discrepant temperature estimates after exhumation. We show that the fluid inclusion thermometer is more sensitive to isotopic alteration than the clumped isotope thermometer and that the mass balance of oxygen between calcite and fluid inclusions determines the sensitivity of both paleothermometers as well as the vulnerability of fluid inclusions (δ18Ofi) to diagenetic overprinting. We applied coupled clumped isotope and fluid inclusion measurements on calcite veins from the External Albanides (Albania), which were formed at depth and subsequently exhumed, in order to compare natural samples to our isotope exchange model. These veins show strongly discrepant calcite-water equilibrium temperatures and clumped isotope temperatures, suggesting the fraction of calcite available for isotope exchange with internal fluids may indeed be a key parameter of diagenetic alteration during exhumation. Even though the clumped isotope temperatures of our samples appear to be insensitive towards internal oxygen isotope exchange, our model predicts that at low burial temperatures, the carbonate clumped isotope thermometer may be susceptible to alteration by diagenetic isotope exchange with fluid inclusions under certain conditions.