The geochemical and ultrastructural properties of thermally altered skeletal carbonate are expected to be compromised to varying degrees by disequilibrium processes between solids and the ambient aqueous fluids. When assessing the alteration history of carbonates, it is important to apply models that quantitatively describe these diagenetic processes on multiple geochemical systems, such that they can be identified in natural samples. Carbonate clumped isotope analysis provides a unique tool for validating such models and can be combined with other geochemical tools/proxies to more comprehensively describe the processes and products. Here, we have analyzed bivalve shells that have undergone hydrothermal alteration (experimental diagenesis) in high water/rock ratio experiments at 130 and 160{degree sign}C, demonstrating that non-linear changes in ∆47 and ∆48 values can be attributed to heterogeneous replacement of precursor carbonates. Importantly, this model predicts decoupled ∆47 and ∆48 values, despite all reactions occurring at clumped isotope equilibrium with respect to the experimental temperature. We demonstrate that the rapid, thermally induced re-equilibration occurs in a “closed system” with minimal exchange with the ambient fluid, similar to the results of heating experiments conducted without an extraneous fluid. Later stages of alteration occur in an “open system” wherein internal fluid is exchanged with the external fluid at a similar rate to recrystallization and neomorphism. In these experiments, some oxygen from the original inorganic-organic composite-biomineral is inherited, indicating restrictions on the availability of fluid oxygen. Our experiments and models validate a novel application for dual-clumped isotopes for reconstructing hydrothermal temperatures and fluid δ18O compositions.

Fluid inclusions in mineralized fracture infillings (i.e. veins) could preserve information about subsurface fluids like temperature and salinity. The isotopic composition of water in these fluid inclusions could provide direct evidence of the provenance of these mineral forming fluids. So far, the isotope compositions of fluid inclusions have been mainly derived from carbonate veins and other precipitates, like speleothems. The aim of this study is to analyse the δ18O and δ2H isotopic compositions of aqueous fluid inclusions of quartz veins using a Cavity Ring-Down Spectroscopy (CRDS) analyser in combination with a moisturized nitrogen background and mechanical crusher. For this study, we analysed δ18O and δ2H values of fluid inclusions in quartz veins from three north-western European locations formed during the Variscan orogeny. Prior to crushing, the fluid-rich quartz fraction was separated from the pure quartz fraction, from other mineral phases and host rock by using conventional heavy liquids and magnet separation. Raman spectrometry detected some rare occurrences of hydrocarbon, methane and nitrogen in the fluid inclusions. The samples were sequentially crushed to elucidate the potential impact of different fluid inclusion assemblages (FIA) on the δ18O and δ2H values. The results from single and sequential mechanical crushing, together with interlaboratory comparisons, exhibit reliable and consistent isotopic patterns across locations with high precision (for δ18O: 1σ SD < 0.8 ‰; for δ2H: 1σ SD < 1.5 ‰). The obtained data trends three different clusters for three study zones, providing evidence for the presence of meteoric-derived fluids in the fold-and-thrust belt of the Variscan orogeny. These findings demonstrate that the CRDS approach can be successfully applied to quartz minerals, investigating fluid pathways within the upper crust and the formation of these secondary minerals.