Novel crushing technique for measuring δ18O and δ2H values of fluid inclusions (H2O) in quartz mineral veins using Cavity Ring-Down Spectroscopy

Fluid inclusions in mineralized fracture infillings (i.e. veins) could preserve information about sub-surface fluids like temperature and salinity. The isotopic composition of water in these fluid inclusions could provide direct evidence of the provenance of these fluids and indirectly about the temperatures and rate of vein formation. 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 δO and δH 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 stepwise crushing. For this study, we analysed δO and δH 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 understand the impact of different fluid inclusion assemblages (FIA) on δO and δH isotopic values. The results from single and stepwise mechanical crushing, together with interlaboratory comparisons, exhibit, reliable and consistent isotopic patterns across locations with high accuracy and precision (for δO 1SD<0.8 ‰; for δH 1SD< 1.5‰). The obtained data aligns with the existing Global Meteoric Water Line, providing evidence for the presence of meteoric fluids in the examined fold-and-thrust belt of the Variscan orogeny. These findings demonstrate that the CRDS approach can be successfully used in other geological research to investigate fluids pathways within the upper crust and the formation of minerals.