New crushing approach for δ 18 O and δ 2 H measurements of fluid inclusion water in quartz mineral veins using Cavity-Ring Down Spectroscopy

Abstract

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-inclusion have been mainly derived from carbonate veins and other precipitates like speleothems. The aim of this study is to analyse the δ O and δ H isotopic compositions of water fluid inclusions of quartz vein using a Cavity Ring-Down Spectroscopy (CRDS) analyser in combination with a moisturized nitrogen background and mechanical stepwise crushing. For this study, we analysed δ O and δ H values of fluid inclusions quartz veins from three north-western European locations formed during the Variscan orogeny. Prior to crushing, the fluid-inclusions rich fraction was separated from the pure quartz fraction from other mineral phases and host rock by using conventional heavy liquid mineral and magnet separation technique to avoid from adsorbed atmospheric water contaminant or “bleeding” effect from intergranular or primary permeability water. Raman spectrometry detected some rare occurrences of hydrocarbon, methane and nitrogen. The samples were sequentially crushed to minimize the influence of (pseudo)secondary on the isotopic signal of the primary fluid-inclusion. 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 δ O 1SD<0.8‰; for δ H 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.