Protocols for bulk off-line fluid inclusion extraction for the analysis of δ13C-CH4 and δ13C-CO2 using a cavity ring-down spectroscopy (CRDS) analyser

Fluid inclusions are a window into deep geological fluids, providing unique access to their nature and composition. The isotopic composition of CO 2 and CH 4 hosted in fluid inclusions is a powerful proxy to assess the origin and transformation of deep geological fluids, giving insights into carbon sources, fluxes, and degassing in a wide variety of geodynamic settings. Over the last 5 decades, techniques have been developed to extract fluid inclusions from their host minerals and measure their bulk composition. These techniques are often challenged by analytical artifacts including high blank levels of CO 2 and CH 4 , fluid re-speciation, gas adsorption, and diffusion. Since these processes may alter the pristine composition of gases liberated from fluid inclusions, rigorous protocols are needed in order to evaluate the isotopic integrity of the extracted volatile species. In this study, we introduce new protocols for bulk off-line fluid inclusion extraction for the analysis of δ 13 C-CH 4 and δ 13 C-CO 2 using a Cavity Ring-Down Spectroscopy (CRDS) analyser (Picarro G2201-i). Two mechanical fluid extraction techniques are compared: ball milling in ZrO 2 jars and sample crushing in a stainless steel sealed tube under a hydraulic press. Blanks and isotopically labelled tests with the ball milling technique suggest that rotation speed, grinding stock filling degree and filling type alter the CH 4 and CO 2 concentrations and isotopic compositions measured by the CRDS analyser. In contrast, the crushing technique does not generate measurable quantities of blank CH 4 and CO 2 . The protocols presented in this study allow to extract, detect, and analyse δ 13 C of CH 4 and CO 2 for concentrations above 10 and 1,000 ppm respectively. Interlaboratory experiments allowed to replicate previously measured δ 13 C-CH 4 values in natural fluid inclusions within 1‰ with both extraction techniques. This study highlights the potential of combining simple bulk off-line fluid inclusion extraction techniques with a CRDS analyser for δ 13 C analysis of CO 2 and CH 4 without gas separation being required.