Knowledge of the carbon and oxygen isotope composition of CO2-rich inclusions in crust- or mantle-derived minerals is crucial for a more quantitative understanding of cycling of volatiles between Earth's deep and surficial reservoirs. In this study, we present a crushing method for analysis of combined carbon and oxygen stable isotope compositions of small (≤ nmole) amounts of CO2 in fluid and melt inclusions. During crushing, the released CO2 was cryo-trapped, and subsequently introduced into a Gas Bench II coupled to a Deltaplus Isotope Ratio Mass Spectrometry (IRMS). Reference standard CO2 introduced in the crusher chamber was analysed to a precision better than ~0.3 and ~ 0.4‰ (1σ) for carbon and oxygen, respectively. Analysis of previously analysed samples validates that the crushing technique can be applied to CO2-rich fluid inclusions in quartz samples as small as 0.2 to 5 mg. CO2 –rich quartz samples were analysed from the UHT-Bakhuis Granulite Belt (Suriname) and Val Nalps (Switzerland) as well as forsterite-rich olivine-hosted melt inclusions from Vulsini volcanic district (Italy). The carbon isotope compositions obtained from the quartz samples allow discrimination between different sources within the crust and mantle. In contrast, the oxygen isotopic signatures of the fluid inclusions containing water appear affected by isotopic exchange between CO2 and H2O in the inclusions, where for the free-water fluid inclusions can reflect the pristine δ18O value. Crushing of olivine grains with CO2 melt-inclusions displayed instantaneous adsorption of the released CO2, which leads to low CO2 yields, and considerable oxygen and carbon isotope fractionation, hampering reliable isotopic determinations. We conclude that the δ13C and δ18O values from CO2-rich inclusions can be used as an indicator of the CO2 source and to evaluate mantle CO2 outgassing and release from storages within Earth's crust. The method presented can be readily applied to determine carbon and oxygen isotopic compositions of CO2 (4.5−10 mol) in small amounts of fluid-inclusion bearing quartz.
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The authors are grateful to Dr. Volker Lüders for providing the quartz samples from Ghana and Argentina and for the unpublished data. The author would like to thank Dr. Emond de Roever for providing the quartz samples from the Bakhuis UHT granulite belt and for sharing information about the samples. We thanks also Dr. Freek Ariese from the LaserLab at the Vrije Universiteit for the access to the Raman spectroscopy. The constructive reviews of two anonymous referees were highly appreciated and helped considerably to improve the paper. The project was funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 759563 ).