Hydrogen and oxygen stable isotope signatures of goethite hydration waters by thermogravimetry-enabled laser spectroscopy

The hydrogen and oxygen stable isotope composition (δ2H and δ18O values) of mineral hydration waters can give information on the environment of mineral formation. Here we present and validate an approach for the stable isotope analysis of mineral hydration waters based on coupling a thermogravimetric analyzer with a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. TGA-IRIS generates δ2H and δ18O values of liquid water samples with precision for δ2H of ± 1.2‰, and for δ18O of ± 0.17‰. For hydration waters in goethite, precision for δ2H ranges from ± 0.3‰ to 1.6‰, and for δ18O ranges from ± 0.17‰ to 0.27‰. The ability of TGA-IRIS to generate detailed water yield data and δ2H and δ18O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. TGA-IRIS analyses of hydrogen isotopes in goethite yields δ2H values that reflect the hydrogen of the OH− phase in the mineral and are comparable to that made by IRMS and found in the literature. In contrast, δ18O values on goethite reflect the oxygen in OH− groups bound to Fe (Fe-OH group), and not the oxygen bound only to Fe (Fe-O group) in the mineral crystal lattice, and may not be comparable to literature δ18O values made by IRMS that reflect the total O in the mineral. TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in goethite, which may allow the mineral to be used as a single mineral geothermometer. TGA-IRIS measurements of hydration waters are likely to open new avenues and possibilities for research on hydrated minerals.