Precise Measurement of the Hydrogen Isotope Composition of Phyllosilicates by Continuous Flow Off-Axis Integrated Cavity Output Spectroscopy

New methodology is presented for analyzing hydrogen isotope ratios (D/H) in phyllosilicate minerals by laser absorption spectroscopy. D/H measurements were carried out using an off-axis integrated cavity output spectroscopy (OA-ICOS) instrument operated in a continuous flow configuration. Water was extracted from minerals in a high temperature quartz column and advanced to the analyzer in a dry air carrier gas stream. We report the first D/H measurements by a laser system for serpentine, muscovite, sericite, talc, and biotite. We also measure kaolinite, gypsum, and small volumes of water. Materials, excluding biotite, were calibrated to within 1.5‰ of IRMS-measured δD_VSMOW values, with an average precision of 1.1‰. Biotite δD measurements were up to 10‰ more positive than established IRMS values, due to partial reduction of evolved waters by Fe in the high temperature column. We provide recommendations for overcoming redox interference for measurements of biotite, and other ferrous materials, by OA-ICOS. Rapid, precise, and accurate analyses were carried out on water volumes as low as 0.25 μL extracted from minerals. With the exception of talc, the time required for thermal dewatering and measurement is 140 s, which translates into a throughput of up to 6 mineral samples per hour, including replicates. By demonstrating high precision, rapid throughput, low cost, and ease of operation, we provide a tool that should enable researchers at institutions with limited funding to routinely measure D/H in hydrous minerals. The protocols presented herein should also be useful to commercial users seeking to produce high density isotope data sets relevant to exploration of hydrothermal ore deposits and geothermal fields.