Simultaneous δ ² H and δ ¹⁸ O analyses of water inclusions in halite with off-axis integrated cavity output spectroscopy

Stable isotope compositions of ancient halite fluid inclusions have been recognized to be valuable tools for reconstructing past environments. Nevertheless, in order to better understand the genesis of halite deposits, it could be of great interest to combine both δ² H and δ¹⁸ O measurements of the water trapped as inclusions in the defects of the mineral lattice. We developed a method combining off-axis integrated cavity output spectroscopy (OA-ICOS) connected on line with a modified elemental analyzer (EA-OA-ICOS) to perform those measurements. The first step was to test the method with synthetic halite crystals precipitated in the laboratory from isotopically calibrated waters. Water isotopic signatures have been measured with conventional techniques, equilibration for δ¹⁸ O and chromium reduction for δ² H. Then, we modified and optimized a conventional EA to connect it online with an OA-ICOS instrument for H₂ O measurements. The technique is first evaluated for calibrated free water samples. The technique is also evaluated for salt matrix effect, accuracy, and linearity for both isotopic signatures. Then, the technique is used to measure simultaneously δ² H and δ¹⁸ O values of halite water inclusions precipitated from the evaporation experiments. Data generated with this new technique appeared to be comparable with those inferred from prior off-line technique studies. The advantages offered by the OA-ICOS technique are the simultaneous acquisition of both isotopic ratios and the substantial reduction of data acquisition time and sample aliquot size. Natural halite samples have been analyzed with this method. Natural halite samples as old as Precambrian have also been analyzed with this method.

Keywords: halite; hydrogen; laser spectroscopy; oxygen; stable isotopes.