Improved online hydrogen isotope analysis of halite aqueous inclusions

Abstract

We demonstrate an improved method based on continuous‐flow elemental analyser pyrolysis isotopic ratio mass spectrometry (CF‐EA‐PY‐IRMS) to measure the 2H/1H ratios of water trapped in halite crystals. Two challenges to overcome are the low hydrogen concentration of samples (10‐50 μmol H2·g−1) and the high chloride concentration released when reacting halite in an elemental analyser. We describe an optimization procedure for determining the 2H/1H ratio of this trapped water with an acceptable accuracy. This technique involves the use of a high‐temperature Cr reactor to quantitatively convert H2O into H2. The initial step was performed on halite crystals precipitated from a water reservoir where 2H/1H ratios were monitored from its initial stage until the end of evaporation. The 2H/1H isotopic analyses were automated online in continuous‐flow mode. Precision of the method was determined for those “synthetic” samples with hydrogen concentrations ranging from 0.2 to 0.5 wt%. 2H/1H isotopic ratios of evaporating waters bracket the compositions of water inclusions. The formation of fluid inclusions is not instantaneous and records the isotopic signature of the residual waters across a time range during which the isotopic values of the water still evolve. This property explains why the δ2HVSMOW standard deviation of ±5‰ (2σ) observed for 10‐mg aliquots of halite exceeds the instrumental error (about ±1.5‰ 2σ) determined on the basis of IAEA‐CH7, NBS 30, and NBS 22 references along with calibrated waters with and without added halite crystals. We also applied this method to Mesoproterozoic (1.4 Ga) and Neoproterozoic (0.8 Ga) halite samples with relatively low hydrogen concentrations (300‐1500 ppm). The measured δ2HVSMOW values for Precambrian waters range from −89‰ to −54‰. We propose that this technique offers a new perspective and great potential for palaeoenvironmental reconstructions based on the 2H/1H analyses of water trapped in halite.

Simultaneous δ2H and δ18O analyses of water inclusions in halite with off‐axis integrated cavity output spectroscopy

“…As it has been pointed out by Rigaudier et al19, 20 and Fourel et al, 24 the standard deviations for the halite fluid inclusion measurements are higher than the standard deviations generated by free water measure-…”

“…δ 2 H measurements were performed using the technique described by Fourel et al 24 We used a system developed by Elementar™ GmbH for specific applications with the Pyrocube™…”

“…[16][17][18][19][20] As far as 2 H/ 1 H measurements are concerned, previous methods were proposed by various authors for samples with high halogen contents. [21][22][23][24] Those methods involved the chromium reduction of water into H 2 gas prior to measurement; however, the δ 18 O was not accessible as oxygen reacts with Cr to produce Cr 2 O 3 .…”

“…As far as δ 18 O measurements are concerned, various pyrolysis techniques have been developed through time, involving the conversion of the oxygen to generate CO. [25][26][27][28][29][30][31][32] Some of those methods based on glassy carbon reactors allowed the simultaneous measurement of δ 2 H and δ 18 O of water samples but were unusable for samples containing high halogen contents as described by Gehre et al 23 and Fourel et al 24 Until recently, samples were prepared off-line prior to transfer reaction gases onto an isotope ratio mass spectrometer equipped with a dual inlet system. CO 2 after being equilibrated with H 2 O, then H 2 obtained by a reduction of H 2 O with Cr.…”

“…This crystallization proceeds once the evaporation of water allowed the oversaturation of sodium chloride. 19,24 The measurement technique we used is based on laser spectroscopy using off-axis integrated-cavity output spectroscopy (OA-ICOS) method. Coupling an elemental analyzer (EA) to an OA-ICOS laser spectroscopy instrument (EA-OA-ICOS) has already been investigated 33,34 for the determination of hydrogen isotopic composition.…”