Simultaneous Analysis of (17) O/(16) O, (18) O/(16) O and (2) H/(1) H of Gypsum Hydration Water by Cavity Ring-down Laser Spectroscopy

Rationale: The recent development of cavity ring-down laser spectroscopy (CRDS) instruments capable of measuring (17) O-excess in water has created new opportunities for studying the hydrologic cycle. Here we apply this new method to studying the triple oxygen ((17) O/(16) O, (18) O/(16) O) and hydrogen ((2) H/(1) H) isotope ratios of gypsum hydration water (GHW), which can provide information about the conditions under which the mineral formed and subsequent post-depositional interaction with other fluids.

Methods: We developed a semi-automated procedure for extracting GHW by slowly heating the sample to 400°C in vacuo and cryogenically trapping the evolved water. The isotopic composition (δ(17) O, δ(18) O and δ(2) H values) of the GHW is subsequently measured by CRDS. The extraction apparatus allows the dehydration of five samples and one standard simultaneously, thereby increasing the long-term precision and sample throughput compared with previous methods. The apparatus is also useful for distilling brines prior to isotopic analysis. A direct comparison is made between results of (17) O-excess in GHW obtained by CRDS and fluorination followed by isotope ratio mass spectrometry (IRMS) of O2 .

Results: The long-term analytical precision of our method of extraction and isotopic analysis of GHW by CRDS is ±0.07‰ for δ(17) O values, ±0.13‰ for δ(18) O values and ±0.49‰ for δ(2) H values (all ±1SD), and ±1.1‰ and ±8 per meg for the deuterium-excess and (17) O-excess, respectively. Accurate measurement of the (17) O-excess values of GHW, of both synthetic and natural samples, requires the use of a micro-combustion module (MCM). This accessory removes contaminants (VOCs, H2 S, etc.) from the water vapour stream that interfere with the wavelengths used for spectroscopic measurement of water isotopologues. CRDS/MCM and IRMS methods yield similar isotopic results for the analysis of both synthetic and natural gypsum samples within analytical error of the two methods.

Conclusions: We demonstrate that precise and simultaneous isotopic measurements of δ(17) O, δ(18) O and δ(2) H values, and the derived deuterium-excess and (17) O-excess, can be obtained from GHW and brines using a new extraction apparatus and subsequent measurement by CRDS. This method provides new opportunities for the application of water isotope tracers in hydrologic and paleoclimatologic research.