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.