Method for simultaneous oxygen and hydrogen isotope analysis of water of crystallization in hydrated minerals

The isotopic composition of water in hydrated minerals, such as gypsum and jarosite, has numerous applications in studies of recent climate change, ore formation, and soil development. However, oxygen and hydrogen isotope analysis of water of crystallization is currently a complex procedure. Commonly used techniques involve offline extraction of water from hydrated minerals and subsequent isotope analysis. Such methods are time-consuming, require relatively large sample sizes, and the stepwise procedure has to be carried out with extreme caution to avoid erroneous results. We present a novel online method for the oxygen and hydrogen isotope analysis of water of crystallization in hydrous minerals. Gypsum (CaSO 4.2H 2O) samples, 2 mg in size, are reacted in a simply modified carbon reducing furnace connected to a continuous-flow mass spectrometer system. Analysis time is less than 10 min/sample. The precision (2 std dev mean) of our method for 2-mg gypsum (30 mumol of H 2O) samples is 0.3 per thousand for oxygen and less than 1.4 per thousand for hydrogen isotope measurements. For oxygen isotope analysis alone, samples as small as 0.2 mg of gypsum can be analyzed with a precision of 0.3 per thousand.