TC/EA-MS online determination of hydrogen isotope composition and water concentration in eclogitic garnet

Abstract

A continuous flow method, by a combination of thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), is presented for determination of both H isotope composition and H2O concentration of garnet from eclogite. Together with biotite NBS-30, the garnet was tested by preheating mineral grains at different temperatures. Preheating at 90°C for 12 h was found to be capable of eliminating adsorption water on sample surface. This results in constant δD values and total H2O contents for the garnet, with weighted means of −93 ± 2‰ and 522 ± 11 ppm (wt), respectively. The garnet that was preheated at 350°C for 4 h also gave constant δD values of −86 ± 6‰ and H2O contents of 281 ± 13 ppm (wt). The latter result for the H2O contents agrees with the H2O contents 271 ± 58 ppm (wt) measured by Fourier transform infrared spectroscopy for quantitative analysis of structural hydroxyl in the same garnet. Stepwise-heating TC/EA-MS analyses for the garnet show that the molecular H2O are depleted in D relative to the structural OH and has higher mobility than the structural OH. Therefore, the TC/EA-MS method can be used not only for quantitative determination of both H isotope composition and H2O concentration of hydrous and anhydrous minerals, but also for the concentration of structural hydroxyl after high-T dehydration.