TC/EA-MS online determination of hydrogen isotope composition and water concentration in eclogitic garnet |
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| Authors: | Bing Gong Yong-Fei Zheng Ren-Xu Chen |
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| Institution: | (1) CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Science, University of Science and Technology of China, Hefei, 230026, China |
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| 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. |
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| Keywords: | Nominally anhydrous mineral Hydrous mineral Hydrogen isotopes Water concentration Analytical technique |
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