METHOD FOR DETERMINATION OF WATER CONTENT IN NATURAL RHYOLITE MELTS BY RAMAN SPECTROSCOPY AND ELECTRON MICROPROBE ANALYSISстатья
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Аннотация:The article presents the development of a method for determining the water content in highly polymerized rhyolite and rhyodacite glasses of melt inclusions. The method involves the simultaneous determination of water content by two methods: Electron Probe X-ray Microanalyzer and Raman spectroscopy for mutual ve-rification of the results. Raman was calibrated using set of etalon synthetic glasses of haplogranite composition (SGS) with a water content of 1.8 to 5.9 wt %. The resulting calibrating was verified on set of reference natural rhyolite obsidian (NRS) with a water content of 4.7 to 9.9 wt. %. Two ratios were chosen as the calibrated parameters: 1) the area of the water and hydroxyl bands in the range of 2900–3800 cm–1 to the area of the silicate band vibration (Si–O and Al–O) in the range of 850–1200 cm–1 (Aw/As), 2) the ratio of the intensities of the water band line in the 3550–3560 cm–1 region and the 480 cm–1 line in the silicate vibration region (I3550/I480). As a result, the calibration equations CH2O = 0.8458AwAs(σ) ± 0.17 wt % and CH2O = = 11.494I3550I480(σ) ± 0.21 wt. %. Comparison of water concentrations in standard glasses, determined by Raman spectroscopy using the ratios Aw/As and I3550/I480 and measured by X-ray microanalysis, showed good convergence. The electron probe X-ray microanalysis was used to determine the water content in glasses by recalculating the excess oxygen content. We tested the method using unheated glassy melt inclusions in minerals from dacitic and rhyolitic pumice from caldera eruption deposits of Okataina volcanic center (New Zealand), Kuril Lake (Kamchatka Peninsula), Vetrovoy Isthmus and Lion’s Past’ Bay (Iturup, Kuril Islands). The ability to use different ratios Aw/As or I3550/I480 minimizes the effect of overlapping host mineral lines on the low-frequency part of the spectrum of melt inclusions. We found that using the I3550/I480 ratio for melt inclusions in pyroxenes gives more reliable results than the Aw/As ratio. At the same time, Aw/As is preferable for inclusions in quartz and plagioclase. The water content in the studied inclusions varies from 0.5 to 7.9 wt. %. The proposed method can be used routinely for reliable measurement of the water content in glasses of melt inclusions with a size of at least 10 μm and a water content of at least 0.5 wt. %.