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Abstract:
Five distinct analytical techniques were compared for the quantification of the H2O (0·1−7·4 wt%) and D2O (up to 5·5 wt% D2O) contents in rhyolitic glasses. The hydrogen concentration in obsidians was measured using nuclear reaction analysis (NRA) and converted into H2O concentration. The bulk water content (H2O+D2O) of synthesised glasses was determined using thermal extraction in conjunction with Karl–Fischer titration (KFT). Unextracted H2O was measured by infrared (IR) spectroscopy after remelting the KFT products at high pressure. These well characterised glasses were subsequently used to calibrate secondary ion mass spectrometry (SIMS), infrared and evolved gas analysis (EGA). The calibrations of SIMS for H2O and D2O show a significant matrix effect at H2O and D2O concentrations larger than ~3 mol/l with the relative sensitivity factor (RSF) decreasing with bulk water content. Molar absorption coefficients for the OH stretching vibration band at 3570 cm−1 and the OD stretching vibration band at 2650 cm−1 in the infrared absorption spectra were reinvestigated. In the range 0·1−6 wt% the H2O content of glasses could be reproduced within analytical error using a constant linear molar absorption coefficient. However, it appears that the absorption coefficients for the 3570 cm−1 band as well as for the 2650 cm−1 band both decrease systematically with concentration of dissolved H2O and D2O (ε 3570=80 (±1) − 1·36(±0·23)CH2Ot and ε 2650=61·2(±1) − 2·52(± 0·30)CD2Ot ). EGA allows quantification of the H2O content as well as the study of dehydration kinetics. It is shown that H2O is released under vacuum already at room temperature from glasses containing >7 wt% H2O.
