Spectroscopic analysis (FTIR, Raman) of water in mafic and intermediate glasses and glass inclusions

Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H2O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH and molecular H2O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H2O content (H2OT=OH-+H2Om) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm-1 band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H2O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H2OT band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H2O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H2O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm-1. We provide new FTIR absorptivity coefficients (e3550) for basalt (62.80 ±0.8 L mol-1 cm-1) and basanite (43.96 ± 0.6 L mol-1 cm-1). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (e3550) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H2O in silicate glasses when the matrix effects are properly considered.