Effect of iron and nanolites on Raman spectra of volcanic glasses: A reassessment of existing strategies to estimate the water content

Abstract

The effect of iron content and iron nanolites on Raman spectra of hydrous geologically-relevant glasses is presented. Current procedures to estimate the water content using Raman spectra were tested to explore potential effects of iron content, its oxidation state, and nanolites on models' reliability. A chemical interval spanning from basalt to rhyolite, including alkali- and iron-rich compositions, with water content up to 5.6 wt% was investigated using two spectrometers. When considering nanolite-free samples, the area of the band at 3550 cm−1 linearly correlates with the sample water content regardless of chemical composition. Using this approach, data were reproduced with a root-mean-square error (RMSE) of ~ 0.15 wt%. Depending on the sample chemistry, water content, and acquisition conditions the laser-induced sample oxidation led to underestimating the water content up to ~ 90% with a long acquisition time (26 min). Normalising the water band region to the silicate band region minimises such a limitation. The area ratio between these bands linearly correlates with the water content and the use of different baseline procedures does not remove the dependence of such a correlation by the iron content and its oxidation state. With this procedure, data were reproduced with a RMSE of ~ 0.16 wt%. For both approaches, the presence of iron nanolites may result in underestimating the water content.