Micro-Raman water calibration in ultrapotassic silicate glasses: Application to phono-tephrites and K-foidites of Alban Hills

This study investigates the potential use of micro-Raman spectroscopy for the quantification of water in ultrapotassic silicate glasses. A calibration was developed using experimental phono-tephritic glasses with water content ranging from ~ 1 to ~ 3 wt%. The calibration curve showed a typical direct proportionality between water content and the ratio of high- (3100–3750 cm −1 ) and low-wavenumber (100–1500 cm −1 ) spectral regions, with a linear fit coefficient m = 1.74. The comparison with the m coefficients available in literature for other silicate compositions showed a deviation of our composition as a function of some major oxides such as FeO, TiO 2 and K 2 O, highlighting the possible influence of the polymerization degree (NBO/T: non-bridging oxygens per tetrahedron) on m coefficient. In this respect, we observed a linear relationship between m coefficient and NBO/T and a positive correlation between the area underneath the silicate region (100–1500 cm −1 ) and NBO/T for the phono-tephrite of this study and for other compositions spanning from basalts to phonolite and rhyolites available in literature. For ultrapotassic natural and experimental glasses characterized by the presence of CO 2 , documented by the carbonate peak at 1062–1092 cm −1 , it has been possible to extrapolate the CO 2 content by using the model of Morizet et al. (2013) obtaining values of ~ 1.1 ± 0.3 and ~ 1.7 ± 0.2 wt%, respectively. The obtained m coefficient was applied to estimate water content of natural phono-tephritic glasses belonging to the Colli Albani Volcanic District. Moreover, we estimated water content also for some natural K-foiditic glasses from the same volcanic district. Since the m coefficient results to be strongly dependent on the chemical composition of the sample of interest, the coefficient estimated for the phono-tephrites of this study could result in significant overestimation or underestimation of the water content of the Colli Albani Volcanic District K-foiditic natural samples. Thus, we extrapolated the m coefficient for the K-foiditic samples by means of an equation obtained in this study as function of the polymerization degree (NBO/T). • Micro-Raman water calibration in ultrapotassic silicate glasses • Extension of the application of Raman protocols to phono-tephritic and K-foiditic compositions • Influence of the polymerization degree (NBO/T) on m coefficient • Constraints on H 2 O and CO 2 of interstitial glasses and melt inclusions in the Colli Albani Volcanic District pre-eruptive system