Retrieving dissolved H2O content from micro-Raman spectroscopy on nanolitized silicic glasses: Application to volcanic products of the Paraná Magmatic Province, Brazil

Determining volatile content in silicate glasses has critical importance to evaluate the original water content of magmas and its evolution from storage to eruption. Here we present a Raman spectroscopic study devoted to defining an optimized analytical procedure for the determination of dissolved water content of nano-crystallized ancient silicic glasses, belonging to the Paraná Magmatic Province (PMP) volcanism in South Brazil (131 to 134 Ma). A set of anhydrous glasses was prepared under atmospheric pressure by melting and quenching of natural rocks. These glasses were later hydrated under high pressure in an Internally Heated Pressure Vessel (IHPV). Even after rapid quenching, most of the dry and hydrous glasses showed prominent Raman peaks at ca. 670-690 cm-1, which are associated with the presence of Timagnetite nanolites. Although large Raman peaks are observed, the nanolites are invisible to standard Scanning Electron Microscopy (SEM) due to their small size and not quantifiable by Xray diffraction (XRD) because of their low abundance. Available models and procedures to estimate water content in glasses are calibrated with crystal-free glasses and were proved inadequate for water measurement in nanolite-bearing glasses. Here we provide new calibration strategies to determine the water content in nanolite-bearing glasses. These strategies are based on the normalization of the intensity and area of the water band (3550 cm-1) to the individual vibrational bands of the silicate framework of the glass, at 350-500 cm-1 and 860-1240 cm-1. Our new calibrations were used to estimate the dissolved water content of glass matrixes of natural dacitic vitrophyres from the Caxias do Sul volcanic suite, whose Raman spectra show nanolite peaks of variable intensity. Our work constitutes a further step ahead toward the definition of complex strategies to determine the volatile content in natural multicomponent and multiphase glasses.