Water Concentrations and Distribution in Evolving Melts as Suggested by Melt Inclusions and Matrix Glasses

Water concentrations in glasses and feldspar crystals from volcanic centers associated with the Torfajokull central volcano of southern Iceland were determined using Fourier transform infrared spectroscopy (FTIR). FTIR spectra were collected using synchrotron-generated radiation at the National Synchrotron Light Source at Brookhaven National Laboratory. The bright synchrotron-generated IR beam permits the analysis of spots as small as 10 mm and collection of maps of water concentration across areas of interest. The goal of this study was to explore the distribution of water between glasses and coexisting feldspar crystals in a range of mingling and non-mingling magmas in a large bimodal volcanic complex. With distance from phenocryst borders, water concentrations of matrix glass generally decrease, but some also subsequently increase, from initial relative highs closest to crystal edges. Water concentrations of matrix glasses vary from a high of ~ 1 wt % in clean glass to a low of ~ 0.5 wt % in more microcrystalline glass, and concentrations grade between adjacent glasses. Differences in water concentration between the glasses may be a result of crystallinity, glass composition, or both. Average water concentrations in melt inclusions vary slightly between different feldspar crystals and throughout single feldspar crystals. Averages of water concentration in melt inclusions in single phenocrysts range from 0.24 to 0.77 wt %, vary within phenocrysts by as much as ~ 0.7 wt %, and are generally higher in inclusions in hornblende. Variations in water concentration amongst melt inclusions and matrix glasses may reflect water behavior during melt evolution as crystallization, and mingling if present, proceed. The determination of water concentration in conjunction with composition in these glasses and crystallizing phases may permit a quantitative understanding of water distribution throughout the crystallization history of complex melts.