Water speciation in silicate melts investigated by Raman spectroscopy: Implication for volcanic processes

In addition to temperature, pressure and main chemical components, volatiles exert a strong influence on the physical properties of magmas. In particular, water plays a fundamental role in the dynamics and evolution of magmas in the deep interior and during volcano eruptions. However, water speciation in silicate melts is not fully understood. Infrared and NMR spectroscopy had provided some valuable informations about the H2O/OH- speciation. Nevertheless, some issues still remain unsolved about the OH-/H2O linkage to the silicate network. Raman spectroscopy already allows quantifying the proportion of water dissolved in an aluminosilicate melt. Raman spectra are composed of i) a low wave number region that corresponds to vibrations of the silicate network (0-1500 cm-1), and ii) a high wave-number region, which corresponds to the OH- stretching vibrations and H2O molecular vibrations (3100-3750 cm-1). We have performed a first set of in situ experiments using a micro-furnace at ambient atmosphere. An evolution of the high wave-number region in function of the time and temperature is observed. New Raman peaks can be distinguished, particularly near 3650-3700 cm-1. In this communication, we will present and discuss these observations between water and the silicate network in melts. Raman spectroscopy provides valuable informations to build a general model of water speciation, distinguishing the OH bonds between tetrahedral species and network modifiers for example. Results from this new model confirmed the amphoteric behavior of the water previously reported from polymeric modelling for instance, and opens new ways to study water in melts.