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