Water and magmas: insights about the water solution mechanisms in alkali silicate melts from infrared, Raman, and 29Si solid-state NMR spectroscopies

Abstract

Degassing of water during the ascent of hydrous magma in a volcanic edifice produces dramatic changes in the magma density and viscosity. This can profoundly affect the dynamics of volcanic eruptions. The water exsolution history, in turn, is driven by the water solubility and solution mechanisms in the silicate melt. Previous studies pointed to dissolved water in silicate glasses and melts existing as molecules (H 2 O mol species) and hydroxyl groups, OH. These latter OH groups commonly are considered bonded to Si 4+ but may form other bonds, such as with alkali or alkaline-earth cations, for instance. Those forms of bonding influence the structure of hydrous melts in different ways and, therefore, their properties. As a result, exsolution of water from magmas may have different eruptive consequences depending on the initial bonding mechanisms of the dissolved water. However, despite their importance, the solution mechanisms of water in silicate melts are not clear. In particular, how chemical composition of melts affects water solubility and solution mechanism is not well understood. In the present experimental study, components of such information are reported via determination of how water interacts with the cationic network of alkali (Li, Na, and K) silicate quenched melts. Results from 29 Si single-pulse magic-angle spinning nuclear magnetic resonance ( 29 Si SP MAS NMR), infrared, and Raman spectroscopies show that decreasing the ionic radius of alkali metal cation in silicate melts results in decreasing fraction of water dissolved as OH groups. The nature of OH bonding also changes as the alkali ionic radius changes. Therefore, as the speciation and bonding of water controls the degree of polymerization of melts, water will have different effects on the transport properties of silicate melts depending on their chemical composition. This conclusion, in turn, may affect volcanic phenomena related to the viscous relaxation of hydrous magmas, such as for instance the fragmentation process that occurs during explosive eruptions.

In situ study at high pressure and temperature of the environment of water in hydrous Na and Ca aluminosilicate melts and coexisting aqueous fluids

“…H 2 O mol reacts mostly with bridging oxygens to form T-OH bonds (T = Al, Si), so that water solution in silicate melts results in depolymerization of their structure [Scholze, 1960;Mysen et al, 1980;Stolper, 1982;Mysen and Virgo, 1986;Kohn et al, 1989;Zotov and Keppler, 1998;Mysen and Cody, 2005;Xue and Kanzaki, 2006;Mysen, 2007;Malfait and Xue, 2010]. In addition, the formation of M-OH species (with M a metal cation, e.g., Na or Ca) has been documented in silicate and aluminosilicate glasses [Xue and Kanzaki, 2004;Mysen and Cody, 2005;Mookherjee et al, 2008;Le Losq et al, 2015b]. Such species may play an important role in determining the effect of water on melt polymerization, because the reaction of water molecules with metal cations does not lead to melt depolymerization [Xue and Kanzaki, 2004].…”

Solubility of water in lunar basalt at low pH2O

“…In reaction (2), O ,௧ could be a bridging oxygen (i.e., an oxygen bridging two tetrahedrally coordinated, network-forming cations), a non-bridging oxygen in an aluminosilicate tetrahedron, or a "free" oxygen; and OH ௧ represents a hydroxyl group bonded to an aluminosilicate polymer (Behrens and Nowak 1997;Kohn 2000) or an alkali or alkaline-earth cation (Le Losq et al 2015). Combining reactions (1) and (2) ↔ 2OH ௧ , which is governed by an equilibrium constant ‫ܭ‬ (Stolper 1982a) that can be approximated by…”

29Si solid state NMR and Ti K-edge XAFS pre-edge spectroscopy reveal complex behavior of Ti in silicate melts

“…2) 29 Si NMR frequency as well as that observed for Si Q 4 (Al0) is most likely due to variation in Si-O-Si bond angle. For example, (Le Losq et al 2015) showed that lithium, sodium, and potassium tetrasilicate glasses, there is a systematic shift in Si-O-Si bond angles in Q4 species, where from Li + to K + the bond angles decreased.…”