Molecular water motional behaviour in volcanic glasses from magmatic and phreatomagmatic explosions

A magma is permeable to the diffusion of volatile species which are in solution in the melt phase. Among volatile components in magmas, water is the most abundant. Decompression, cooling and mineral crystallization cause water exsolution out of the melt, and in supersaturation conditions the pore spaces filled by water molecules may become the sites for the formation of hydration bubbles. Such bubbles may expand through the progressive drainage of water from the surrounding melt by diffusion. In the condition of non-equilibrium degassing, as in highly viscous magma or during a rapid ascent, continuous bubble nucleation occurs and the magma becomes increasingly fragmented by pore steam explosions, thus having the conditions for an explosive burst. Explosive hydrovolcanic eruptions may also occur as a consequence of the interaction of magma with external water, at the proper water/magma ratio and depending on the P,T conditions. During hydration, the reactions of aqueous solutions that enters the melt may allow ion-exchanges among cations and proton. In the present study, volcanic glasses from deposits related to magmatic and phreatomagmatic explosions of the Agnano-Monte Spina (AMS) eruption, in the Campi Flegrei caldera (CFc), have been investigated.