1H MAS NMR characterization of hydrous species in volcanic glasses from the Phlegrean Volcanic District (South Italy)

The state of water in volcanic glasses separated from pumices and shards of alkali-trachyte composition from the Agnano-Monte Spina eruption (4.1 ka) in the Phlegrean Volcanic District (South Italy) has been investigated by 1H Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy. The surface analysis of pumice glasses indicates that they correspond to a highly vesiculated, disordered foam characterized by several generations of vesicles both of sub-spherical and strained shape, suggesting a sudden increase of the melt structural relaxation time while bubbles were still deforming. Shards, characterized by low vesicularity, are typically composed of angular or sub-rounded clasts of glass, the latter showing, in some cases, evidence of surface pitting. The 1H resonance patterns of shards can be fitted in terms of isolated H2O molecules and OH groups. The shards with the highest water content and rounded shape, supposed to be the product of phreatomagmatic explosions, are characterized by the higher H2O/OH molar ratio (2.6), attributed to the interaction of the melt with external water. In the angular shard, previously interpreted as due to magmatic fragmentation, the molar H2O/OH ratio is significantly lower (0.7).