~(27)Al MAS NMR spectroscopy on volcanic glasses from the Neapolitan Yellow Tuff (Southern Italy): inference on Al coordination and reactivity of aluminosilicate melts during phreatomagmatic explosions

摘要

Trachytic and alkali-trachytic volcanic glasses from the deposits of the Neapolitan Yellow Tuff eruption, in the Campi Flegrei Caldera (Southern Italy), were studied by back-scattered electron microscopy, electron microprobe analyses and ~(27)Al magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Both trachytic and alkali-trachytic glasses have homogeneous chemical composition in terms of the major element oxides The ~(27)Al MAS NMR experiments indicate that trachytic glasses are characterized by ~(27)Al chemical shift in the frequency region of tetrahedral Al, and do not reveal any detectable difference in terms of Al site distribution between glasses from phreatoplinian and more magmatic explosions. Some of the alkali-trachytic glass fragments show microscopic evidence of corrosion, with less than 1 mum holes distributed on the glass surface, outside and within vesicles. The ~(27)Al MAS NMR spectrum on the alkali-trachytic glass shows the contribution of an ~([6])Al resonance, in addition to the ~([4])Al resonance. A possible explanation is that the transformation of Al from four- to six-fold occurred during the penetration of aqueous fluids into the glass/melt structure, yielding aluminum hydrolysis by the reaction of water with Al-O bonds and the formation of Al species in octahedral coordination.