The structure of dense and vesiculated volcanic glass fragments from the Astroni tephra (Phlegraean fields, Italy) explored by spectroscopic techniques: implications on bubble expansion and dynamics of magma ascent

The structure of coexisting dense and vesiculated glasses in the tephra of Astroni, a volcano (age range: 4.1-3.8 ka before present) located in the Campanian Region, South Italy, was investigated by means of Si-29, Al-27 and H-1 solidstate nuclear magnetic resonance, electron spin resonance and Mossbauer spectroscopy. The dense glass is characterized by incomplete bubble expansion. The vesiculated glass shows the occurrence of smaller bubbles expanding into larger cavities of sub-spherical shape, and fractures around expanding bubbles. The spectroscopic investigations indicate a similar glass structure with most of the silicon in Q(4) structural units and possible Al substitution in the next nearest tetrahedra. The structural similarity suggests that the melts/glasses probably developed the different vesicularity during different ascent processes from depth to the surface which characterized this phase of the Astroni eruption.