Re-Mobile Magma Chambers

1)There is evidence for some active volcanoes to have explosive eruptions not alerted by clear geophysical indicators as deformation, gravity changes and seismicity due to the mass transport of fresh magma. 2) Several active central volcanoes show high rigidity anomalies along the main eruptive axis indicating the lack of molten masses within the conduit and shallow chamber(s). 3) Recent data on age determination of phenocrysts from very well constrained explosive eruptions testify an older crystallisation age coinciding with the ones from preceeding major eruption(s). From these evidences altogether, it is, therefore, possible that a still hot, but rheologically immobile magma chamber melts again gaining eruptive capabilities. For this process to occur, it is not necessary an increase of temperature (energetically very expensive because of providing of the latent heat of fusion) in the solid body, but the diffusion of a hot gas (water) sparging from below that lowers the melting temperature of the system. A relevant consequence for the volcanic hazard assessment is these re-juvenated melts will be fully hydrated and therefore can vesiculate on decompression and erupt explosively. To test the dynamics of this event, few exploratory hydration and vesiculation experiments have been carried out on drilled cores from a previously dehydrated glass, as part of a trachytic sample from the basal spatter-flow unit of Campanian Ignimbrite. The hydration experiments have been carried out at P of 100 MPa and T of 950 and 1000 (°C) for a time ranging from 1 to 72 h. The decompression experiments have been performed at 970°C from 100 to 50 MPa at 10 and 5 MPa/min decompression rates and then quenched isobarically. All the experimental products gave clear glass devoid of crystals. The full hydration level of the whole experimental charge which remelted completely, was reached within 24 h. The 10 MPa/min decompression experiments gave a homogeneous nucleation of vesicles up to 5 micron indicating a critical supersaturation pressure less than 50 MPa. Future experiments on bubbles growth after decompression will be performed on trachybasaltic composition in order to estimate the relative efficiency of water diffusion in the hydrating glass and in the melt.