Origin of silicic water-poor obsidian and non-explosive activity during the Mt Pilato-Rocche Rosse eruptions, Italy

2017-02-14T01:03:43Z (GMT) by Cabrera, Agustin Pablo
Rhyolite eruptions often begin explosively and change into effusion of lava and the glasses of both contrasting eruptive deposits commonly have vesicle and water content heterogeneously distributed. The distribution of bubbles in magma and water content in silicic melt is thought to play a key role on the style and flow dynamics of volcanic eruptions, however the source of these heterogeneities remain unclear. The Mt. Pilato-Rocche Rosse sequences on the north-eastern corner of Lipari Island, Italy, represent an explosive-effusive eruptive event. The exceptionally preserved glasses on these deposits provide a rare opportunity to study processes associated with dewatering and flow dynamics of melts thought to be formed during the same explosive and effusive volcanic cycle. The Rocche Rosse lava is texturally heterogeneous, yet compositionally homogeneous, except in terms of water-content. The study reveals that the flow evolved dynamically during surface emplacement between obsidian and pumiceous lava locally, foaming into pumice, and degassing into obsidian as a function of variations in water content and dynamic pressures. Thus, the final and current textural configuration observed in silicic flows does not reflect a simple history of output from the conduit, but a complex history of dehydration of the magma in the conduit and during flow. This study shows for the first time evidence of water loss associated with melt fracturing during rhyolite explosive eruptions. Results indicate that dynamic pressure (P) and temperature (T) variations along faults probably play an important role during magma autobrecciation and that rising rhyolite magma can indeed degas while fracturing, prior to its welding and forming dense obsidian magma. It is proposed that pervasive fracturing is a potential degassing mechanism to defuse rhyolite explosive eruptions. Further studies on the deposits that from the Mt Pilato-Rocche Rosse sequences reveal that magma fracturing with consequent water loss and healing of the melt occurred repeatedly and pervasively, both during the explosive and the effusive silicic eruptions. Findings presented here indicate that increased permeability, due to syn-eruptive magma fracturing, leads to progressive magma dewatering and explains the formation of dense water-poor obsidian and the temporal evolution from explosive to effusive rhyolite eruptions. Analysis on the distribution of water content on the glass of the Rocche Rosse lava shows that glass from hinge zones at or near the surface of folded lava records water content values that imply significant viscosity variations over distances at the scale of 1mm. Results also reveal that locally the values of water content are notably less than those predicted for surface pressures and, therefore, require sub-atmospheric pressures. It is proposed that these values are associated with a complex and varied history of dynamic pressures resulting from divergent and convergent flow of viscous magma. This process would be common to any flowing lava, and helps explain well-documented local variations in water content of silicic glass. The significant variation in viscosity resulting from water content variations would also trigger further flow perturbations that might reinforce in a positive feedback loop, further water fluctuations through variations in dynamic pressure, thus explaining the significant water-content variability.