Vesiculation in magmas from Stromboli and implications for normal Strombolian activity and paroxysmal explosions in basaltic systems

Abstract

We performed a series of X-ray tomographic experiments and lattice Boltzmann permeability simulations on pyroclastic products from explosive activity at Stromboli between December 2004 and May 2006. We reconstructed the 3-D textures of vesicles to investigate the relationship between the nature of vesiculation in the erupted products and the dynamics of gas transport in the shallow conduit in order to derive implications for the eruptive behaviour of basaltic volcanoes. Scoriae from normal Strombolian explosions display remarkably consistent vesicle volume distributions fit by power-laws with an exponent of 1 (±0.2). We ascribe the origin of such distributions to the combined effect of coalescence and continuous nucleation events in the steady-state, shallow magma system that supplies normal Strombolian activity. Volume distributions and textures of vesicles in pumice clasts from the 5 April 2003 and 15 March 2007 paroxysmal activity are markedly different from those in the scoriae. Besides a powerlaw function with a higher exponent, portions of these distributions can be also fit by an exponential function, suggesting the attempt of the system to reach near-equilibrium conditions. The investigated pumice clasts also lack the large, connecting vesicles responsible for the development of degassing pathways in the Stromboli magma that erupts the scoriae. This testifies to a decreased degassing efficiency of the magma associated with paroxysmal explosions and potential overpressure build-up at depth. By comparison with degassing experiments on basaltic melts, we derive a time constraint on the order of minutes to hours for the incubation of paroxysms at Stromboli.