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

Abstract

[1] 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 behavior 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 power law 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 buildup 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.

Lava effusion — A slow fuse for paroxysms at Stromboli volcano?

“…Withdrawal of this threshold magma volume tapped a small batch of LP magma which then ascended the conduit. The timescale of its transport to the surface could only have been from hours to days ( [Calvari et al, 2006], [Calvari et al, 2010], [Harris et al, 2008] and [Polacci et al, 2009]). The volumes involved in the paroxysms, i.e.…”

“…The steady supply of magma is associated with a bi-flow regime in the conduit, sustained degassing and frequent Strombolian eruptions (sensu strictu), punctuated roughly every 4 to 5 yr by much stronger explosions, commonly referred to as paroxysms (Barberi et al, 1993). These explosions erupt the same highly porphyritic (HP), high-density, crystallised magma associated with typical Strombolian activity and residing within the conduit but mixed with variable amounts ( [Lautze and Houghton, 2007] and [Polacci et al, 2009]) of less-porphyritic (LP), low-density, volatilerich magma ascending directly from an intermediate storage zone (at 6-9 km depth ( Fig. 1; [Di Carlo et al, 2006], [Métrich et al, 2005] and [Pichavant et al, 2009]).…”

A model of degassing for Stromboli volcano

“…The LP magma is thought to coexist with a substantial (∼2 wt.%) fraction of CO 2 -rich gas bubbles at reservoir conditions (Burton et al, 2007a,b). Observations on erupted pumices strongly suggest that, prior to a paroxysm, the LP magma is rapidly decompressed, maintaining virtually unchanged his "deep" petrological (Métrich et al, 2010) and textural (Polacci et al, 2009) properties.…”

Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography

“…Because vesicle size, shape and number density in xeno-pumice are ultimately a function of (i) the volatile content of the source material and (ii) the physical conditions the material was exposed to, the vesicle textures observed in the individual samples can probably offer insight into evolutionary processes during xeno-pumice formation and vesiculation (cf. Song et al 2001;Polacci et al 2008Polacci et al , 2009. From thin section analyses Meletlidis et al (2012) and Sigmarsson et al (2013) found that the xenopumice cores and the basanitic crust are unimodally vesiculated, which they suggest to be a result of a single vesicle nucleation event in an aphyric magma.…”

“…Multiple modes in VSD diagrams can thus be produced by either secondary nucleation events or vesicle coalescence, or by a combination of both (e.g. Bai et al 2008;Polacci et al 2009). These two processes are generally distinguished by the principle that secondary nucleation would produce a higher number density of small bubbles, whereas coalescence would produce a higher number density of larger bubbles (Toramaru 2014).…”