Primary and secondary fragmentation of crystal-bearing intermediate magma

Abstract

Crystal-rich intermediate magmas are subjected to both primary and secondary fragmentation processes, each of which may produce texturally distinct tephra. Of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes. One way to address this question is by fragmenting pyroclasts under controlled conditions. We fragmented pumice samples from Soufriere Hills Volcano (SHV), Montserrat, by three methods: rapid decompression in a shock tube-like apparatus, impact by a falling piston, and milling in a ball mill. Grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns, and that the fractal dimension increases from a minimum of ~2.1 for decompression fragmentation (primary fragmentation) to a maximum of ~ 2.7 by repeated impact (secondary fragmentation). To assess the details of the fragmentation process, we quantified the shape, texture and components of constituent ash particles. Ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts. We also quantify the extent to which the matrix is separated from the crystals, which shows that secondary processes efficiently remove adhering matrix from crystals, particularly during milling (abrasion). Furthermore, measurements of crystal size distributions before (using x-ray computed tomography) and after (by componentry of individual grain size classes) decompression-driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution, but also that free crystals are smaller in the fragmented material than in the original pumice clast. Taken together, our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation. Critically, however, our extension of previous analyses to characterization of shape, texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin. We illustrate this application with examples from SHV deposits.

Distal Enhanced Sedimentation From Volcanic Plumes: Insights From the Secondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits

“…At Spurr, the origin of the bimodality of the TGSD is not known. The fine modes are often related to a secondary process of fragmentation, such as comminution in pyroclastic density currents (PDCs) (Bernard & Le Pennec, ; Jones et al, ), which can produce significant fine ash enrichment in some fallout deposits (Bonadonna et al, ; Eychenne et al, ; Eychenne et al, ). The August and September eruptions, however, produced only small volume PDCs (<3 km runout distances) (Miller et al, ) and are thus unlikely sources of abundant fine ash.…”

Causes of fragmented crystals in ignimbrites: a case study of the Cardones ignimbrite, Northern Chile

“…Very large ignimbrite eruptions may generate conditions that impose extreme pressure gradients (ΔP) within erupting magma, which may in turn affect fragmentation (e.g., Gottsmann et al 2009;Cashman and Giordano 2014). For insight into the role of ΔP on fragmentation we look to experimental shock tube studies (e.g., Alidibirov and Dingwell 1996;Spieler et al 2004;Mueller et al 2008;Jones et al 2016). Here magma fragmentation is caused by tensile fractures within the magma and occurs at a threshold ΔP with an efficiency determined by fracture strength and magma permeability.…”

“…These studies show that addition of crystals increases the bulk magma strength, but that the crystals may fracture if the pressure drop is large and rapid (e.g. Jones et al 2016). An additional mechanism of crystal fragmentation is ΔP-induced explosion of crystalhosted melt inclusions (e.g.…”

“…We suggest that the on average 12% dilated space between the crystal fragments was created during the eruption and is related to volatiles in melt inclusions coming out of solution, expanding and breaking the crystal. Such, isotropic crystal fragmentation by dilation pushes apart crystal fragments in all directions and suggests a very large pressure drop that exceeds the tensile strength of the crystals; shock tube experiments suggest that ΔPs of tens of MPa are required (Alidibirov and Dingwell 1996;Spieler et al 2004;Mueller et al 2008;Jones et al 2016). Models of explosive volcanic flows in conduits calculate rapid ΔPs of this magnitude, with the largest and most rapid ΔPs occurring at and above the fragmentation level (decreases of several tens of MPa over ca.…”

“…Magma, especially crystal-rich magma, can respond brittlely to rapid stress perturbations in magmatic systems (e.g. Spieler et al 2004;Lavallée et al 2007;Gottsmann et al 2009;Huber et al 2011;Jones et al 2016). Magma decompression also causes over pressurisation and explosion of melt inclusions trapped in phenocrysts (Tait 1992;Best and Christiansen 1997;Zhang 1998;Williamson et al 2010).…”