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Pyroclast microtextures reveal complex fragmentation history in mafic magmas (Cumbre Vieja, Stromboli, Etna, et al.)

Jacopo Taddeucci1, Piergiorgio Scarlato1, Elisabetta Del Bello1, Daniele Andronico2, Francesco Pennacchia1, Antonio Mangiacapre3, Gianmarco Buono4, Lucia Pappalardo4, Danilo M. Palladino3

  • Affiliations:  1Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, Roma, Italy; 2Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy; 3Sapienza-Università di Roma, Dipartimento di Scienze della Terra, Rome, Italy; 4Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 207

  • Programme No: 3.2.30

  • Theme 3 > Session 2

Abstract

Magma fragmentation is a key driver of explosive volcanic eruptions. Fragmentation is often simplified as a single process occurring at some well-defined interval in time and space (e.g., the often-invoked 'fragmentation depth'), but mounting experimental, observational, and textural evidence is now uncovering a more complex picture. Here, we present an ensemble of textural evidence from mafic pyroclasts illustrating the complexity of magma fragmentation during a range of explosive activities. Angular to smooth contours of pyroclasts from the same deposit indicate fragmentation both before and after quenching, with transitional features witnessing fragmentation at the fragile-viscous boundary. Multiple fragmentation events are revealed by the incorporation and re-fragmentation of pyroclast-in-pyroclast. Broken crystals within intact glass reveal the passage of brittle cracks through the magma and their subsequent viscous healing. Crack healing and agglutination of clasts are testified by the formation of sutures, marked by the presence of iron-rich and micro-oxides surfaces. A quantification of these features, performed by Focused Ion Beam Scanning Electron Microscopy and X-ray microtomography, reveals that: i) all these features are common to all study volcanoes; ii) abundance of broken crystals and angular contours increases in parallel; iii) broken crystals are more abundant in finer (ash-sized) pyroclasts; and iv) different eruption styles do not result in obvious textural changes. All these evidences point to a fragmentation history that is always complex spatially and temporally, involving magma at variable temperature, vesicularity and crystallinity, and controlled by both viscous and brittle processes of repeated fracturing, healing, welding, and re-fracturing.