Protracted magma fragmentation during mafic explosive eruptions.

Magma fragmentation generates pyroclasts and drives explosive volcanic eruptions. Mounting evidence suggests that fragmentation is not a single event but a multistep process occurring under variable and currently poorly defined conditions. Here, we present microtextural features of fresh, glassy pyroclasts that shed new light on the variety of conditions under which mafic magmas fragment during explosive activity of different styles. By integrating scanning electron microscopy, X-ray microtomography, and image analysis, we describe and quantify microtextures related to magma fragmentation in pyroclasts from recent eruptions of Etna and Stromboli (Italy) and Cumbre Vieja (Spain) volcanoes. Pyroclast contours range from smooth to ragged, with intermediate features indicating that fragmentation occurred before, after, and right at the viscous to brittle transition. The presence of pyroclasts reincorporated into magma and then re-fragmented records multiple break-up and fusing events. Broken crystals within intact glass reveal the propagation of brittle cracks through magma and the subsequent viscous healing of the melt. Both crack healing and clast fusing leave behind identifiable sutures. During the Cumbre Vieja eruption, the abundance of these microtextures changed with changing eruption style. Sutures peak in lava fountain deposits, suggesting deep and hot fragmentation favoring frequent re-welding of cracks and pyroclasts. Broken crystals and incorporated pyroclasts are more frequent in less vigorous jetting activity, dominated by shallower, cooler fragmentation closer to quenching temperature. We propose that mafic magma fragments through multiple fracturing and healing events that protract across a range of temperatures and environments and reflect in the abundance of specific pyroclast microtextures.