Mafic explosive volcanism at Llaima Volcano: 3D x-ray microtomography reconstruction of pyroclasts to constrain shallow conduit processes

Abstract Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity still being explored. Eruption style determined ability of gas escape as magma ascends, which tends be easier in low-viscosity, magmas. If permeability sufficiently high reduce bubble overpressure during ascent, volatiles may from magma, inhibiting violent activity. In contrast, if low retain phase within drive fragmentation. Rapid ascent induce disequilibrium crystallization, increasing viscosity affecting network with consequences for permeability, hence, explosivity. To explore strongly volcanism, we combine microlite textural analyses synchrotron x-ray computed microtomography 10 pyroclasts 12.6 ka Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify crystal size distributions (CSD), number densities, porosity, interconnectivity, density, geometrical properties porous media investigate role degassing processes at use an analytical technique estimate tortuosity combing Kozeny-Carman relationship, factor, pyroclast vesicle textures. The groundmass our samples composed up 44% plagioclase microlites, > 85% < µm length. addition, identify two populations vesicles samples: (1) a convoluted interconnected produced extensive coalescence smaller (> 99% pore volume), (2) population very small completely isolated (< 1% porosity). Computed ranges 3.0 × ?13 6.3 ?12 m 2 , lower than similarly eruptions Tarawera (1886; New Zealand) Etna (112 BC; Italy). combination CSDs, 3D textures evidence rapid induced conditions, promoting syn-eruptive crystallization microlites shallow conduit. interpret increased simultaneously forcing bubbles deform they grew together, resulting permeable highly tortuous vesicles. Using densities (0.1-3 ?3 4 per mm 3 ), obtain minimum average decompression rate 1.4 MPa/s. Despite reached percolation threshold, propose suppressed outgassing overpressures, leading Further, using porosity samples, estimated 5 MPa could have been sufficient fragment magma. Other report similar rate, implying control more generally.