Shallow crystallization of eruptive magma inferred from volcanic ash microtextures: a case study of the 2018 eruption of Shinmoedake volcano, Japan

The occurrence of groundmass crystals reveals the shallow conduit process of magmas, which affects the behavior of eruptions. Here, we analyzed groundmass microtextures of ash samples from the 2018 eruption of Shinmoedake volcano, Japan, to evaluate the change of magma ascent conditions during the eruption sequence. The eruptive activity changed from ash venting (Phase 1: March 1–6) to lava effusion with continuous ash-laden plumes (Phase 2: March 6–9) and then shifted to Vulcanian explosions (Phase 3: March 10–April 5). Non-juvenile particles were abundant in Phase 1, whereas juvenile particles were dominant in Phases 2 and 3. Vesicular juvenile particles were more abundant in Phase 2 than Phase 3. The lower microlite crystallinity and groundmass SiO2 concentrations of the vesicular particles indicate that they were sourced from magma that ascended rapidly. Abundant nanolites were observed in the black interstitial glass of juvenile particles under an optical microscope, whereas few nanolites were observed in the transparent ones. The presence of nanolites can be explained by the dehydration of silicate melt, as well as cooling and oxidation between fragmentation and quenching. Temporal changes in the ash componentry show that the eruption activity started from the erosion of the pre-existing vent plug (Phase 1), shifted to the simultaneous eruption of bubble-bearing and outgassed magmas (Phase 2), and concluded with explosions of the stagnant lava (Phase 3), thereby demonstrating the sequence of vent opening and extrusion and stagnation of magma. Therefore, ash microtextures are valuable for monitoring the shallow conduit process of eruptive magma.

Using apatite records of volatile budget and magma ascent rates to investigate eruption dynamics

<p>One of the major challenges faced by volcanologists to investigate controls on eruption dynamics is to quantify both pre-eruptive volatile budgets and timescales of magma ascent. Indeed, petrological investigations of the two parameters usually rely on different methods/analytical techniques that are not always applicable/accessible. Recent studies have shown that the abundance and zoning pattern of F, Cl, and OH in apatite can be used to determine both pre-eruptive volatile budget and magma degassing rates that can, under some conditions, be related to magma ascent rates ([1],[2]).</p><p>Here we apply the two methods to apatite in the Rabaul 2006 eruption deposits (Papua-New-Guinea). This was a VEI-4 eruption and occurred in three main phases: (1) a sub-plinian onset followed 12h after its start by (2) a mixed strombolian-effusive phase, which subsequently evolved into (3) discrete vulcanian explosions. We sampled deposits of the three phases: (1) pumices, (2) fragments of lava flow, and (3) fragments of cow-pad bombs.</p><p>We calculated pre-eruptive water contents using apatite included in clinopyroxene as they keep a better record of reservoir conditions from the time of entrapment. We found that the magma that fed the sub-plinian phase contained the highest water content of about 2 wt.%, while magmas that fed the lava flow and the vulcanian phase were drier, with 0.2 to 0.5 wt.% less H<sub>2</sub>O. X-ray maps acquired with an EPMA show that only apatite crystals in the groundmass of the vulcanian and effusive deposits are zoned in F and Cl at the crystal rims, whereas those from the sub-plinian deposits and included in clinopyroxenes are not zoned. This indicates that the zoning is related to syn- or immediately pre-eruptive changes of Cl-F-H<sub>2</sub>O during magma ascent towards the surface and can thus be modelled as diffusive reequilibration of the crystal and the melt. We obtained maximum diffusion timescales of <8 hours for the unzoned apatite in sub-plinian deposits, timescales of 20–22 hours for apatite in vulcanian deposits, and 600–1500 hours for those in the lava flow. Thus, the time scales increase with decreasing explosivity of the eruptions, as it could be expected if magma ascent rate played the key role of eruption dynamics. However, the degassing timescales of the effusive phase are significantly longer than the eruption duration itself, which can be explained if the magma started rising in the system 1–3 months prior to the onset of the eruption. The volatile-rich, fast-rising magma that fed the initial sub-plinian phase propagated through, disturbed and remobilized the shallower, more degassed batch of magma, which was erupted during the following effusive phase. Deeper, volatile-poor magma that kept moving up the open conduit, was responsible for the late vulcanian explosions.</p><p>Our results show that apatite is a powerful tool for probing slight changes in magma volatile chemistry and ascent rates that can vary between different phases of the same eruption and produce different eruption styles.</p><p> </p><p>[1] Li and Costa, 2020, GCA [2] Li et al. 2020, EPSL</p>

Micro-Textural Controls on Magma Rheology and Vulcanian Explosion Cyclicity

Understanding the relationship between degassing, crystallization processes and eruption style is a central goal in volcanology, in particular how these processes modulate the magnitude and timing of cyclical Vulcanian explosions in intermediate magmas. To investigate the influence of variations in crystal micro-textures on magma rheology and eruption dynamics, we conducted high-temperature (940°C) uniaxial compression experiments at conditions simulating a shallow volcanic conduit setting on eight samples of high-crystallinity andesite with variable plagioclase microlite populations from the 2004 to 2010 Vulcanian explosions of Galeras volcano, Colombia. Experiments were conducted at different strain rates to measure the rate-dependence of apparent viscosities and assess the dominant deformation processes associated with shear. Variations in plagioclase micro-textures are associated with apparent viscosities spanning over one order of magnitude for a given strain rate. Samples with low numbers of large prismatic microlites behaved consistently with published rheological laws for crystalline dome samples, and displayed extensive micro-cracking. Samples with high numbers of small tabular microlites showed a lower apparent viscosity and were less shear-thinning. The data suggest a spectrum of rheological behavior controlled by concurrent variations in microlite number, size and shape. We use previously published micro-textural data for time-constrained samples to model the apparent viscosity of magma erupted during the 2004–2010 sequence of Vulcanian explosions and compare these results with observed SO2 fluxes. We propose that variations in magma decompression rate, which are known to produce systematic textural differences in the plagioclase microlite cargo, govern differences in magma rheology in the shallow conduit. These rheological differences are likely to affect the rate at which magma densifies as a result of outgassing, leading to magmatic plugs with a range of porosities and permeabilities. The existence of magmatic plugs with variable physical properties has important implications for the development of critical overpressure driving Vulcanian explosions, and thus for hazard assessment during volcanic crises. We suggest a new conceptual model to explain eruption style at andesitic volcanoes based on micro-textural and rheological differences between “plug-forming” and “dome-forming” magma. We advance that existing rheological laws describing the behavior of andesitic magma based on experiments on dome rocks are inappropriate for modeling large Vulcanian explosions (∼106 m3), as the magma involved in these eruptions lacks the characteristics required to form exogenous lava domes.