Abstract
In the 2011 eruption of Shinmoedake of the Kirishima volcano group, sub-Plinian eruptions were followed by lava extrusion with intermittent Vulcanian explosions. The interstices of microlites and relatively large nanolites (> 0.4 nm width) in the groundmass of pyroclasts (“groundmass interstices”) were studied to reveal shallow magmatic processes that lead to different eruption styles. The pumice of the sub-Plinian eruption has the least differentiated groundmass interstices composition. The groundmass interstices of the dense juvenile fragments of the Vulcanian explosions are divided into two groups. The first group is the most differentiated as expected from their lava-like texture, whereas the second group is less differentiated and covers the range of sub-Plinian pumice. The Vulcanian pumice overlaps the dense juvenile fragments despite their high vesicularity. This seemingly contradictory relationship of composition in the interstitial groundmass indicates the clastogenic lava origin of the less-differentiated group of dense juvenile fragments. In contrast, magmas stagnated in the shallow conduit maintained elevated temperature and water content, allowing groundmass interstices to differentiate by microlite growth. These magmas then erupted as pumice in the Vulcanian explosions and were effused as lava that was fragmented by subsequent explosions to be later sampled as a dense juvenile fragment. The groundmass crystallinity increased by 9.1 vol.% in 5–45 days between the sub-Plinian and Vulcanian eruptions, increasing initial melt and magma viscosity from 106.1–7.4 and 107.0–8.3 Pa s to 106.9–8.4 and 108.2–9.7 Pa s, respectively. This viscosity increase by nanolite crystallization could have facilitated the stress fragmentation of conduit magma, leading to Vulcanian explosions. Post-fragmentation expansion of the sub-Plinian pumice could have been suppressed by this viscosity increase, resulting in their low vesicularity. Late-stage groundmass differentiation can thus control shallow magmatic processes.
Highlights
Recent petrologic studies on volcanic eruption dynamics have revealed that the explosive/effusive transition is often determined by shallow-level processes including depressurization of conduit magmas (Castro and Gardner 2008), viscosity increase through dehydration-induced crystallization of matrix melt (Clarke et al 2007; Preece et al 2016), and rewelding of Editorial responsibility: S
The lightest-toned area under the optical microscope was clearly recognized by SEM because of the lower groundmass crystallinity (Figs. 3c and 4b and e)
We investigated late-stage groundmass differentiation by microlites and nanolites in the Shinmoedake 2011 eruption, which produced sub-Plinian eruptions and subsequent lava extrusion with intermittent Vulcanian explosions followed by frequent Vulcanian explosions
Summary
Recent petrologic studies on volcanic eruption dynamics have revealed that the explosive/effusive transition is often determined by shallow-level processes including depressurization of conduit magmas (Castro and Gardner 2008), viscosity increase through dehydration-induced crystallization of matrix melt (Clarke et al 2007; Preece et al 2016), and rewelding of Editorial responsibility: S. The crystallization conditions of microlites have been investigated through decompression experiments that simulate magma ascent from a magma chamber. Schlinger et al (1986) investigated the Fe-oxide ultrananolites in a welded tuff during cooling, and Sharp et al (1996) described the pyroxene nanolites in obsidian formed after the emplacement. Mujin et al (2017) revealed that nanolites of plagioclase, pyroxene, and Fe oxide in the quenched andesitic pumice and dense juvenile fragments of the 2011 eruption of Shinmoedake formed syn-eruptively in the shallow conduit during magma ascent. They found that the CSDs of plagioclase and the number density of magnetite in the submicrometer scale recorded the transition of eruption styles. Rock magnetic and mineralogical characteristics were focused in these studies, whereas chemical differentiation of groundmass resulted from nanoscale crystallization has not been investigated despite its potential to control the eruption dynamics through its effects on magma rheology
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