Role of Nanolite Crystallization in Magma Fragmentation During Ash Eruptions
Abstract
An ash eruption is a typical eruption mode for basaltic-andesitic magmas, which can be a long-lasting event that expels a large cumulative volume, and therefore, serves as a safety valve in volcanic systems. It is often characterized by the continuous emission of black sandy ash without explosion sounds, and is distinguished from other explosive eruptions. However, the fragmentation mechanism that shapes ash eruption characteristics is unclear. Here we conducted a nanoscale analysis of volcanic ash samples from Aso Volcano, Japan, and explored the relationship between their textural properties and nanoscale crystals in groundmass.
We analyzed ash fallout samples acquired in situ on March 15, 2015, at 5.4 and 6.4 km southeast of Nakadake Crater. We also examined scoria from three Strombolian phases that occurred between Dec. 2014 and Jan. 2015 for comparison. The ash samples were first classified into brown glassy ash that was dominated by spongy foam and black ash, as well as liberated crystals and accidental lithic fragments. The former two comprised 75% of the total particles, of which 90% was black ash. The black ash was further divided into ash with a glassy luster (36%), which typically had a tear drop shape, and ash with a metallic luster (64%), which primarily had polyhedral shapes. Our FE-SEM analysis reveals that the groundmass interstice of the brown glassy ash consists of "real" glass at the atomic scale with ultrananolites (Mujin et al., 2017 Am. Mineral.) around plagioclase microlites, whereas that of the black ash with a glassy luster has a very high ultrananolite crystallinity. The black ash with a metallic luster has a pilotaxitic (diktytaxitic) texture. It should be noted that real glass is very rare, even in quenched scoria with a quite glassy luster. We therefore infer that: 1) the crystallization of ultrananolite might have affected magma fragmentation in ash and Strombolian eruptions, 2) the rapid growth of interfingered microlites and nanolites in mafic melts, represented by pilotaxitic texture, is essential for the formation of black sandy ash, and 3) such crystallization embrittlement may be responsible for continuous fragmentation by shear deformation of the magma and erosion by gas flow. We emphasize that shallow magmatic processes can be better understood via nanoscale groundmass observations.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V44A..06N
- Keywords:
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- 8404 Volcanoclastic deposits;
- VOLCANOLOGYDE: 8428 Explosive volcanism;
- VOLCANOLOGYDE: 8445 Experimental volcanism;
- VOLCANOLOGYDE: 8486 Field relationships;
- VOLCANOLOGY