5:15 PM - 6:30 PM
[SVC31-P08] Shallow conduit process of Strombolian eruption: A case study of Izu-Omuroyama monogenetic volcano, Japan
Keywords:mafic magma, Strombolian eruption, microlite crystallization, Shallow conduit process, Higashi-Izu monogenetic volcano group, eruption style
Izu-Omuroyama volcano is one of the monogenetic volcanoes in the Higashi-Izu monogenetic volcanic field (HIMVF) and was formed by the Strombolian eruption that occurred at 4ka. The eruption formed the largest scoria cone in HIMVF and simultaneously effused abundant lava flows. In this study, we performed quantitative textural analyses of minerals and chemical analyses of glasses in the groundmass of the Izu-Omuroyama scoria to clarify the shallow-conduit processes inducing the branch of the scoria-forming explosion and the lava effusion.
The scoria of the cone-forming stage was investigated. BSE images were acquired using FE-EPMA at ERI. We quantified volume fractions (Fs), number densities (NDs), and size distributions (CSDs) by image analyses for plagioclase (80 samples) and mafic minerals (40 samples), respectively. Because it is difficult to discriminate olivine and pyroxene in BSE images, we treated these minerals collectively as mafic minerals.
The groundmass of the scoria is composed of plagioclase, olivine, pyroxene, and glass. Fs of plagioclase and mafic minerals vary ~5-40 vol.% and ~3-20 vol.%, respectively. NDs are ~2000-20000 mm-1 for both minerals. An-rich, coarse and An-poor, fine plagioclase grains are found; the former and the latter are called microphenocryst and microlite, respectively. We discriminated against the microphenocrysts and the microlites based on CSDs. Inflected CSDs are found for both plagioclase and mafic minerals in the scoria; they are successfully explained by combinations of two linear CSD relations. The inflected sizes are determined for each sample. We regarded the mineral grains coarser and finer than the inflected size as microphenocrysts and microlites, respectively.
For microphenocrysts, Fs vary ~5-40 vol.% for plagioclase and ~0.4-20 vol.% for mafic minerals, respectively. NDs are 20-1000 mm-2 for both minerals. Positive relations are found between F and ND for both minerals. The ND of plagioclase microphenocryst increases from lower to higher units of the stratigraphy. The NDs of the higher units are similar to those of the lava flow, indicating that the groundmass texture of the lava flow can be explained by the crystal growth of microphenocryst without microlite crystallization. For microlites, Fs vary ~5-40 vol.% for plagioclase and ~2-20 vol.% for mafic minerals, respectively. The NDs are 1000-20000 mm-2 for both minerals. Contrary to the scoria case, the ND of plagioclase microphenocryst decreases from lower to higher units of stratigraphy. Fs of microphenocrysts are lower than ~40 vol.%, whereas those of all groundmass minerals are higher than ~40 vol.%. Previous experimental studies show that plagioclase-bearing magma behaves as solid when the F of suspended crystal is larger than ~40 vol.%, suggesting that the liquid-solid rheological transition was induced by microlite crystallization. The depth condition of microlite crystallization is constrained to be < 400m based on the chemical compositions of interstitial glasses and MELTS simulation.
Our results suggest that microlite crystallization at < 400m depth induced solidification of magma, which resulted in brittle fragmentation and explosion at the summit vent. On the other hand, the branch of the conduit to the flank vent occurred at a deeper depth than that of microlite crystallization, resulting in lava flow effusion. The increase and decrease of NDs of microphenocrysts and plagioclase microlite, respectively, from the lower to the higher stratigraphic units can be attributed to the resupply of the deep-derived hot magma, which is consistent with the abundant lava effusion following the scoria cone formation.
The scoria of the cone-forming stage was investigated. BSE images were acquired using FE-EPMA at ERI. We quantified volume fractions (Fs), number densities (NDs), and size distributions (CSDs) by image analyses for plagioclase (80 samples) and mafic minerals (40 samples), respectively. Because it is difficult to discriminate olivine and pyroxene in BSE images, we treated these minerals collectively as mafic minerals.
The groundmass of the scoria is composed of plagioclase, olivine, pyroxene, and glass. Fs of plagioclase and mafic minerals vary ~5-40 vol.% and ~3-20 vol.%, respectively. NDs are ~2000-20000 mm-1 for both minerals. An-rich, coarse and An-poor, fine plagioclase grains are found; the former and the latter are called microphenocryst and microlite, respectively. We discriminated against the microphenocrysts and the microlites based on CSDs. Inflected CSDs are found for both plagioclase and mafic minerals in the scoria; they are successfully explained by combinations of two linear CSD relations. The inflected sizes are determined for each sample. We regarded the mineral grains coarser and finer than the inflected size as microphenocrysts and microlites, respectively.
For microphenocrysts, Fs vary ~5-40 vol.% for plagioclase and ~0.4-20 vol.% for mafic minerals, respectively. NDs are 20-1000 mm-2 for both minerals. Positive relations are found between F and ND for both minerals. The ND of plagioclase microphenocryst increases from lower to higher units of the stratigraphy. The NDs of the higher units are similar to those of the lava flow, indicating that the groundmass texture of the lava flow can be explained by the crystal growth of microphenocryst without microlite crystallization. For microlites, Fs vary ~5-40 vol.% for plagioclase and ~2-20 vol.% for mafic minerals, respectively. The NDs are 1000-20000 mm-2 for both minerals. Contrary to the scoria case, the ND of plagioclase microphenocryst decreases from lower to higher units of stratigraphy. Fs of microphenocrysts are lower than ~40 vol.%, whereas those of all groundmass minerals are higher than ~40 vol.%. Previous experimental studies show that plagioclase-bearing magma behaves as solid when the F of suspended crystal is larger than ~40 vol.%, suggesting that the liquid-solid rheological transition was induced by microlite crystallization. The depth condition of microlite crystallization is constrained to be < 400m based on the chemical compositions of interstitial glasses and MELTS simulation.
Our results suggest that microlite crystallization at < 400m depth induced solidification of magma, which resulted in brittle fragmentation and explosion at the summit vent. On the other hand, the branch of the conduit to the flank vent occurred at a deeper depth than that of microlite crystallization, resulting in lava flow effusion. The increase and decrease of NDs of microphenocrysts and plagioclase microlite, respectively, from the lower to the higher stratigraphic units can be attributed to the resupply of the deep-derived hot magma, which is consistent with the abundant lava effusion following the scoria cone formation.