5:15 PM - 7:15 PM
[SVC33-P01] Eruptive sequence and magma storage conditions of Plinian eruptions at Karakunidake,
Kirishima Volcano Group, Japan
Keywords:Kirishima, Plinian eruption, Magma storage, eruptive sequence
Plinian eruptions are among the most explosive volcanic events, characterized by tall eruption columns. Some Plinian eruptions conclude in a short period, whereas others persist over an extended time with multiple eruptive phases. However, the factors controlling this variability in eruption sequences remain poorly understood. To address this, it is crucial to investigate the pre-eruptive conditions of magma reservoirs, including chemical composition, temperature, depth (pressure), viscosity, and heterogeneities, and their relationships to eruption sequences. The Kobayashi Pumice Eruption, which occurred at Karakunidake in the Kirishima volcanic group approximately 16,000 years ago, represents a typical case of multiple Plinian phases. The tephra deposits consist of alternating layers of pumice fall and volcanic ash, indicating Plinian phases repeated. Although the modern magma supply system has been elucidated through geophysical observations, the magma storage conditions at the time of the Kobayashi Pumice Eruption remain unclear. This study aims to reconstruct the pre-eruptive magma storage conditions and evaluate their influence on eruption dynamics through geological and petrological analyses.
A field survey was conducted at nine outcrop locations on the northern and eastern slopes of the summit, where stratigraphic descriptions and sample collection were carried out. Thin sections were analyzed under a polarizing microscope to examine phenocrysts. Whole-rock compositions of pumice and pyroclastic flow scoria were analyzed using XRF, while phenocrysts, melt inclusions, and matrix glass were analyzed via FE-EPMA. Magma temperature was estimated using geothermometers for two pyroxenes and Fe-Ti oxides, and water content was estimated via a plagioclase-melt hygrometer. Magma storage depth was inferred through volatile solubility modeling.
Pumice samples exhibited high crystallinity, high vesicularity, and a microlite-poor matrix. In contrast, scoria samples had lower crystallinity, lower vesicularity, and a microlite-rich matrix. Plagioclase (Plg), clinopyroxene (Cpx), orthopyroxene (Opx), and Fe-Ti oxides are identified in all units as phenocryst minerals. Many phenocrysts displayed zoning, resorption textures, and sieve textures, suggesting a history of magma mixing. The crystallinity determined by point counting was 32 vol.% for scoria and 40-50 vol.% for pumice, implying that the magma originated from a crystal-rich reservoir resembling a crystal mush. Whole-rock compositions of scoria, which were less affected by weathering, and mass balance-derived estimates for pumice samples indicated basaltic andesite compositions with SiO2 58-62 wt.%. Plg compositions ranged from An# 55-85 in scoria and An# 55-92 in pumice. Cpx had Mg# 68-84 in scoria and Mg# 70-78 in pumice, while Opx had Mg# 66-70 regardless of units. The glass compositions of attached melt on phenocrysts and melt inclusions systematically differed between Unit B/I and Unit F within the fall pumice. This compositional variation is difficult to explain solely by fractional crystallization within a single magma chamber, suggesting that two silicic magmas mixed relatively shortly before the eruption. Furthermore, the groundmass glass of scoria had a slightly more mafic composition than that of pumice, indicating that scoria originated from a different magma reservoir than the pumice. Temperature and pressure estimated from two-pyroxene thermometry and plagioclase-melt hygrometry indicate that major phenocrysts crystallized at 950±50°C and 3.5±0.3 km for scoria, and 910±20°C and 3.9±0.6 km for pumice. Based on the above discussion, a model was proposed that the Kobayashi Pumice Eruption originated from not a single magma chamber but multiple magma pockets existing under conditions like a mush-like magma reservoir. Additionally, the results suggest a close relationship between changes in eruption style and variations in magma supply sources.
A field survey was conducted at nine outcrop locations on the northern and eastern slopes of the summit, where stratigraphic descriptions and sample collection were carried out. Thin sections were analyzed under a polarizing microscope to examine phenocrysts. Whole-rock compositions of pumice and pyroclastic flow scoria were analyzed using XRF, while phenocrysts, melt inclusions, and matrix glass were analyzed via FE-EPMA. Magma temperature was estimated using geothermometers for two pyroxenes and Fe-Ti oxides, and water content was estimated via a plagioclase-melt hygrometer. Magma storage depth was inferred through volatile solubility modeling.
Pumice samples exhibited high crystallinity, high vesicularity, and a microlite-poor matrix. In contrast, scoria samples had lower crystallinity, lower vesicularity, and a microlite-rich matrix. Plagioclase (Plg), clinopyroxene (Cpx), orthopyroxene (Opx), and Fe-Ti oxides are identified in all units as phenocryst minerals. Many phenocrysts displayed zoning, resorption textures, and sieve textures, suggesting a history of magma mixing. The crystallinity determined by point counting was 32 vol.% for scoria and 40-50 vol.% for pumice, implying that the magma originated from a crystal-rich reservoir resembling a crystal mush. Whole-rock compositions of scoria, which were less affected by weathering, and mass balance-derived estimates for pumice samples indicated basaltic andesite compositions with SiO2 58-62 wt.%. Plg compositions ranged from An# 55-85 in scoria and An# 55-92 in pumice. Cpx had Mg# 68-84 in scoria and Mg# 70-78 in pumice, while Opx had Mg# 66-70 regardless of units. The glass compositions of attached melt on phenocrysts and melt inclusions systematically differed between Unit B/I and Unit F within the fall pumice. This compositional variation is difficult to explain solely by fractional crystallization within a single magma chamber, suggesting that two silicic magmas mixed relatively shortly before the eruption. Furthermore, the groundmass glass of scoria had a slightly more mafic composition than that of pumice, indicating that scoria originated from a different magma reservoir than the pumice. Temperature and pressure estimated from two-pyroxene thermometry and plagioclase-melt hygrometry indicate that major phenocrysts crystallized at 950±50°C and 3.5±0.3 km for scoria, and 910±20°C and 3.9±0.6 km for pumice. Based on the above discussion, a model was proposed that the Kobayashi Pumice Eruption originated from not a single magma chamber but multiple magma pockets existing under conditions like a mush-like magma reservoir. Additionally, the results suggest a close relationship between changes in eruption style and variations in magma supply sources.