| Budget Amount *help |
¥3,700,000 (Direct Cost: ¥3,700,000)
Fiscal Year 2001: ¥200,000 (Direct Cost: ¥200,000)
Fiscal Year 2000: ¥200,000 (Direct Cost: ¥200,000)
Fiscal Year 1999: ¥3,300,000 (Direct Cost: ¥3,300,000)
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| Research Abstract |
In order to understand the structure and evolution of boundary layers in magma chambers, we analyzed the growth history of all phenocrysts included in air-fall tephra deposits on the basis of zoning patterns of the phenocrysts. Tokyo Pumice, emanated from Hakone Volcano, Japan, has a larger amount of phenocrysts and higher proportions of mafic minerals in stratigraphically higher levels. The zoning patterns and crystallization temperatures of both orthopyroxene and clinopyroxene phenocrysts show the presence of 3 types of magmas with different temperatures which mixed together to form the observed chemical zonings. Prior to the eruption, the magma chamber was density-stratified consisting of at least two layers. The eruption first tapped the viscous upper layer that withdrew the lower layer and mixed with it within the conduit.
Spreading- and subduction-related plutonic complexes of the Oman Ophiolite were investigated as the best candidates for fossil boundary layers. The roof zones of the axial magma chamber are composed of doleritic, pegmatitic, massive and laminated gabbros in descending order. Doleritic gabbros solidified under the roof of the chamber, while massive gabbros crystallized on the chamber floor, that were compacted and reoriented while being buried under crystal mush, giving rise to the formation of the laminated gabbros. The interstitial melts trapped between the compacted cumulus minerals crystallized as zoned rims. In contrast, the crystals on the chamber floor kept in contact with the main reservoir, resulted in orthocumulates of the massive gabbros. On the contrary, subduction-related plutons are heterolithologic and experienced repeated injection and mixing of magmas and crystal mush. Such occurrence is consistent with the magma chamber model of arc volcanoes as deduced from the Quaternary Akagi Volcano, Japan (Umino and Horio, 1998).
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