新富士火山初期の大きなソレアイトマグマだまりにおける結晶分化(<特集>火山のモデル(II))

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Climactic effusions (〜40km^3) of tholeiite lavas of Younger Fuji Volcano (Izu-Mariana Arc) occurred between 13000-8000 yBP. The mode of occurrence of these effusions differs greatly from the volcanic phenomena characterizing the Fuji Volcano eruptive history (from 80,000 years ago up to present), mainly represented by small scoria eruptions. Less fractionated and larger amounts of lavas erupted in the beginning and were followed by more fractionated and smaller amounts of lavas. In the more highly fractionated magma, there is a wide variation (0-40 vol %) in the amount of phenocrysts. The bulk rock K_2O concentration proportionally increases with the increse of the amount of groundmass. Plagioclase phenocrysts coexist with olivine phenocrysts in most cases, and are relatively enriched in comparison with mafic minerals. The maximum size of phenocrystic plagioclase reaches 5-10 mm and is 2 to 3 times larger than that reached in other stages. These facts are consistent with a model that crystal settling in a slowly cooling large magma chamber is the major fractionation mechanism for the climactic effusions. The tholeiite lavas belong to the new magma series, the parental magma of which is slightly richer in incompatible elements than the old magma series. The feeding of the new magma series into the new magma chamber is estimated to have started about 10,000 years prior to the climactic effusions. The delay of the eruptions is due to the difficulty to make a new conduit and allow the growth of the slowly cooling large magma chamber. At the same time, a small magma chamber for the old series magma was still alive and led to intermittent eruptions until about 3,000 yBP.
特定非営利活動法人日本火山学会の論文
1991-07-15

新富士火山初期の大きなソレアイトマグマだまりにおける結晶分化(<特集>火山のモデル(II))

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