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Origin of phenocrysts and eruption mechanism -in case of Unzen Volcano-.

斑晶的起源和喷发机制-以云仙火山为例-。

基本信息

批准号：
07454134
负责人：
NAKADA Setsuya
金额：
$ 4.29万
依托单位：
University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1996
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-07454134/
关键词：
magma chamber dacite boundary layr enclaves phenocryst Unzen Volcano disequilibrium recycling of crystal 雲仙火山普賢岳溶岩噴出率マグマ混合

项目摘要

Dacite lavas of the 1991-95 eruption at Unzen contain phenocrysts all indicating their disequilibrium with melt, and include basaltic to andesitic enclaves. The origin of the disequilibrium and the existence of enclaves can be explained by the process associated during cooling of a magma chamber, rather than by magma mixing prior to or during eruption. The magma chamber consisted of hot convecting rhyodacite melt in the interior and the boundary layr against the country rock. The boundary layr had the crystal-dominant ("chilled") margin outside and the crystal mush inside, increasing proportion of rhyolite melt inward. Phenocrysts are either the crystals which had precipitated once at the margin when conductive flux in the country rock was large initially, and, then, were disaggregated by melting when heat flux from magma exceeded the conductive flux, or the restites of the wallrock which survived melting. All crystals incorporated into the convecting interior melt were resorbed before eruption. Enclaves represent pieces of either the chilled margin or crystal mush in the boundary layr. Overgrowth rims were formed on phenocrysts by degassing accompanied with eruption. It is unlikely that the idea of magma mixing prior to or during eruption is supported always by disequilibrium phenocryst assemblage/texture or the existence of mafic enclaves in porphyritic lavas.

1991年至1995年在安岑喷发的英安岩熔岩中含有斑晶，表明它们与熔体不平衡，并包括玄武岩至安山岩包体。这种不平衡的起源和包体的存在可以通过岩浆室冷却过程来解释，而不是通过喷发前或喷发期间的岩浆混合来解释。岩浆房由内部热对流流纹岩熔体和围岩边界层组成。边界层外有以晶体为主的(“冷”)边缘，内有晶体碎屑，向内增加流纹岩熔体的比例。斑晶既可以是围岩初期导热通量较大时在边缘析出的晶体，当岩浆热流超过导热通量时熔融分解，也可以是围岩中残存下来的残留物。所有进入对流内部熔体的晶体在喷发前都被吸收了。包体代表边界层中的冰冷边缘或晶体碎屑。脱气作用伴随着喷发，在斑晶上形成了凸起边缘。岩浆在喷发前或喷发期间混合的想法不太可能总是由斑岩熔岩中不平衡的斑晶组合/结构或镁铁质包体的存在所支持。