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 - 95年爆发的dacite熔岩含有表晶，都表明它们与熔体不平衡，并包括玄武岩到安第斯山脉的飞地。不平衡的起源和飞地的存在可以通过在岩浆腔室冷却过程中相关的过程来解释，而不是在喷发前或爆发过程中岩浆混合。岩浆室包括在内部和边界上对乡村岩石的热门曲面融化。边界的外部有晶体主导（“冰冷”）边缘，内部的晶体糊为糊状，增加了节纹石的比例向内融化。当乡村岩石的导电通量最初是较大时，现场晶体是在边缘沉淀一次的晶体，然后当岩浆中的热通量超过导电通量时，通过熔化，或者被分解，或者是熔岩的恢复液的熔点，而熔岩的恢复液的可融化。在喷发前将所有融合到对流内部熔体中的晶体均已吸收。飞地代表边界层中冷缘或晶体糊的碎片。通过脱气并伴有喷发，在表晶上形成了过度生长的轮辋。在喷发前或在喷发过程中混合岩浆混合的想法始终是由斑岩熔岩中的不平衡态晶体组合/纹理或镁铁质飞地的存在所支持的。