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All phenocrysts analyzing project ― Structure and evolution of boundary layers in magma chambers

全斑晶分析项目——岩浆房边界层的结构和演化

基本信息

批准号：
11640475
负责人：
UMINO Susumu
金额：
$ 2.37万
依托单位：
Shizuoka University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640475/
关键词：
magma chambers roof zone melt lens crystal mush phenocryst Oman Ophiolite Hakone Volcano Tokyo Pumice 結晶ネットワーク島弧火山結晶マッシュ赤城火山

项目摘要

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).

为了了解岩浆房边界层的结构和演化，根据斑晶的分带模式，分析了气降火山灰矿床中斑晶的生长历史。产自日本箱根火山的东京浮岩，在较高的地层层次上，具有较多的斑晶和较高比例的镁铁质矿物。斜方辉石和单斜辉石斑晶的分带模式和结晶温度表明，存在3种不同温度的岩浆，它们混合在一起，形成所观察到的化学分带。在喷发之前，岩浆房是由至少两层组成的密度分层。喷发首先利用粘性上层撤回下层和混合与它在导管。蔓延和俯冲相关的深成复合体的阿曼蛇绿岩进行了调查，作为最佳候选人的化石边界层。轴向岩浆房的顶带由粗玄质辉长岩、伟晶岩、块状辉长岩和层状辉长岩组成。白云质辉长岩在洞顶下固化，而块状辉长岩在洞底结晶，在被埋在晶体泥下时被压实和重新定向，从而形成层压辉长岩。被困在致密的积云矿物之间的间隙熔体结晶为带状边缘。相反，在腔室底部的晶体保持与主储层接触，导致了巨大的辉长岩的正累积。与此相反，俯冲相关的岩体是异岩性的，并经历了反复注入和混合的岩浆和晶体泥。这种产状与从日本第四纪赤城火山中推导出的弧火山岩浆房模型一致（Umino和Horio，1998年）。