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

为了了解岩浆室边界层的结构和演化，在分析气落式温体矿床中所有斑晶的分带特征的基础上，对斑晶的生长历史进行了分析。东京浮石发源于日本箱根火山，其斑晶数量较多，基性矿物比例较高，地层水平较高。正辉石和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，这些岩浆混合在一起形成了所观察到的化学分带。在喷发之前，岩浆房是密度分层的，至少由两层组成。火山喷发首先撞击黏稠的上层，将下层抽出，并在导管内与之混合。阿曼蛇绿岩的扩张和俯冲相关的深成杂岩被认为是化石边界层的最佳候选。轴向岩浆房顶带由高到低依次为白云岩、伟晶岩、块状辉长岩和层状辉长岩。斑岩型辉长岩在硐室顶板下凝固，块状辉长岩在硐室底板上结晶，块状辉长岩被压实、重定向，埋于结晶泥下，形成层状辉长岩。压实的积云矿物之间的间隙熔体结晶成带状的边缘。相反，岩室底板上的晶体与主储层保持接触，形成块状辉长岩的正积层。相反，与俯冲有关的岩体是异岩性的，经历了岩浆和晶体浆液的反复注入和混合。这种产状与日本第四纪赤城火山（Umino and Horio, 1998）推断出的弧型火山岩浆室模型相一致。

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).