A microstructural analyses of groundmass of dacite lavas, and their flowage and solidification processes.

英安岩熔岩基质及其流动和凝固过程的微观结构分析。

基本信息

批准号：
11640447
负责人：
MIYAKE Yasuyuki
金额：
$ 2.24万
依托单位：
Shinshu 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 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640447/
关键词：
Dasite lava flowage groundmass cavitation Unzen Daisen Yakedake Hyaloclastite キャビテイション

项目摘要

Such dacite lava domes as, Unzen-Fugendake, Daisen-Misen, Yakedake, and Miocene subaqueous dome at Katsurajima, Shimane Peninsula were investigated. Especially their microstructures and occurrences of cavities are studied. Following four results are obtained.1. Lavas that emplaced on steep slopes contain voids formed by mechanical processes. The voids vary from having ragged margins to sharp margins, and ragged margin voids are interpreted as failures in the ductile crystal mush magma. Voids with sharp margins are interpreted as brittle cracks formed when the rock had attained homogenous strength. These dilatant structures have been observed in upper, lower, proximal and distal parts of the lava flows. The dilatantancy and brittle flow structures observed are characteristic of dacite emplaced on the steep slopes.2. Pores in dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) have characteristics unlike vesicles. Most of the pores have ragged walls marked by the protru … More sion of groundmass crystals and phenocrysts. The shapes of these pores range from irregular to planar. Many pores occur in 'pressure shadows' adjacent to plenocrysts. Some pores are crack-like and pass around phenocrysts and have blunt terminations. Most of the pores are interpreted as the result of failure of the magma by flow during ductile-brittle transition. This phenomenon, known as cavitation, is well established in hydraulics and the material sciences. Cavitation of the lavas in this study was promoted by steep slope of emplacement, subtle mechanical contrasts between bands, and strain intensification around rigid crystals and lithic fragments. Cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening. Cavitation is inferred to be critical to the initiation of lava-front pyroclastic flows.3. Dacite magma emplaced subaqueously show band structures caused by rapid cooling by water. In the case of Katurajima lava dome in Shimane Peninsula, Miocene in age, the bands consists of mafic recessive zone and felsic prominent zones, SiO2 variation being58-74wt.%. Such a wide range variation, along with the groundmass structures are interpreted by combination of rapid cooling by magma and magma differentiation at the vent.4. Subaqueous dacite lavas provide an example of hyaloclastite that were formed by invasion of water into the hot rock body. Such an invasive hyaloclastites can be distinguished from the formerly well described subaqueous autobrecciated lavas. In the shallow water, hyaloclastite can be formed not only by quench-contraction of hot rock but also by small explosion induced by magma-water interaction. Less

对岛根半岛桂岛的云仙-风玄岳、大山-弥山、烧岳和中新世水下穹窿等英安岩熔岩穹窿进行了研究，获得了以下四个结果。陡坡包含由机械过程形成的空隙，这些空隙的范围从具有锯齿状边缘到尖锐边缘，并且锯齿状边缘空隙被解释为失败。具有尖锐边缘的韧性晶体糊状岩浆被解释为当岩石达到均匀强度时形成的脆性裂缝，这些膨胀结构在熔岩流的上部、下部、近端和远端都被观察到。日本三座火山（云仙、烧岳和大山）英安岩熔岩的孔隙具有不同的特征。大多数孔隙具有参差不齐的壁，其特征是基底晶体和斑晶的突出，许多孔隙出现在与全晶相邻的“压力阴影”中。类似并绕过斑晶并具有钝端部，大多数孔隙被解释为岩浆在流动过程中破裂的结果。这种被称为空化的现象在水力学和材料科学中得到了充分证实，在这项研究中，熔岩的空化是由陡峭的坡度、带之间微妙的机械对比以及刚性晶体和周围的应变强化所促进的。熔岩中的岩屑碎片可以从岩浆中提取挥发物，导致过早脆化和弱化，这对于熔岩前缘火山碎屑的形成至关重要。 3、水下侵位的英安岩岩浆显示出由于水的快速冷却而形成的带状结构，以中新世岛根半岛的香岛熔岩穹丘为例，带状结构由镁铁质隐性带和长英质突出带组成，SiO2变化范围为58-74wt。如此广泛的变化以及基底结构可以通过岩浆快速冷却和喷口处岩浆分异的结合来解释。4。水下英安岩熔岩提供了通过水侵入热岩体形成的玻璃碎屑岩，这种侵入性玻璃碎屑岩可以与之前描述的水下自角砾岩熔岩区分开来。在浅水中，玻璃碎屑岩不仅可以通过淬火形成。 -热岩收缩，但也由岩浆-水相互作用引起的小爆炸引起。