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。

• 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

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

研究了岛根半岛胜岛的云仙-富玄岳、大山-美仙、屋岳和中新世水下穹隆等英安岩熔岩穹隆。重点研究了它们的微观结构和空洞的出现情况。得到以下四个结果.位于陡坡上的熔岩含有由机械过程形成的空洞。这些空洞从边缘不规则到边缘尖锐不等，边缘不规则的空洞被解释为韧性晶体糊状岩浆的失败。具有尖锐边缘的空隙被解释为岩石达到均匀强度时形成的脆性裂缝。在熔岩流的上部、下部、近端和远端都观察到了这些构造。所观察到的流动构造是英安岩在陡坡上侵位的特征.日本三个火山（云仙、屋岳和戴森）的英安岩熔岩中的孔隙具有不同于泡的特征。大多数气孔的孔壁粗糙， ...更多信息 基质晶体和斑晶的锡永。这些孔的形状范围从不规则到平面。许多气孔出现在与脾晶相邻的“压力阴影”中。有些孔隙呈裂纹状，绕过斑晶，末端钝。大多数孔隙被解释为岩浆在韧-脆性转变过程中流动破坏的结果。这种现象被称为气穴现象，在水力学和材料科学中得到了很好的证实。在这项研究中的熔岩空化促进了陡峭的斜坡侵位，微妙的机械带之间的对比，以及周围的刚性晶体和岩屑的应变强化。熔岩中的空化作用会从岩浆中提取挥发物，导致过早脆化和弱化。空穴被推断为熔岩前缘火山碎屑流的启动的关键。英安岩岩浆在水下侵位，呈现水快速冷却的带状构造。以中新世时代的岛根半岛的香土岛熔岩穹丘为例，条带由镁铁质隐性带和长英质突出带组成，SiO2变化为58 - 74wt%。这种大范围的变化，沿着基底构造的变化，是岩浆快速冷却和岩浆在喷口分异作用的综合结果。水下英安岩熔岩提供了一个水侵入热岩体形成的玻璃碎屑岩的例子。这种侵入性的玻璃碎屑岩可以与以前描述的水下自角砾熔岩区别开来。在浅水区，玻璃碎屑岩既可由热岩的急冷收缩形成，也可由岩浆-水相互作用引起的小爆炸形成。少

项目成果

Smith, J.V.: "Textural evidence for dilatant (shear thickening) rheology of magma at high crystal concentrations."Jour.Volcanol.Geothermal Res.. 99. 1-7 (2000)

Smith, J.V.：“高晶体浓度下岩浆膨胀（剪切增稠）流变学的结构证据。”Jour.Volcanol.Geo Thermal Res.. 99. 1-7 (2000)

Smith,J.V.,Miyake,Y.and Oikawa,T.: "Interpretation of porosity in dacite domes as ductile brittle failure textures"Journal of Volcanology and Geothermal Research. (in print).

Smith, J.V.、Miyake, Y. 和 Oikawa, T.：“将英安岩圆顶中的孔隙度解释为延性脆性破坏纹理”火山学与地热研究杂志。

Smith J. V.: "Textural evidence for dilatant (shear thickening) rheology of magma at high crystal concentrations."Jour. of Volcanology and Geothermal Res.. in press.

Smith J. V.：“高晶体浓度下岩浆膨胀（剪切增稠）流变学的结构证据。”

Smith, J.V.: "Dilatancy and brittle flow structures in dacite lavas emplaced on steep slopes in Japan."EOS. 80. F1090 (1999)

Smith, J.V.：“日本陡坡上英安岩熔岩的剪胀性和脆性流动结构。”EOS。

Smith,J.V.: "Textural evidence for dilatant (Shear thickening) rheology of magma at high crystal concentrations."Jour nal of Volcanology and Geothermal Research. 99. 1-7 (2000)

Smith, J.V.：“高晶体浓度下岩浆膨胀（剪切增稠）流变学的结构证据。”火山学和地热研究杂志。