PETROLOGICAL STUDY ON MAGMA STORAGE AND MOVEMENT IN THE UPPER CRUST

上地壳岩浆储存和运动的岩石学研究

• 批准号: 09640565
• 负责人: NAKADA Setsuya
• 金额: $ 1.98万
• 依托单位: The University of Tokyo
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 1997
• 资助国家: 日本
• 起止时间: 1997 至 1998
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-09640565/
• 关键词: magma chamber; degassing; crystallinity; eruption pulses; ascent of magma; crystal mush; 溶岩ドーム

This research focuses on magmatic processes in magma reservoir and conduit.Porphyritic volcanic rocks contain glomerocrysts and the fragments as phenocrysts. This suggests their entrapment from crystal mush zone in the reservoir margins. Dacite of the latest eruption at Unzen is an example of such porphyritic volcanic rocks, which were mainly studied in this research. Texture of the phenocrysts and the chemical relationship between phenocrysts and groundmass including Sr and Nd isotopic ratios indicate their disequilibrium with the surrounding melt before and during eruption. At Unzen, phenocrysts probably represent crystals from the mushy zone of the magma reservoir, however, where the wall (intrusive rock) rocks were melted due to heating rather than crystallization of the melt occurred due to cooling. Distribution of isotope ratios in individual phenocrysts, with micro isotopic analysis near future, should prove the petrological model.During ascent of hydrous felsic magma in the conduit, melt's crystallization occurs due to degassing. The smaller the magma ascent rate, the more complete both water-loss from the melt and the increase in crystallinity. These facts cause the apparent viscosity of magma increase extremely. As a result, magma became too viscous to move in the top of the conduit. Fragmentation and compaction of foamy and brittle magma may occur easily, and the top of the conduit would be sealed with the compacted lava like a plug. Then, eruption stops. If such sealing is complete, successive crystallization below the plug builds up excess magmatic pressure, and eruption would resumes. The petrological characteristics of the Unzen dacite support this model. The higher the crystallinity of groundmass, the lower the rate of lava effusion. Fragmentation and compaction of foamy magma and the accompanied escape of volcanic gas may have generated low-frequency earthquakes that had occurred in the conduit top during eruption.

本文主要研究岩浆储库和岩浆通道中的岩浆作用。斑状火山岩中含有球粒晶和作为斑晶的碎屑。这表明它们是从储层边缘的晶体泥带中捕获的。云仙最近一次喷发的英安岩是这种斑状火山岩的一个例子，这是本研究的重点。斑晶的纹理和斑晶和基质之间的化学关系，包括Sr和Nd同位素比值表明它们与周围的熔体不平衡之前和喷发过程中。在云仙，斑晶可能代表来自岩浆库糊状区的晶体，然而，其中壁（侵入岩）岩石由于加热而熔化，而不是由于冷却而发生的熔体结晶。单斑晶的同位素比值分布及微量同位素分析结果证实了这一岩石学模式：长英质岩浆在通道中上升过程中，由于脱气作用，熔体发生结晶作用。岩浆上升速率越小，熔体失水和结晶度增加越完全。这些因素导致岩浆的表观粘度急剧增加。结果，岩浆变得过于粘稠，无法在管道顶部移动。泡沫状和脆性岩浆很容易破碎和压实，压实后的熔岩会像塞子一样将管道顶部封闭。然后，喷发停止。如果这种封闭作用完成，岩塞下面的连续结晶作用会产生过量的岩浆压力，火山喷发就会恢复。云仙英安岩的岩石学特征支持这一模式。基质的结晶度越高，熔岩喷出率越低。泡沫状岩浆的破碎和压实以及伴随的火山气体的逸出可能产生了在喷发期间在管道顶部发生的低频地震。

项目成果

Fujii, T.and S.Nakada: "The 15-September 1991 pyroclastic flows at Unzen Volcano (Japan) : a flowing model." Journal of Volcanology and Geothermal Research. Vol.89 (in press). (1999)

Fujii, T. 和 S.Nakada：“1991 年 9 月 15 日云仙火山（日本）的火山碎屑流：流动模型。”

Nakada, S.and Y.Motomura: "Petrology of the 1991-95 eruption at Unzen : effusion pulsation and groundmass crystallization." Journal of Volcanology and Geothermal Research.89(印刷中). (1999)

Nakada, S. 和 Y. Motomura：“1991-95 年云仙火山喷发的岩石学：喷发脉动和基质结晶。”火山学和地热研究杂志。89（出版中）。

Nakada, S.et al.: "Researchers Discuss Mt.Unzen, a Decade Volcano" EOS,Transactions, American Geophysical Union. Vol.78 No.45. 505-507 (1997)

Nakada, S.et al.：“研究人员讨论云仙山，十年火山”EOS，交易，美国地球物理联盟。

中田節也: "「マグマと地球」4年間続いた雲仙火山の噴火" 第12回「大学と科学」公開シンポジウム組織委員会編, 120-130 (1998)

中田节也：《岩浆与地球：云仙火山四年的喷发》第12届“大学与科学”公共研讨会组委会编辑，120-130（1998）

中田節也ほか: "雲仙国際ワークショップ報告" 火山. 42・3. 239-244 (1997)

Setsuya Nakata 等人：“云仙国际研讨会报告”42・3（1997）。