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Petrological research on magma behavior during volcanic eruptions

火山喷发过程中岩浆行为的岩石学研究

基本信息

批准号：
12304033
负责人：
NAKADA Setsuya
金额：
$ 29.88万
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 (A)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2002
项目状态：
已结题

项目摘要

Decompression and degassing process during volcanic eruptions magma becomes vesiculated and crystallization of melt occur. As a result, diversities of melt compositions and rock texture appear. In the research, we aimed at understanding of magmatic behaviors during eruptions, by reading the above petrological characteristics. Samples from Miyakejima, Usu, and Unzen volcanoes were investigated.During the 2000 eruption at Miyakejima, basaltic andesite magma that generated eruption was replaced with phyric basalt magma in the early August. Melt inclusions from these magmas with different bulk compositions have the chemistry not distinct largely to each other, including volatile contents. The method to calculate oxygen fugacity on volcanic glass was developed, and the pressure was estimated to be near NNO for both magmas. Grain size analysis of the tephra showed large fragmentation and small dispersion, characterizing phreatomagmatic eruption rather than magmatic eruption.The magma ascent speed was estimated roughly, based on crystallinity and vesicularity of the 2000 eruption products at Usu ; where.two-stage vesiculation events were proposed. As fragmentation of lavas occurred before and after quenching near surface, petrological distinguishing of one from the another becomes important for discussion.The extents of crystallinity in groundmass and degassing, the latter of which was estimated from porosity of lavas, are linked intimately to depths of low-frequency earthquakes during the 1991-95 eruption at Unzen. We expected the genetic relationship of low-frequency earthquakes with fragmentation of foamy magma and the associated flowage of volatiles. We introduced a model in which magma more enriched in volatile can be broken by larger strain rate, accompanied by high decompression rate.

火山喷发时的减压脱气过程使岩浆发生囊化，熔体发生结晶。因此，熔体成分和岩石结构呈现多样性。在研究中，我们旨在通过阅读上述岩石学特征来了解火山喷发期间的岩浆行为。研究人员对三宅岛、乌苏和云仙火山的样本进行了调查。在2000年的三宅岛火山喷发中，产生喷发的玄武岩安山岩岩浆在8月初被玄武岩岩浆取代。这些岩浆的熔融包裹体具有不同的体积组成，它们的化学成分（包括挥发性成分）在很大程度上并不不同。建立了火山玻璃上氧逸度的计算方法，估计两个岩浆的压力接近NNO。岩屑粒度分析显示碎裂大、分散小，具有岩浆喷发而非岩浆喷发的特征。根据2000年乌苏火山喷发产物的结晶性和囊泡性，粗略估算了岩浆上升速度；在哪里。提出了两阶段的起泡事件。由于熔岩在近地表淬火前后都有碎裂，因此岩石学上的区分就变得非常重要。地面物质的结晶度和脱气程度（后者是根据熔岩的孔隙度估计的）与1991-95年云岑火山喷发期间低频地震的深度密切相关。我们期望低频地震与泡沫岩浆破碎和相关挥发物流动的成因关系。本文提出了一种大应变速率、高减压速率的高挥发性岩浆破碎模型。