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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Structure of the shallow magma chamber of the active volcano Mt. Zao, NE Japan: Implications for its eruptive time scales
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Structure of the shallow magma chamber of the active volcano Mt. Zao, NE Japan: Implications for its eruptive time scales

机译：活火山的浅岩浆室结构。日本东北部藏王市：对爆发时间尺度的影响

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Zao volcano is one of the representative active stratovolcanoes in northeast Japan, having many historical eruption records, as the last one occurred in 1940. Since 2013 to present, precursory phenomena, such as volcanic tremors and deformation of the summit area, have been detected in this volcano; thus, it is urgently necessary to thoroughly characterize the magma feeding system beneath the volcano. Our aim is to understand the magma chamber structure and dynamics associated with the recent activity (ca. similar to 2 ka) of Zao volcano in NE Japan. We conducted petrological and geochemical studies in the representative pyroclastic products and youngest lavas of its recent activity. The products are divided into the GSN (Goshikidake-nanbu) pyroclastics, GST (Goshikidake-tobu) pyroclastics and GS lavas (Furikodaki lava and Goshikidake-nanpo lava and associated pyroclastic deposits).The rock samples are calc-alkaline olivine-bearing pyroxene andesites originated as the result of mixing between felsic (containing Mg-poor pyroxenes + anorthite (An)-poor plagioclases) and mafic (containing forsterite (Fo)-rich olivines + An-rich plagioclases magmas). Thermodynamic modelling indicates that the temperature (T)-pressure (P)-water activity (H2O) conditions of the felsic (ca. 60-61 wt% SiO2, 968-1000 degrees C, 1.6-2.0 kbar and 1.8-2.2 wt% H2O) and the mafic (ca. 48.0-50.5 wt% SiO2, 1113-1170 degrees C, 2.4 kbar and 1.0-2.2 wt% H2O) end members are similar between the GSN, GST pyroclastics and GS lavas.Compositional zoning of orthopyroxene phenocrysts suggests the formation of two mixed magmas, namely the main mixed and the Mg-rich mixed ones. The mafic magma from a deeper source entered repeatedly into a shallower felsic magma chamber. As a result, the main mixed magma developed within the chamber. The Mg-rich-mixed magma tentatively developed by the injection and simultaneous mixing of both the main mixed magma and the injected mafic one, merging later on into the main mixed magma.The Fe-Mg diffusion modelling on orthopyroxenes reveal that the injections occurred from 60 days to 1.7 years for thin-darker-zone types, and multiple injections of mafic magma occurred from 2.5 years to 150 years for broad darker-zone types until the eruptions. The thin-darker-zone type orthopyroxene phenocrysts are abundant in the GSN and GST pyroclastics but rare in the GS lavas. Instead, the broad-darker-zone type orthopyroxenes are rare in the GSN and GST pyroclastics and abundant in the GS lavas. Thus, the main mixed layer was more evolved for the GS lavas than for the GSN and GST pyroclastics. The injected mafic magmas for the GS lavas were merged more extensively than the GSN and GST pyroclastics, resulting in a larger release of volatile components that triggered the effusive episodes. Given that local precursory phenomena have been currently detected in Zao volcano, we are confident that any research to thoroughly characterize the magma feeding system under this active volcano is essential for society to better react to any potential volcanic hazard in the near future. (C) 2019 Elsevier B.V. All rights reserved.
机译：Zao火山是日本东北部代表性的活跃平流火山之一，有许多历史喷发记录，最后一次是1940年发生的。自2013年至今，先兆现象如火山震颤和山顶区域变形等已被发现。这个火山因此，迫切需要对火山下方的岩浆供给系统进行全面描述。我们的目的是了解与日本东北部Zao火山近期活动（约2 ka）相关的岩浆腔结构和动力学。我们对代表性的火山碎屑产品及其最近活动的最年轻熔岩进行了岩石和地球化学研究。产品分为GSN（Goshikidake-nanbu）火山碎屑岩，GST（Goshikidake-tobu）火山碎屑岩和GS熔岩（Furikodaki熔岩和Goshikidake-nanpo熔岩及相关的热碎屑矿床）。起源于长英质（含镁含量低的辉石+钙长石（An）的斜长石）和镁铁质（含镁橄榄石（Fo）的富橄榄石+富含鞍石的长石浆）混合而成。热力学模型表明长英质的温度（T）-压力（P）-水活度（H2O）条件（约60-61 wt％SiO2、968-1000摄氏度，1.6-2.0 kbar和1.8-2.2 wt％在GSN，GST火山碎屑岩和GS熔岩之间，镁铁质（约48.0-50.5 wt％SiO2、1113-1170摄氏度，<2.4 kbar和1.0-2.2 wt％H2O）的端基相似。隐晶形成两个混合岩浆，即主要混合岩浆和富镁混合岩浆。来自较深来源的镁铁质岩浆反复进入较浅的长英质岩浆室。结果，在室内形成了主要的混合岩浆。通过注入和同时混合主要混合岩浆和注入的镁铁质岩浆，初步开发出富含镁的混合岩浆，然后合并到主要混合岩浆中。在邻辉石的Fe-Mg扩散模型表明，注入发生于对于较暗的区域类型，<60天到1.7年不等，对于较深的较深区域类型，铁素体岩浆多次注入发生在2.5年到150年之间，直到喷发为止。 GSN和GST火山碎屑岩中存在较暗的暗区型邻苯二酚酚醛树脂，而GS熔岩中稀有。取而代之的是，在GSN和GST火山碎屑中稀有较暗的区域型邻苯二茂，而在GS熔岩中则丰富。因此，与GSN和GST火山碎屑岩相比，GS熔岩的主要混合层演化更多。与GSN和GST火山碎屑岩相比，注入GS熔岩的铁镁质岩浆合并的范围更大，导致释放大量挥发性成分，从而触发了喷发性发作。鉴于目前在Zao火山中已经发现了局部先兆现象，因此我们有信心对这一活跃火山下的岩浆供给系统进行全面表征的任何研究对于社会在不久的将来更好地应对任何潜在的火山灾害至关重要。 （C）2019 Elsevier B.V.保留所有权利。

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