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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption, New Zealand
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Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption, New Zealand

机译：区分潜水岩浆和岩浆岩灰的危险；对2012年8月6日山火山爆发机制的见解。新西兰汤加里罗喷发

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摘要

The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lack of obvious juvenile components in its proximal ballistic deposits imply that the eruption was caused by the sudden decompression of a sealed, hot hydrothermal system. Strong magmatic signals in pre- and post-eruption gas emissions indicate that fresh magma had intruded to shallow levels shortly before this eruption. Here we examine the volcanic ash produced during the August eruption with the aim of determining whether juvenile magma was erupted or not The widely applied criteria for identifying fresh juvenile pyroclasts provided inconclusive results. The Te Maari ash sorting and trend towards a unimodal grain-size distribution increase with distance along the dispersal axis. Proximal to intermediate sites showing polymodal grain-size distributions can be related to the re-fragmentation of different pre-existing lithologies, overlapped erupted pulses and transport mechanisms, and to particle aggregation. Between 69 and 100 vol.% of particles coarser than 3 φ and 45-75 vol.% of grains finer than 3 φ were sourced from the pre-existing, commonly hydrothermally altered, vent-area lavas and pyroclasts. Free crystals (pyroxene ＞ plagioclase ＞ magnetite ＞ pyrite) make up 0-23 vol.% of particles coarser than 3 φ, and 22-41 vol.% of grains finer than 3 φ. Brown to black fragments of fresh glass are a small (1-15 vol.%), but notable, component Under SEM, these blocky, glassy particles are poorly vesicular, and irregularly shaped, some with flu-idal or bubble-wall surfaces, and others with fragmented stepped surfaces and fine adhering ash. In thin section, they contain variable amounts of microlites within an isotropic groundmass. The range in silica content of the microprobe-analysed glass is very wide (56-77 wt.%) and cannot be correlated to any specific particle textural type. These chemically and texturally diverse glassy fragments are identical to mechanically broken pieces of country rock lavas and pyroclasts; both their diversity, and their match with vent country rocks, argue strongly against a "juvenile" origin for the glassy fragments. We conclude that rising magma provided only heat and gas into the overlying, sealed vapour-dominated hydrothermal system. A landslide from this area led to a rapid decompression and ash was produced by top-down hydrothermal explosions. Careful attention must be paid to the combination of compositions and textures of fine ash particles in such situations, as well as to the context of their source vent, in order to be confident that new magma has reached the surface.
机译：2012年8月6日蒂玛阿里火山喷发的弱地球物理前兆。汤加里罗（Tongariro）以及在其近端弹道沉积物中缺乏明显的幼体成分，表明火山喷发是由密封的热液热系统突然减压引起的。喷发前和喷发后气体排放中的强烈岩浆信号表明，在喷发前不久，新鲜岩浆侵入到浅层。在这里，我们检查了八月喷发过程中产生的火山灰，目的是确定是否喷出了少年岩浆。用于鉴定新鲜的少年火成岩的广泛应用的标准提供了不确定的结果。沿着分散轴的距离，Te Maari灰的分选和单峰粒度分布的趋势会增加。显示多峰粒度分布的近端至中间位点可能与不同的现有岩性的重新碎裂，重叠的喷发脉冲和传输机制以及颗粒聚集有关。大于3φ的颗粒中有69至100体积％的颗粒，小于3φ的颗粒中有45至75体积％的颗粒来自先前存在的，通常经过热液改造的排风熔岩和火山碎屑。游离晶体（辉石>斜长石>磁铁矿>黄铁矿）占0-23 vol。％大于3φ的颗粒，占22-41 vol。％小于3φ的颗粒。新鲜玻璃的棕色至黑色碎片很小（占体积的1-15％），但值得注意的是，在SEM下，这些块状玻璃状颗粒的囊泡性很差，形状不规则，有些带有流体壁或气泡壁表面，以及其他具有碎裂的阶梯状表面和细密的灰烬。在薄截面中，它们在各向同性地基内包含数量可变的微岩。经微探针分析的玻璃的二氧化硅含量范围非常宽（56-77重量％），并且不能与任何特定的颗粒质地类型相关。这些化学和质地不同的玻璃碎片与机械破碎的乡村岩石熔岩和火山碎屑相同；它们的多样性以及与发泄的乡村岩石的匹配都强烈反对玻璃碎片的“少年”起源。我们得出的结论是，上升的岩浆仅将热量和气体提供给上覆的，密封的，以蒸气为主的热液系统。来自该地区的滑坡导致了快速的减压，并且自上而下的热液爆炸产生了灰烬。在这种情况下，必须特别注意细灰颗粒的组成和质地的组合，以及它们的源喷口，以确保新的岩浆已经到达表面。

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  • 来源
    《Journal of Volcanology and Geothermal Research》 |2014年第1期|397-414|共18页
  • 作者

    Natalia Pardo; Shane J. Cronin; Karoly Nemeth; Marco Brenna; C. Ian Schipper; Eric Breard; James D.L. White; Jonathan Procter; Bob Stewart; Javier Agustin-Flores; Anja Moebis; Anke Zernack; Gabor Kereszturi; Gert Lube; Andreas Auer; Vince Neall; Clel Wallace;

  • 作者单位

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand,Japan Agency for Marine Earth Science and Technology (JAMSTEC) 2-15 Natsushima-cho Yokosuka, Kanagawa 237-0061, Japan;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Geology Department, University of Otago, PO Box 56, Dunedin 9054, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

    Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand;

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  • 原文格式 PDF
  • 正文语种 eng
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  • 关键词

    Te Maari; Hydrothermal; Juvenile; Tephra; Molten-fuel coolant interaction; Glass;

    机译：Te Maari;水热少年;特非拉熔融燃料冷却液的相互作用;玻璃;

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