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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars
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Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars

机译：小体积玄武岩火山的浅水管道系统，2：火山灰锥和马尔斯地壳异岩的证据

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Monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood. This paper explores these relationships, building upon earlier studies of exposed shallow plumbing (in the upper hundreds of meters of crust) beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes, such as scoria cones and maars, respectively. Eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the San Francisco Volcanic Field (Arizona, USA). Well-constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types. Sedimentary xenolith contents at scoria cones are <10~(-3) and commonly <10~()-4 (volume fraction of xenoliths) and are composed almost entirely of fragments from the uppermost sedimentary formation (Kaibab Formation, depth interval ~50-200 m). These xenolith contents are consistent with conduits or dikes that widen mainly in the uppermost tens of meters of the crust, as observed at exposed plumbing systems of eroded scoria cones. Sedimentary xenolith contents in tephra ring deposits at one of the studied maar volcanoes also are typically <10"3 (volume fraction) and the remaining fraction is dominated by dasts of pre-maar volcanic rocks that formed an -50 thick surface layer over the sedimentary formations; the second studied maar has much higher xenolith contents but this also appears to be dominated by the shallowest unit. The maars' tephra ring deposits contain xenoliths from all of the major sedimentary units beneath the volcanoes (to depths of ~1200 m) but the abundances and proportions of xenoliths are not consistent with the volumes of sub-volcanic units that would be disrupted assuming dimensions that are commonly observed in exposed maar plumbing systems (diatremes). These differences illustrate the different mechanisms for conduit/dike widening in magmatic versus phreatomagmatic eruptions. Namely, ascending eruptive mixtures driven by magmatic volatiles widen their relatively shallow conduits by erosion and mechanical failure of the walls and ejection of the resulting xenoliths. Maar-forming eruptions produce wide and deep diatremes mainly by mechanical disruption of country rock during many discrete magma-water explosions at varying depths in the subsurface. Deep explosions cause debris jets that may not erupt, and the disrupted country rock and juvenile material gradually churns and mixes within the diatreme. Only a small fraction of the debris is ejected from the crater by especially strong and/or shallow explosions, to form tephra ring deposits.
机译：单相玄武质火山记录了复杂的喷发过程，人们对这些过程与浅埋管道之间的关系知之甚少。本文以早期的研究为基础，探索了这些关系，这些研究是由岩浆喷发过程和喷发岩浆过程所产生的火山下面的裸露浅层管道（在地壳的上百米处）和火山喷发过程所产生的，分别是鳞屑锥和泥浆。在美国旧金山火山场（美国亚利桑那州）的三个火山岩锥和特菲拉环上的两个马氏岩周围描述了喷发相和异岩体的丰度。受到严格约束的火山下沉积地层对不同类型的异岩的起源深度提供了限制。渣ria锥上的沉积异岩体含量<10〜（-3），通常<10〜（）-4（异岩体的体积分数），几乎全部由最上层沉积岩层（Kaibab组，深度间隔〜50-）组成。 200 m）。这些异岩内容物与导管或堤坝一致，导管或堤坝主要在地壳的最上层数十米处变宽，如在侵蚀的鳞屑锥的裸露管道系统中观察到的那样。在其中一个研究的玛尔火山中，特非拉环沉积物中的沉积异物含量通常也小于10“ 3（体积分数），其余部分由前玛尔火山岩的碎屑所占主导，在沉积物中形成了-50厚的表层玛尔的特非拉环沉积物中含有来自火山下面所有主要沉积单元（约1200 m深度）的玛瑙，但第二个研究的玛尔的异岩体含量高得多，但似乎也被最浅的单元所控制。假设在裸露的马勒管道系统（直径）中通常观察到的尺寸，异种岩的丰度和比例与将被打断的亚火山单元的体积不一致，这些差异说明了岩浆岩和岩脉中导管/堤坝拓宽的不同机制岩浆喷发：即由岩浆挥发物驱动的上升喷发混合物，由于侵蚀作用而扩大了其相对较浅的导管n以及壁的机械故障和生成的异种石的喷出。形成Maar的喷发主要是由于在地下深处不同深度发生的许多不连续的岩浆水爆炸过程中，对乡村岩石的机械破坏而产生了广泛而深刻的危害。深度爆炸会造成碎片喷流，这些喷流可能不会爆发，而破裂的乡村岩石和少年物质会逐渐在杂物间搅动和混合。特别是强烈和/或较浅的爆炸，只有一小部分碎片从火山口中喷出，形成蒂法拉环沉积物。

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