Explosive origin of silicic lava: Textural and δD-H2O evidence for pyroclastic degassing during rhyolite effusion
Explosive origin of silicic lava: Textural and δD-H2O evidence for pyroclastic degassing during rhyolite effusion
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DOI：
10.1016/j.epsl.2014.08.012
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发表时间：
2014-11-01
影响因子：
5.3
通讯作者：
Schipper, C. Ian
中科院分区：
文献类型：
--
作者：
Castro, Jonathan M.;Bindeman, Ilya N.;Schipper, C. Ian
火山学中一个长期存在的挑战是解释为什么岩浆的爆炸性喷发会让位给熔岩。一个被广泛引用的观点是，爆炸性到溢出性的转变体现了两阶段脱气历史，其中熔岩是初始爆炸性、封闭系统脱气后非爆炸性、开放系统气体释放的产物。对流纹岩喷发的直接观察表明，喷涌流纹岩实际上是高度爆炸性的，因为它们与猛烈的火山爆发和火山碎屑喷泉同时从一个共同的喷口喷发数月。这种爆炸性和溢出性的重叠表明，火山碎屑过程在使硅质岩浆充分脱气以生成熔岩方面发挥着关键作用。在这里，我们使用精确的H-同位素和岩浆H2O的测量和纹理的证据来证明，从爆炸（S）-熔岩渗出的结果是脆性变形的直接产物，促进批量脱气到瞬态火山碎屑通道（凝灰岩），重复和爆炸性地从流出的熔岩排气。我们的测量结果表明，具体地说，D/H比和H2O含量的一个广泛的套件爆炸和溢出样品柴滕火山（含水炸弹，普林尼火山碎屑，凝灰岩脉，熔岩）定义了一个单一的和连续的脱气趋势，连接湿爆炸火山碎屑（类似于1.6重量%）H2O，Δ D = -76.4700）至干黑曜石熔岩（类似于0.13重量% H2O，Δ D = -145.7份/千）。这种地球化学模式是最适合分批脱气模型，包括小的重复封闭系统脱气步骤，然后脉冲的蒸汽提取。这种脱气机制是通过凝灰岩脉的作用而实现的，凝灰岩脉通过抽取已经呈泡状或角砾状的岩浆，使一批批溶出的气体能够迅速地、爆炸性地从相对孤立的封闭系统域和大片的导管岩浆中逸出，从而使它们具有远距离连通性。尽管凝灰岩脉使岩浆脱气并能够流出，但它们仍然是剧烈气体和粒子输送的途径，因此当它们被堵塞或焊接关闭时，有可能驱动爆炸。因此，传统上被认为是相对温和的熔岩喷发过程，呈现出一种特别危险的火山爆发形式。(C)2014爱思唯尔有限公司版权所有。
A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava. Here we use precise H-isotope and magmatic H2O measurements and textural evidence to demonstrate that effusion results from explosion(s)-lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels (tuffisites) that repetitively and explosively vent from effusing lava. Our measurements show, specifically that D/H ratios and H2O contents of a broad suite of explosive and effusive samples from Chaiten volcano (hydrous bombs, Plinian pyroclasts, tuffisite veins, and lava) define a single and continuous degassing trend that links wet explosive pyroclasts (similar to 1.6 wt.% H2O, delta D = -76.4700) to dry obsidian lavas (similar to 0.13 wt.% H2O, delta D = -145.7 parts per thousand). This geochemical pattern is best fit with batched degassing model that comprises small repeated closed-system degassing steps followed by pulses of vapour extraction. This degassing mechanism is made possible by the action of tuffisite veins, which, by tapping already vesicular or brecciated magma, allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed-system domains and large tracts of conduit magma by giving them long-range connectivity. Even though tuffisite veins render magma degassed and capable of effusing, they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut. Thus the effusion of silicic lava, traditionally thought to be relatively benign process, presents a particularly hazardous form of explosive volcanism. (C) 2014 Elsevier B.V. All rights reserved.