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首页> 外文期刊>Journal of Volcanology and Geothermal Research2012V243-244NOCT,15 >Insights into the dynamics of mafic magmatic-hydromagmatic eruptions from volatile degassing behaviour: The Hverfjall Fires, Iceland
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Insights into the dynamics of mafic magmatic-hydromagmatic eruptions from volatile degassing behaviour: The Hverfjall Fires, Iceland

机译：挥发性脱气行为对镁铁质岩浆-岩浆喷发动力学的洞察力：冰岛Hverfjall火灾

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摘要

The style and intensity of hydromagmatic activity is governed by a complex interplay between the relative volumes of magma and water that interact, their relative viscosities, the depth of subsurface explosions, the substrate properties, and the vent geometry. Fundamental questions remain, however, regarding the role of magmatic vesiculation in determining the dynamics of magma-water interaction (MWI). Petrological reconstructions of magmatic degassing histories are commonly employed to interpret the pre- and syn-eruptive conditions during 'dry' magmatic eruptions, but the application of similar techniques to hydromagmatic activity has not yet been fully explored. In this study, we integrate glass volatile measurements (S, CI, H2O and CO2) with field observations and microtextural measurements to examine the relationship between degassing and eruptive style during the Hverfjall Fires fissure eruption, Iceland. Here, coeval fissure vents produced both 'dry' magmatic (Jarobaosholar scoria cone complex) and variably wet hydromagmatic (Hverfjall tuff ring) activity, generating physically distinct pyroclastic deposits with contrasting volatile signatures. Matrix glass volatile concentrations in hydromagmatic ash (883 +/- 172 [1 sigma] ppm S; 0.45 +/- 0.03 [1 sigma] wt% H2O;= 20 ppm CO2) are consistently elevated relative to magmatic ash and scoria lapilli (418 +/- 93 [1 sigma] ppm S; 0.12 +/- 0.48 Pal wt% H2O; CO2 below detection) and overlap with the range for co-erupted phenocryst-hosted melt inclusions (1522 +/- 127 [1 sigma] ppm S; 165 +/- 27 [1 sigma] ppm CI). Measurements of hydromagmatic glasses indicate that the magma has degassed between 17 and 70% of its initial sulfur prior to premature quenching at variably elevated confining pressures. By comparing volatile saturation pressures for both magmatic and hydromagmatic glasses, and how these vary through the eruptive stratigraphy, we place constraints on the conditions of MWI. Crucially, our data demonstrate that the magma was already vesiculating when it encountered groundwater at depths of 100-200 m, and that the external water supply was sufficient to maintain MWI throughout the eruption with no significant vertical or lateral migration of the fragmentation surface. We propose that development of an in-vent water sediment slurry provides a mechanism through which the elevated confining pressures of similar to 1.6-2.6 MPa (or up to 6 MPa accounting for uncertainty in CO2 below analytical detection) could be maintained and buffered throughout the eruption, whilst enabling vertical mixing and ejection of fragmented juvenile and lithic material from a range of depths. Importantly, these results demonstrate that the volatile contents of hydromagmatic deposits provide valuable records of (1) the environment of MWI (e.g., groundwater versus surface water, vertical migration of the fragmentation level) and (2) the state of the magma at the time of fragmentation and quenching. We further suggest that the volatile content of tephra glasses provides a reliable alternative (or additional) indicator of a hydromagmatic origin, particularly for reduced Ocean Island Basalts where late-stage volatile saturation and degassing (S, H2O) occurs over a pressure range relevant to typical MWI environments. (C) 2018 Elsevier B.V. All rights reserved.
机译：岩浆活动的类型和强度取决于相互作用的岩浆和水的相对体积，它们的相对粘度，地下爆炸的深度，基质的性质和喷口的几何形状之间的复杂相互作用。然而，关于岩浆囊泡作用在确定岩浆-水相互作用（MWI）动力学中的作用仍然存在根本问题。岩浆脱气历史的岩石学重建通常被用来解释“干”岩浆喷发期间的喷发前和喷发条件，但尚未充分探索类似技术在水岩浆活动中的应用。在这项研究中，我们将玻璃挥发物测量值（S，CI，H2O和CO2）与野外观测和微观结构测量结果进行了综合，以研究冰岛Hverfjall火灾喷发期间脱气与喷发样式之间的关系。在此，同卵裂隙喷口既产生“干”岩浆（Jarobaosholar灰岩锥复合体）活动，又产生不同程度的湿水岩浆（Hverfjall凝灰岩环）活动，产生了物理上明显的火山碎屑沉积物，形成了鲜明的挥发性特征。相对于岩浆灰和角砾石lap（883 +/- 172 [1 sigma ppm S; 0.45 +/- 0.03 [1 sigma] wt％H2O; <= 20 ppm CO2），基体玻璃的挥发物浓度持续升高（ 418 +/- 93 [1 Sigma ppm S; 0.12 +/- 0.48 Pal wt％H2O； CO2低于检测值]，并且与共同出晶的隐晶母熔体夹杂物的范围重叠（1522 +/- 127 [1σ]） ppm S； 165 +/- 27 [1 Sigma ppm CI）。对水力岩浆玻璃的测量表明，岩浆在变高的围压下进行过早淬火之前，已对其初始硫的17％至70％进行了脱气。通过比较岩浆玻璃和岩浆玻璃的挥发性饱和压力，以及它们在喷发地层中的变化，我们对MWI的条件施加了限制。至关重要的是，我们的数据表明，岩浆在遇到100-200 m深度的地下水时已经处于囊泡状态，并且外部供水足以在整个喷发过程中维持MWI，而破碎面没有明显的垂直或横向移动。我们认为，开发本发明的水沉积泥浆提供了一种机制，通过该机制可以在整个过程中维持和缓冲类似于1.6-2.6 MPa（或高达6 MPa的高限制围压，考虑到低于分析检测的二氧化碳不确定性）。喷发，同时可以从一定深度垂直混合和碎片化的少年和石质物质的喷射。重要的是，这些结果表明，水成岩沉积物的挥发性含量提供了以下方面的有价值的记录：（1）MWI的环境（例如，地下水与地表水，碎片水平的垂直迁移）和（2）当时的岩浆状态破碎和淬灭。我们进一步建议，提夫拉玻璃的挥发物含量是水磁成因的可靠替代（或附加）指标，特别是对于减少的大洋洲玄武岩而言，后期挥发物饱和和脱气（S，H2O）发生在与压力有关的压力范围内。典型的MWI环境。 （C）2018 Elsevier B.V.保留所有权利。

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