The Timing and Nature of Volcanological Processes as Captured by Bimodal Composition Eruptions

一个关键的，但仍然没有完全解决火山参数是涉及地壳过程导致岩浆演化和喷发的时间段。 这些可能在不同风格、构造背景和成分的岩浆系统中差异很大。有许多方法可以潜在地获得地壳岩浆过程的时间尺度。 这一建议提出了一种新的方法，根据U系列不平衡年龄之间的原始和演化的岩浆同时爆发的对比。 这种“双峰”喷发的证据在火山学文献中已经司空见惯。所涉及的岩浆可能是通过各种过程和一系列时间尺度形成的，这是本次调查的一个重要方面。三个相互关联的问题是：（a）岩石学：形成演化岩浆的机制;（B）火山学：触发较冷、粘性、化学演化岩浆（英安岩和流纹岩）的喷发;（c）时间尺度：这些过程在不同环境中需要多长时间。这项研究将审查岩浆演化和几种提出的利用双峰组合物喷发引发火山喷发的机制之一（即玄武岩注入触发机制）。 这种喷发提供了复杂和不断演变的岩浆系统过程的独特快照，因为两个端元岩浆同时喷发，因此在空间和时间上密切相关。 三个工作假设将测试有关的时间尺度和过程中捕获的4个历史（或地质最近）双峰组成喷发，被认为是代表各种方案的岩石学演化和镁铁质岩浆的作用，造成或协助化学演化岩浆喷发。 要研究的火山爆发是：1875年阿斯克亚火山; 1724年克拉夫拉火山; 1902年圣玛利亚火山; 1943-1952年帕里克斯火山。虽然有许多其他例子（例如，维苏威火山、赫克拉火山、波波卡特佩特火山、科利马火山、诺瓦鲁普塔火山、圣海伦斯，内华达德尔鲁伊斯，蒙特塞拉特，皮纳图博），这里提出的火山爆发被选中，因为他们已经在以前的火山学和岩石学研究，有很好的时间控制喷发本身，有一些接近组成的端元物理分离浮石或熔岩，并有可用的样品类型适合与U系列不平衡的内部矿物等时线。 将对喷发沉积物进行火山学检查，并对样品进行岩石学和地球化学检查，以尽可能全面地了解所涉及的火山/岩浆过程。 还将审查第五次喷发（大约1305年新西兰塔拉韦拉山“Kaharoa”喷发）的时间。

RubinEAR-0106463One critical yet still incompletely resolved volcanic parameter is the time period involved in crustal processes leading to both magmatic evolution and eruption. These likely vary greatly amongst magmatic systems of differing style, tectonic setting and composition. There are a number of ways to potentially get at the time scales of crustal magmatic process. This proposal suggests a novel approach based upon U-series disequilibrium age contrasts between primitive and evolved magmas that erupted simultaneously. Evidence for such " bimodal" composition eruptions has become commonplace in the volcanological literature. The magmas involved may have formed through a variety of processes and over a range of time scales, which is an important aspect of this investigation. Three inter-related issues are: (a) petrological: mechanism of formation of evolved magmas; (b) volcanological: eruption triggering of relatively cool, viscous, chemically evolved magmas (dacite and rhyolite); and c) time scales: how long do these processes take in different settings. This study would examine both magmatic evolution and one of several proposed mechanisms for initiating silicic eruptions (i.e. the basalt injection trigger) using bimodal composition eruptions. Such eruptions provide a unique snapshot of processes in complex and evolving magmatic systems because the two end members magmas were erupted simultaneously and therefore are closely-related in space and time. Three working hypotheses would be tested regarding time scales and processes captured in 4 historical (or geologically very recent) bimodal-composition eruptions that are believed to represent various scenarios for petrological evolution and the role of mafic magma in causing or aiding in eruption of chemically evolved magmas. The eruptions to be studies are: 1875 Askja; 1724 Krafla; 1902 Santa Maria; and 1943-1952 Paricutin. Although there are numerous others examples (e.g., historical or recent eruptions of Vesuvius, Hekla, Popocatepetl, Colima, Novarupta, Mt. St. Helens, Nevado del Ruiz, Montserrat, Pinatubo), the eruptions proposed here were picked because they have been previously well-studied volcanologically and petrologically, have excellent temporal control on the eruptions themselves, have something close to compositional end members physically separable from pumices or lavas, and have available sample types suited for dating with U-series disequilibrium by internal mineral isochrons. Eruption deposits will be examined volcanologically, and samples will be examined petrographically and geochemically to arrive at as complete a picture as possible for the volcanic/magmatic processes involved. Time permitting a 5th eruption (the circa 1305 Mt Tarawera "Kaharoa" eruption in New Zealand) would also be examined.