A sub-volcanic chemical pump: experimental and theoretical investigations of volatile element transport beneath volcanoes

火山爆发是由储存在地下岩浆体中的火山气体释放引起的。这些气体可以携带丰富的微量元素，其中一些被释放到大气中，一些通过次火山岩浆体重新分布，另一些则集中沉淀为热液矿床。因此，了解火山气体如何运输微量元素以及它们去了哪里，对于我们理解火山动力学和热液成矿都至关重要。这一建议建立在之前对美国圣海伦火山的大量研究工作的基础上。这项工作记录了一系列证据，证明某些挥发性微量元素，如Li和Pb，可以有效地从岩浆体的一部分转移到另一部分。为什么它们从岩浆体的某些部分释放出来，而在其他部分积聚，目前还不清楚。其原因是微量元素在火山气体中的溶解能力没有得到很好的限制。一个关键的未知因素是共存的硅酸盐熔体和火山气体之间微量元素的分配，这是压力、温度和气体成分的函数。一个复杂的问题是，一些气体从岩浆深处释放出来时是均匀的，但在较浅的压力下凝结成两个不同的相。同样，尽管有人提出这种类型的蒸汽冷凝是热液矿体形成的关键，但微量元素如何在这些分离相之间分配并没有得到很好的约束。根据我们在圣海伦斯火山的研究，我们假设在大喷发之后岩浆体中的压力下降可能首先导致蒸汽的凝结，然后随着岩浆在喷发结束时重新加压而使蒸汽重新溶解。这构成了一种新型的“化学泵”，我们想通过这个提议来测试它。为此，我们计划使用高压和高温实验来确定共存的熔体和蒸汽之间选定的微量元素的分配，并将其与火山晶体中被称为熔体内含物的微小玻璃球中捕获的微量元素的进一步研究结合起来。研究熔体包裹体的优点在于，它们在喷发前有效地捕获了包括微量元素在内的喷发前挥发性物质，从而为岩浆储存的条件以及气体在岩浆中如何移动提供了线索。我们的实验研究将与火山过程的动态模型相结合，使用由访问研究员开发的新的二维计算机代码。该代码在解释火山行为的许多神秘特征方面取得了相当大的成功，但以前从未应用于火山喷发期间和之后发生的气体逸出、气体再分配和气体保留问题。我们认为熔体包裹体中挥发性微量元素的浓度是次火山脱气过程的宝贵档案。解开这些档案需要综合的地球化学、岩石学和理论研究。

Explosive volcanic eruptions are driven by the release of volcanic gases stored in underground magma bodies. These gases can carry an abundance of trace elements, some of which are released into the atmosphere, some are redistributed through the sub-volcanic magma body and others are concentrated and precipitated as hydrothermal ore deposits. Understanding how volcanic gases transport trace elements and where they go is therefore of central importance in our understanding of both volcano dynamics and hydrothermal ore formation. This proposal builds on considerable previous work on Mount St. Helens volcano in the USA. This work has documented a number of lines of evidence that certain volatile trace elements, such as Li and Pb, are efficiently moved from one part of the magma body to another. Why they are released from some parts of the magma body and accumulate in others is not well understood. The reason for this is that the ability of trace elements to dissolve in volcanic gases is not well constrained. A key unknown is the partitioning of trace elements between coexisting silicate melts and volcanic gas as a function of pressure, temperature and gas composition. A compounding problem is that some gases are homogeneous when released from magma at depth but condense to two separate phases at shallower pressure. Again, how trace elements partition between these separate phases is not well constrained, although it has been proposed that this type of vapour condensation is key to the formation of hydrothermal ore bodies. Based on our studies at Mount St. Helens we have hypothesised that the pressure drop in a magma body following major eruptions may lead first to condensation of vapour and then the redissolving of this vapour as the magma becomes repressurised at the end of the eruption. This constitutes a novel sort of 'chemical pump', which we want to test out by means of this proposal. To do this we plan to use high pressure and temperature experiments to determine the partitioning of selected trace elements between coexisting melt and vapour and to combine this with further studies of trace elements trapped in tiny globules of glass, known as melt inclusions, in volcanic crystals. The virtue of studying melt inclusions is that they effectively trap the pre-eruptive volatile inventory, including trace element, prior to eruption and therefore hold clues to the conditions under which magma was stored and how gases move around within this magma. Our experimental studies will be allied to dynamical models of volcanic processes using a new 2D computer code developed by the Visiting Researcher. This code has had considerable success in explaining many enigmatic features of volcano behaviour but has never before been applied to the problems of gas escape, gas redistribution and gas retention that occur during and after an eruption. We propose that the concentration of volatile trace elements in melt inclusions is a valuable archive of sub-volcanic degassing processes. Unlocking this archive requires the kind of integrated geochemical, petrological and theoretical study proposed here.