Magma mush eruptibility: the lifetime of mobile magma

火山作用--地球内部熔融岩石的产生和喷发--是板块构造最明显的表现之一。地壳的增长发生在岩浆在地壳内储存和凝固，或者在地球表面喷发。俯冲带的喷发活动可能具有爆炸性和高度破坏性，是一种重要的自然灾害，一旦发生，将对生命、健康和金融稳定产生影响。火山学家面临的主要挑战之一是准确预测这种喷发行为。过去火山活动的大量证据表明，大量的岩浆可以在一次事件中喷发。然而，用于地球表面以下成像的地球物理技术无法区分储存在地壳内的大量熔体（岩浆液体）。相反，熔化物可能被储存为“晶体泥”，即火山晶体的积累，仅被少量的熔化物分离，难以成像。然而，具有低熔体含量的晶体糊状物表现得像固体并且不能喷发。因此，研究人员认为，这些糊状物含有“可喷发”的透镜体，它们具有较高的熔体含量，但仍然足够薄，无法通过地球物理技术来解析。如果是这样的话，那么整个糊状区域中晶体和液体的大规模空间重组可能会改变其整体物理行为，从而使其迅速变得可爆发。与此相反，其他学者预测地壳内更富含液体（可能爆发）的糊状物体的长期存在。在这种情况下，缺乏目前观察到的地球物理信号的大型，熔体丰富的岩浆体可能只是由于岩浆活动的短暂性。为了取得进展，关于可喷发糊状区域的寿命的更多信息至关重要。这项提议将开发一种新的方法来确定熔融富集区的寿命，使我们能够解决目前的冲突。关于火山系统的时间“年代学”信息通常记录在矿物锆石中，其中含有对时间敏感的放射性元素。锆石年代学表明，水晶泥可以持续很长一段时间（例如100万年），但这些测量结果具有很大的不确定性。更易喷发、富含熔体的“移动的岩浆”的寿命更难研究，因为它发生在锆石可能不稳定的较高温度下。然而，这些信息是地球物理观测和火山学之间的关键联系，地球物理观测记录了地壳状态的快照，火山学记录了很长一段时间内岩浆过程的信息。该项目将开发一种新方法，通过分析高温下从熔体中生长的晶体，直接确定移动的岩浆结晶的寿命。具体来说，我们将涉及的硅酸盐矿物斜长石，几乎所有的俯冲带岩浆生长的纵横比（长度/宽度），结晶可用的时间。我们的初步工作表明，在俯冲带喷发的富水、富硅岩浆的纵横比和时间之间存在很强的关系。利用高温实验、对年代久远的斜长石晶体的分析和数学方法，研究小组将得出一个适用于所有岩浆环境的普遍关系。我们将应用该方法的中间俯冲带火山系统，有最近的地球物理信息，以重新评估地下岩浆管道系统的架构。最后，我们将把我们的晶体尺度观测与现有的地球物理信息和年代学数据集相结合，为熔体的分布和我们的能力带来新的见解。这将导致新的约束条件的识别，识别和糊状管道系统的定义在未来。

Volcanism - the generation and eruption of molten rock from within the earth's interior - is one of the most visible manifestations of plate tectonics. Growth of the earth's crust occurs either when magma is stored and solidified within the crust, or is erupted at the earth's surface. Eruptive activity at subduction zones can be explosive and highly disruptive, and represents an important natural hazard, with implications for life, health and financial stability when it occurs. One of the major challenges facing volcanologists is the accurate forecasting of this eruptive behaviour. Abundant evidence of past volcanic activity shows that large volumes of magma can be erupted in a single event. However, geophysical techniques used to image below the earth's surface fail to distinguish large volumes of melt (magmatic liquid) stored within the crust. Instead, melt may be stored as "crystal mush", i.e. an accumulation of volcanic crystals separated by only small amounts of melt that is hard to image geophysically. However, a crystal mush with low melt content behaves like a solid and cannot be erupted. Researchers therefore suggest that the mush contains 'eruptible' lenses that have higher melt content, yet remain thin enough to be unresolved by geophysical techniques. If so, then wholesale spatial reorganisation of crystals and liquid in the whole mushy region could change its overall physical behaviour, such that it quickly becomes eruptible. In contrast, other scholars predict a prolonged existence of more liquid-rich (potentially eruptible) mush bodies within the crust. In this case, the lack of currently observed geophysical signals for large, melt-rich magma bodies may simply result from the ephemeral nature of magmatism. To make progress, more information about the longevity of eruptible mushy regions is essential. This proposal will develop a new method to determine the lifetime of melt-rich regions, enabling us to resolve this current conflict. Time 'chronology' information about volcanic systems is commonly recorded in the mineral zircon, which contains radioactive elements that are sensitive to time. Zircon chronology shows that crystal mushes can persist over long time periods (e.g. 100s kyr), but these measurements hold significant uncertainties. The lifetime of the more eruptible, melt-rich 'mobile magma' is much harder to investigate, because it occurs at higher temperatures where zircon may not be stable. However, this information is a critical link between geophysical observations, which record a snapshot of the state of the earth's crust, and volcanology, which records information about magmatic processes over very long times. This project will develop a new method to determine the lifetime of mobile magma crystallisation directly by analysing crystals that grow from melt at high temperatures. Specifically, we will relate the aspect ratio (length/ width) of the silicate mineral plagioclase, which grows from almost all subduction zone magmas, to the time available for crystallisation. Our preliminary work suggests a strong relationship between aspect ratio and time for water-rich, silica-rich magmas that erupt at subduction zones. Using high-temperature experiments, analysis of well-dated plagioclase crystals, and mathematical approaches, the team will derive a universal relationship that can be applied to all magmatic environments. We will apply the method to intermediate subduction zone volcanic systems that have recent geophysical information, in order to re-evaluate the architecture of the subterranean magma plumbing systems. Finally, we will integrate our crystal-scale observations with existing geophysical information and chronology datasets, to bring new insights into the distribution of melt and our ability to see it geophysically. This will lead to novel constraints on the identification, recognition and definition of mushy plumbing systems in future.