The structure and rheology of crystal mushes

当熔融岩石（岩浆）冷却时，它会结晶，在岩浆体的边缘形成“晶体糊状”。这种糊状物含有晶体和火山液体的混合物。持续的冷却将剩余的液体转化为晶体，糊状层不断增长，直到整个身体变成固体。这种浆液层对压力的反应方式（它的强度，或“流变性”）控制着所有规模的火山过程，从超级火山下大型岩浆室的演化，到大洋中脊岩浆的喷发，熔岩流的就位，以及危险火山喷发的动力学和爆炸性。因此，如果我们要了解许多火山过程，对糊状流变学的了解是至关重要的。了解液体和颗粒的混合物如何对外部应力作出反应，对于许多问题也很重要，包括制作冰淇淋、浇筑混凝土和理解泥浆流动的行为。在了解含有少量悬浮晶体（50%液体）的岩浆和含有很少熔体（<5%液体）的岩石的流变学方面取得了很大进展。然而，对于含有约10-50%液体的糊状的流变学研究相对较少，而且由于流变学变化强烈，很难从以前的研究中扩大或缩小。这种中间情况很重要，因为许多火山过程都涉及含有这些中间液体成分的岩石。例如，由于缺乏合适的流变学数据，我们不知道晶体糊状压实在控制玄武岩岩浆演化中的重要性。我们建议通过将实验结果与对天然晶体糊状结构的观察相结合，研究具有中间液体含量的晶体糊状的体流变学来解决这一差距。糊状物的流变性取决于它的晶体尺度结构。例如，颗粒的大小和形状对糊状的硬度和强度有影响。因此，我们将专注于对糊状结构进行量化，这也将有助于我们将自然结果和实验结果联系起来。首先，我们将描述和量化晶体糊状的微观结构，我们可以确定没有变形，使用自然辉长岩的例子。一旦我们知道了典型辉长岩浆糊的结构，我们就会设计简单的实验，使用低温模拟材料来模拟辉长岩浆糊。这些实验将向我们展示当它变形时糊状结构是如何变化的，以及各种参数（例如粒度、形状和液体量）是如何影响糊状强度和它变形的方式的。我们最后将检查已经变形的天然岩石，以校准我们的结果，并确定压实等过程的重要性。通过这种方式，我们将建立对岩浆冷却和结晶过程中的流变学的定量理解。研究结果将广泛适用于地球科学的其他领域，也将与化学工程、食品加工和冶金等领域的一系列问题有关。

As molten rock (magma) cools, it crystallizes and a 'crystal mush' forms on the margins of the magma body. The mush contains a mixture of crystals and volcanic liquid. Continued cooling steadily converts the remaining liquid to crystals, and the mush layer grows until the entire body is solid. The way this mush layer responds to stress (its strength, or 'rheology') controls volcanic processes on all scales, from the evolution of large magma chambers under super-volcanoes, to the eruption of magma at mid-ocean ridges, the emplacement of lava flows, and the dynamics and explosivity of hazardous volcanic eruptions. An understanding of mush rheology is therefore vital if we are to understand many volcanic processes. Knowing how mixtures of liquid and particles respond to external stresses is also important to a wide range of problems, including making ice-cream, pouring concrete and understanding the behaviour of mud-flows.Much progress has been made in understanding the rheology of magmas with few suspended crystals (>50% liquid) and of rocks containing very little melt (<5% liquid). However, relatively little has been done to investigate the rheology of mushes with approximately 10-50% liquid, and it is difficult to scale up or down from previous studies because the rheology changes strongly. This intermediate case is important because many volcanic processes involve rocks with these intermediate liquid contents. For example, we don't know how important crystal mush compaction is in controlling basalt magma evolution, because of a lack of suitable data on rheology. We propose to address this gap by investigating the bulk rheology of crystal mushes with intermediate liquid contents, by combining experimental results with observations on the structure of natural crystal mushes. The rheology of a mush depends on its crystal-scale structure. For example, the size and shape of the particles has an effect on how rigid and strong the mush is. We will therefore focus on quantifying the mush structure, which will also help us to link together natural and experimental results. Firstly we will describe and quantify the microstructure of crystal mushes that we can be sure haven't been deformed, using natural examples of gabbro. Once we know the structure of a typical gabbro mush, we will design simple experiments using low-temperature analogue materials that mimic the gabbro mush. These experiments will show us how the mush structure changes when it is deformed and how various parameters (e.g. grain size, shape and the amount of liquid) affect the mush strength and the way it deforms. We will finally examine natural rocks that have been deformed, in order to calibrate our results and determine the importance of processes such as compaction. In this way we will build a quantitative understanding of rheology during cooling and crystallisation of magma. The results will have broad applicability for other areas of Earth science, and will also be relevant to a range of problems in chemical engineering, food processing and metallurgy.