The structure and rheology of crystal mushes

当熔岩(岩浆)冷却时，它会结晶，并在岩浆体的边缘形成“晶体糊状物”。泥浆中含有晶体和火山液体的混合物。持续冷却会将剩余的液体稳定地转化为晶体，糊状层会不断增长，直到整个身体变成固体。这种糊状层对压力的反应方式(其强度或“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，到大洋中脊的岩浆喷发，熔岩流的侵位，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，对泥浆流变学的了解是至关重要的。了解液体和颗粒的混合物对外部应力的反应对一系列问题也很重要，包括制作冰淇淋、浇注混凝土和了解泥石流的行为。在理解很少悬浮晶体的岩浆(50%液体)和含有极少熔体的岩石(5%液体)的流变学方面已经取得了很大进展。然而，对液体含量约为10-50%的蘑菇的流变性研究相对较少，而且由于流变学的强烈变化，很难从以往的研究中放大或缩小。这种中间情况很重要，因为许多火山过程涉及具有这些中间液体含量的岩石。例如，由于缺乏合适的流变学数据，我们不知道晶体碎屑压实作用在控制玄武岩岩浆演化中的重要性。我们建议通过结合实验结果和对天然晶体蘑菇结构的观察来研究中等液体含量的晶体蘑菇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，粒子的大小和形状会影响糊状物的刚性和强度。因此，我们将把重点放在量化MUSH结构上，这也将有助于我们将自然结果和实验结果联系起来。首先，我们将使用辉长岩的天然例子来描述和量化我们可以确定没有变形的晶体泥的微观结构。一旦我们知道了典型辉长岩的结构，我们将使用模拟辉长岩的低温模拟材料来设计简单的实验。这些实验将向我们展示糊化结构在变形时是如何变化的，以及各种参数(例如，颗粒大小、形状和液量)如何影响糊化强度和它的变形方式。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。这些结果将广泛适用于地球科学的其他领域，也将与化学工程、食品加工和冶金领域的一系列问题相关。

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.