Regime change: convection and crystallisation of magma

多组分液体的冷却，如岩浆（也包括冰淇淋、盐或海水），会导致凝固成一系列令人眼花缭乱的形式。通常形成的固体与液体的成分不同（例如，纯冰与盐水）。这意味着在冷却过程中，残余液体的成分和温度总是在变化，从而引起液体密度的变化。这些密度的变化可以驱动液体中的对流，并对结晶系统中质量和热量的传递方式产生深远的影响。当冷却速度是温和的凝固发生在寒冷的边界，当冰在一个平静的冬日池塘上形成。相反，当冷却速率非常高时，液体中强烈的对流会使结晶远离冷边界，在旋转的内部形成一连串的结晶。在熔融岩石（岩浆）体中，对流运动重新分配质量的方式对残余液体改变成分的方式有重要影响。这在确定任何喷发的熔岩流的最终成分（以及由此产生的爆炸性）方面起着至关重要的作用。结晶的方式也会影响地表喷发的岩浆通道冻结到足以阻止更多岩浆沿其流动的速度。理解对流如何控制岩浆在地壳岩浆房中演化的另一个重要原因是，我们能够推断在难以接近的地球深处发生的过程的唯一方法是检查喷发的熔岩的成分。该项目将涉及使用盐水溶液为真实岩浆体创建小规模的台式模拟物。我们将能够控制储罐中的冷却和凝固速度，并直接观察发生了什么以及晶体在哪里形成——这在真正的岩浆中是不可能的。我们将利用最近的一些新发现，将我们的实验结果与玄武岩、岩浆侵入岩的自然例子进行比较，这意味着我们可以利用颗粒形状、内部成分变化和致密矿物的空间分布的细节，解码完全凝固的玄武岩侵入岩和流动中留下的结晶风格记录。这些微观结构标志将使我们能够计算出岩浆体内的液体在凝固过程中是对流的还是静态的。这些发现提供了一个令人兴奋的机会，可以在理解这些天体冷却的基本过程方面取得真正的进展。

The cooling of poly-component liquids, such as magma (and also ice-cream and salt- or sea-water), can drive solidification in a bewildering array of styles. Often the solid that forms is of a different composition from the liquid (e.g. pure ice from salt water). This means that the composition and temperature of the residual liquid is always changing during cooling, causing changes in the density of the liquid. These density changes can drive convection in the liquid, and can have profound effects on the way in which mass and heat are transported within the crystallising system. When cooling rates are gentle solidification occurs from the cold boundaries as when ice forms on the pond on a still winter's day. In contrast, when cooling rates are very high, vigorous convection in the liquid can drive crystallization away from the cold boundaries, forming a flurry of crystallization in the swirling interior. In the context of bodies of molten rock (magma) the way convective motion can re-distribute mass has significant effects on the way the residual liquid changes composition. This plays a vital role in determining the final composition (and hence the explosivity) of any erupted lava flows. The style of crystallization also affects how quickly a magma conduit feeding a surface eruption will freeze sufficiently to prevent more magma travelling along it. A further important reason to understand how convection controls the way magmas evolve in crustal magma chambers is because the only way we can make deductions about processes occurring in the inaccessible deep Earth is by an examination of the composition of erupted lavas.The project will involve creating small-scale, bench-top analogues for real magma bodies using salt-water solutions. We will be able to control the cooling and solidification rates in our tanks and watch directly what happens and where the crystals are forming - something that is not possible in real magmas. We will compare our experimental results with natural examples of basaltic, magmatic intrusions by taking advantage of some recent new discoveries that mean we can decode the record of crystallization style left in fully-solidified basaltic intrusions and flows using details of grain shape, internal compositional variations and the spatial distribution of dense minerals. These microstructural markers will enable us to work out whether the liquid in the magma bodies convected or was static during solidification. These discoveries provide an exciting opportunity to make real progress in understanding the fundamental processes at work as these bodies cooled.