The importance of crystal exchange and magma mixing in volcanic systems : eruption-triggering mechanisms and timescales

岩浆混合已被证明是触发火山爆发的一个重要过程。触发过程可能与岩浆混合时伴随气泡形成而导致的压力增加有关。由于岩浆是复杂的液体，它们的相互作用也不是简单的。但大多数岩浆都含有晶体，这些晶体可以用来记录岩浆相互作用的历史，就像黑匣子包含飞机飞行的详细记录一样。晶体可以像树木年轮一样解读--外缘（以及结晶最后阶段形成的微小晶体或“微晶石”）反映了岩浆喷发前或喷发期间的环境，而晶体核心则反映了岩浆相互接触之前存在的过去环境。当岩浆相互作用时，有三个重要的结果：1）存在于前体岩浆中的晶体可能从一种液体转移到另一种液体，伴随着液体的某种程度的混合; 2）当液体试图混合时，它们通常不完全混合，并形成岩浆团或一种岩浆在另一种岩浆中的“包体”。在包体中发现的许多晶体起源于现在被视为宿主的岩浆。形成这些包体的趋势以及大小、形状和丰度由原始液体的成分差异控制。在任何情况下，包体形成是液体完全混合之前的中间步骤。因此，火山岩中包体的保存给了我们一个非常有用的系统“快照”，使我们能够测量晶体的分布，它们的大小和成分3）岩浆混合过程本身导致结晶条件的变化，通常由于冷却和冷却的组合而促进包体中微晶的形成（相对于更演化的主岩浆）和由于从液体中损失挥发物（气泡）而升高液相线。由于晶体有能力在岩浆混合发生时锁定环境变化的记录，那么我们可以：1。测量晶体和液体（现在固化为玻璃）的化学成分，并使用平衡关系（如Fe-Mg或Ca-Al分配）来确定不同生长阶段的液体成分，以及晶体何时从一种液体转移到另一种液体。使用“扩散时钟”的化学梯度在晶体中响应平衡条件的变化，以确定多久前爆发（当扩散有效地停止）的晶体被转移。由于晶体转移标志着岩浆混合的最早阶段，而这种混合可能是喷发的触发因素，因此这些时间尺度可以帮助我们预测未来的喷发。测量包体和主岩中晶体的大小和形状，以确定是否优先夹带特定类型的晶体。我们打算对两个自然的近代火山系统进行这些研究; Kameni（希腊圣托里尼）和Lassen（加州，美国），其中有大量的地球化学。岩石学和火山学的工作已经完成，以确定该系统，并在混合结构和包体保存完好。与天然样品的工作平行，我们计划通过进行实验来模拟岩浆混合过程中的晶体交换，从相反的方向来解决这个问题。这些实验将使我们能够评估哪些标准（晶体形状？液体粘度？）是控制晶体交换的最重要因素。我们期望我们对自然系统的测量能够为我们在实验中建立的条件提供信息，最终我们期望在它们之间得出简单的经验关系来描述这种交换。这项工作将与同事们正在开发的数值模型相结合，这些模型迫切需要一些现实的边界条件。

Magma mixing has been shown to be an important process in triggering volcanic eruptions. The triggering process is likely related to the increase in pressure due to bubble formation which accompanies magma mixing. Because magmas are complex liquids, their interaction is also not straightforward. But most magmas contain crystals and these can be used to record the history of magmatic interaction, in much the same way as a black box contains the detailed record of an aircraft's flight. Crystals can be read rather like tree rings - the outer rims (and the tiny crystals or 'microlites' which form at the last stage of crystallisation) reflect the magma environment immediately before or during eruption, while crystal cores reflect past environments which existed before the magmas came in contact with each other. When magmas interact there are three important consequences; 1) crystals which existed in the precursor magmas may be transferred from one liquid to another, accompanied by some degree of mixing of the liquids 2) as the liquids try to mix they commonly do so incompletely, and form magmatic blobs or 'enclaves' of one magma in the other. Many crystals found in the enclaves originated in the magma which is now seen as the host. The tendency to form these enclaves, and the sizes, shapes and abundances are controlled by the difference in composition of the original liquids. In any case enclave formation is an intermediate step before complete mixing of the liquids. As such the preservation of enclaves in volcanic rocks gives us a vitally useful 'snapshot' of the system allowing us to measure the distribution of crystals, their sizes and compositions 3) the magma mixing process itself leads to a change in crystallisation conditions, typically promoting the formation of microlites in the enclaves due to a combination of cooling (relative to the more evolved host magma) and raising of the liquidus due to loss of volatiles (bubbles) from the liquid. Since crystals have the capacity to lock in the record of the changing environment as magma mixing takes place, then we can; 1. Measure the chemical compositions of the crystals and liquids (now solidified to glass) and use equilibrium relationships (such as Fe-Mg or Ca-Al partitioning) to establish what the liquid compositions were at various stages of growth, and therefore when crystals were transferred from one liquid to another 2. Use the 'diffusion clock' of chemical gradients in the crystals responding to changes in equilibrium conditions to determine how long before eruption (when diffusion effectively stops) the crystals were transferred. Since the crystal transfer marks the earliest stages of magma mixing, and this mixing may be the trigger for an eruption, then these timescales can help us predict future eruptions 3. Measure the sizes and shapes of crystals in enclaves and host rock to see whether a particular type of crystal is preferentially entrained We intend to carry out these studies on two natural recent volcanic systems; Kameni (Santorini, Greece) and Lassen (California, USA) where a great deal of geochemical. Petrographic and volocanological work has already been done to characterise the system, and where mixing textures and enclaves are well-preserved. In parallel to the work on natural samples, we plan to approach the problem from the opposite direction by carrying out experiments to simulate crystal exchange during magma mixing. These experiments will allow us to evaluate which criteria (crystal shape? liquid viscosities?) are most important in controlling crystal exchange. We expect our measurements from natural systems to inform the conditions we build into the experiments, and ultimately we expect to derive simple empirical relationships among them to describe this exchange. This work will then interface with numerical models being developed by colleagues which badly need some realistic boundary conditions.