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喷发的一个重要过程。触发过程可能与伴随岩浆混合而形成的气泡导致的压力增加有关。因为岩浆是复杂的液体，它们之间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。晶体可以被解读得很像树轮--外环(以及在结晶的最后阶段形成的微小晶体或‘微石’)反映了岩浆喷发前或喷发期间的环境，而晶体核心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，有三个重要后果：1)存在于前驱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，并在另一种岩浆中形成岩浆团或“包体”。在包体中发现的许多晶体起源于岩浆，现在被视为宿主。形成这些包体的倾向、大小、形状和丰度受原始液体组成的不同所控制。在任何情况下，飞地的形成都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为我们提供了一个极其有用的系统快照，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(相对于更演化的宿主岩浆)和由于液体中挥发物(气泡)的损失而导致的液线上升，促进了包体中微石的形成。由于晶体有能力锁定岩浆混合发生时环境变化的记录，因此我们可以：1.测量晶体和液体(现已凝固为玻璃)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组成，从而确定当晶体从一种液体转移到另一种液体时；2.使用晶体中化学梯度的‘扩散时钟’响应平衡条件的变化来确定晶体在喷发(扩散有效停止时)之前转移了多久。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3.测量包体和围岩中晶体的大小和形状，以确定特定类型的晶体是否优先被夹带。我们打算对最近的两个自然火山系统进行这些研究：Kameni(希腊圣托里尼)和Lassen(美国加利福尼亚州)，那里有大量的地球化学。岩石学和火山学研究已经完成，以描述该系统的特征，以及混合结构和飞地保存完好的地方。在对天然样品进行研究的同时，我们计划从相反的方向解决这个问题，通过进行实验来模拟岩浆混合过程中的晶体交换。这些实验将使我们能够评估哪些标准(晶体形状？液体粘度？)在控制晶体交换方面是最重要的。我们希望我们从自然系统中获得的测量结果能为我们在实验中建立的条件提供信息，最终我们希望得到它们之间的简单经验关系来描述这种交换。然后，这项工作将与同事开发的数值模式对接，这些模式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.