Transport of Lithophile Elements in Magmatic-Hydrothermal Fluids

长期以来，人们一直认为富含H2O(水)和富CO2(碳酸盐)的流体在广泛的地质过程中发挥着基础性的作用。尤其重要的是，这种流体能够选择性地将金属等化学成分从一个地质储集层输送到另一个地质储集层。因此，水溶液和碳酸盐流体在世界上一些最重要的矿藏的形成中起着关键作用。岩浆热液矿床是地壳浅层富挥发分岩浆体冷却分相的结果。流体的化学成分取决于它们所喷出的岩浆的性质，而岩浆的性质又受岩浆活动的构造环境的影响。与破坏性板块边缘有关的岩浆作用倾向于以水流体为主，而碳酸盐流体在板内岩浆作用中更为普遍。流体中还含有各种阴离子(如F、Cl、S等)或阴离子络合物(如CO3、SO4等)，它们在金属的运输中起着重要的作用。尽管人们普遍认识到流体在成矿中的重要性，但令人惊讶的是，我们对流体的物理化学了解很少，这反过来又限制了我们预测矿床可能形成的方式和地点的能力。我们的无知很大程度上源于研究高温水溶液或碳酸盐流体的实验困难。与硅酸盐或碳酸盐熔体不同，流体在室温和压力下不会冷却成固体，因此很难在化学或物理上对其进行表征。我们开创了一种新的实验方法来解决这个问题，其中使用激光穿透冻结的实验胶囊的壁面，直接分析冻结的液体，在切片过程中没有污染的风险。将激光与电感耦合等离子体质谱仪相结合，意味着我们可以分析冻结的液体中的各种微量元素。共存的硅酸盐或碳酸盐熔体可以被淬灭，并从胶囊中回收用于随后的分析。我们可以系统地改变流体的组成和浓度，使我们能够探索金属在流体中如何络合的关键控制。通过观察熔体-流体分配随流体组成的变化，我们可以推测存在的金属-配体络合物的类型。然而，我们不能在压力和温度下直接观察到这些。为此，我们开发了另一种实验方法，在电阻加热的钻石砧座压力室中，将已知组成的小液滴保持在两个钻石的扁平尖端之间。这些钻石对同步加速器产生的X射线是透明的，这意味着可以在高压和温度升高的情况下原位研究溶液。这种方法使我们能够评估基于划分实验所做的预测。最后，我们可以使用计算量子化学(经典和从头算分子动力学)来预测在高温高压下阳离子在流体中的水化和络合作用。最近一种被称为偏氰胺的技术的实现使我们能够从分子动力学模拟中获得形成金属络合物的自由能，从而得到平衡常数。总而言之，我们正在从三个截然不同但相辅相成的方向来处理金属运输问题。就其本身而言，每一种方法都有局限性；这些方法结合在一起，将使我们能够在与矿体形成完全相同的物理条件下生成水和碳酸盐流体的综合图景。我们将从研究一类重要但相对简单的金属开始，即碱、碱土和稀土，尽管我们的方法最终可以扩展到涵盖所有经济上重要的金属。

It has long been recognized that H2O-rich (aqueous) and CO2-rich (carbonic) fluids play a fundamental role in a wide range of geological processes. Of particular importance is the ability of such fluids to selectively transport chemical components, such as metals, from one geological reservoir to another. As a consequence, aqueous and carbonic fluids play a key role in the formation of some of the most economically important ore deposits in the world. Magmatic-hydrothermal ore deposits result from the cooling and phase-separation of volatile-rich magma bodies in the shallow crust. The chemistry of the fluids depends on the nature of the magma from which they exsolved, which in turn is influenced by the tectonic setting of the magmatism. Magmatism associated with destructive plate margins tends to be dominated by aqueous fluids, whereas carbonic fluids are more prevalent in intraplate magmatism. Fluids also contain a variety of anions (e.g. F, Cl, S etc) or anionic complexes (CO3, SO4 etc) which play an important role in metal transport. Despite the universal recognition of the importance of fluids in ore formation, we have surprisingly little understanding of their physical chemistry, which in turn limits our ability to predict how and where ore deposits may form. A large part of our ignorance stems from the experimental difficulties of studying high-temperature aqueous or carbonic fluids. Unlike silicate or carbonate melts, fluids do not quench to a solid at room temperature and pressure, making it difficult to characterise them chemically or physically. We have pioneered a novel experimental approach to this problem, in which a laser is used to drill through the walls of a frozen experimental capsule, directly analysing the frozen fluid, without risk of contamination during sectioning. Coupling the laser to an ICP-MS apparatus means that we can analyse the frozen fluid for a wide variety of trace elements. The coexisting silicate or carbonate melt can be quenched and retrieved from the capsule for subsequent analysis. We can systematically vary the composition of the fluid and its concentration, allowing us to explore the key controls on how metals are complexed in fluids. By looking at the variations in melt-fluid partitioning with fluid composition we can hypothesise about the types of metal-ligand complexes that are present. We cannot, however, directly observe these at pressure and temperature. To do this, we have developed an alternative experimental methodology in which a small droplet of fluid of known composition is held between the flattened tips of two diamonds in a resistance-heated diamond anvil pressure cell. The diamonds are transparent to synchrotron-generated X-rays, meaning that the solution can be studied in situ at elevated pressure and temperature. This approach allows us to evaluate the predictions made on the basis of the partitioning experiments. Finally, we can use computational quantum chemistry (classical and ab initio molecular dynamics) to predict the hydration and complexation of cations in fluids at at elevated pressure and temperature. Recent implementations of a technique call metadyanamics enables us to derive free energies and, hence, equilibrium constants, for the formation of metal complexes from molecular dynamical simulations. In summary, we are approaching the problem of metal transport from three quite different, but complementary directions. In its own right, each approach has limitations; in combination these approaches will enable us to generate a comprehensive picture of aqueous and carbonic fluids under precisely the same physical conditions as ore bodies form. We will begin by studying an important, but relatively simple, class of metals, the alkalis, alkaline earths and rare earths, although our methodology can ultimately be extended to encompass the entire range of economically important metals.