Transport of Lithophile Elements in Magmatic-Hydrothermal Fluids

人们早就认识到，富H2O（水）和富CO2（碳）流体在广泛的地质过程中发挥着重要作用。特别重要的是这种流体选择性地将化学成分（例如金属）从一个地质储层输送到另一个地质储层的能力。因此，水和碳流体在世界上一些经济上最重要的矿床的形成中起着关键作用。岩浆热液矿床是地壳浅部富挥发分岩浆体冷却分相作用的产物。这些流体的化学性质取决于它们所喷出的岩浆的性质，而岩浆的性质又受到岩浆活动的构造背景的影响。与破坏性板块边缘有关的岩浆活动往往以水相流体为主，而碳酸相流体在板内岩浆活动中更为普遍。流体中还含有各种阴离子（如F、Cl、S等）或阴离子络合物（CO3、SO4等），它们在金属迁移中起着重要作用。尽管人们普遍认识到流体在成矿过程中的重要性，但令人惊讶的是，我们对其物理化学的了解甚少，这反过来又限制了我们预测矿床如何形成以及在何处形成的能力。我们的无知很大一部分源于研究高温水或碳流体的实验困难。与硅酸盐或碳酸盐熔体不同，流体在室温和压力下不会淬火成固体，因此很难通过化学或物理方法将其溶解。我们开创了一种新的实验方法来解决这个问题，其中使用激光钻穿冷冻实验舱的壁，直接分析冷冻流体，而没有切片过程中污染的风险。将激光耦合到ICP-MS设备意味着我们可以分析冷冻流体中的各种微量元素。共存的硅酸盐或碳酸盐熔体可以被骤冷并从胶囊中取出用于随后的分析。我们可以系统地改变流体的成分及其浓度，使我们能够探索金属如何在流体中络合的关键控制。通过观察熔体-流体分配随流体组成的变化，我们可以假设存在的金属-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这些。要做到这一点，我们已经开发出一种替代的实验方法，其中一个小液滴的已知组成的流体之间的两个金刚石的电阻加热的金刚石砧压力单元的扁平尖端举行。金刚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高温下原位研究该溶液。这种方法使我们能够评估基于分区实验的预测。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）来预测在高压和高温下流体中阳离子的水合和络合。最近实施的一项技术称为metadyanxide使我们能够获得自由能，因此，平衡常数，从分子动力学模拟金属络合物的形成。总之，我们从三个完全不同但互补的方向来研究金属输运问题。就其本身而言，每种方法都有局限性;结合这些方法将使我们能够在与矿体形成完全相同的物理条件下生成含水和碳酸流体的全面图片。我们将开始研究一种重要但相对简单的金属，即碱金属、碱土金属和稀土金属，尽管我们的方法最终可以扩展到包括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.