Coupled models of magma/mantle dynamics: melt transport at mid-ocean ridges and subduction zones

在地质时期，地球已经分化成一个铁的核心，一个硅酸盐地幔和一个气态大气。地幔进一步分化形成了地壳，这是一层薄薄的硅酸盐岩石，支撑着地球上的大多数生命。地幔的分异发生在部分熔融时。更多的易熔成分被转移到岩浆中，岩浆漂浮并上升到表面，在那里它从火山喷发出来。火山爆发的产物进入大气层和地壳，导致这些储层发生化学变化，对人类生活产生重要影响。虽然地壳和大气层通常可以观察到，但岩浆形成的火山源区在地幔中太深，无法直接观察。因此，基于流体力学和热力学的数学模型可以模拟深部条件，是研究硅酸盐地球分异过程的重要工具。我的工作包括开发和使用数学模型和大规模计算来研究地幔熔融和熔体输送的过程。这些模型是基于一个理论，调用通过结晶地幔的孔隙岩浆流动来解释熔体运输。间接的地质和地球化学证据表明，熔体输送是迅速的，垂直速度为每年10米或100米。这里提出的研究试图调和这些和其他间接的观察与多孔熔体输送理论。当岩浆流局部进入高渗透性通道时，预计会出现快速的孔隙速度-这种局部化是反应性流动的结果，其中流体在流动时溶解固体地幔基质。这一条件预计将由地幔中的岩浆来满足。因此，所提出的研究的目的之一是将反应流和随之而来的通道化的流体通量的计算模型，作为测试的熔体输送的多孔流理论。另一个重要但间接的观测结果与地幔内岩浆的动力学有关，那就是火山在俯冲带中的位置，在俯冲带中，洋壳和岩石圈形成并沉入地幔。潜没作用必然导致火山活动，随之而来的是对人类的危害。最近的研究表明，从火山到俯冲地壳顶部的深度与下沉板块的下降速度有关。新的模型表明，地幔中的熔体输送过程控制着俯冲带火山的位置，尽管这些模型没有明确计算熔体输送。为了在这个基本问题上取得进展，我建议扩展目前的模拟来处理俯冲带熔化的热力学复杂性：熔化区中存在水。这里提出的模型往往是复杂的：它们必须始终包括地幔对流和岩浆运输的流体力学，熔化和冻结的热力学，以及热量和化学物质的运输过程。我以前的工作已经证明了这种模型的开发，验证和解释的能力。牛津大学拥有超级计算设施，在所要求的支持下，将为拟议的工作提供极好的资源。通过继续推进地幔中熔体传输的理论，并继续部署和解释大规模模拟，拟议的工作将产生关于火山的不可接近的源区的新见解，从而对地球的化学分化。

Over geological time, the Earth has differentiated into a iron core, a silicate mantle and a gaseous atmosphere. The mantle has further differentiated to form the crust, a thin outer layer of silicate rock that supports most life on Earth. Differentiation of the mantle occurs when it partially melts. More fusible components are transfered to the magma, which is buoyant and rises to the surface, where it is erupted from volcanoes. The products of eruptions enter the atmosphere and the crust, leading to chemical changes in these reservoirs with important implications for human life. While the crust and the atmosphere are generally accessible to observation, the source regions of volcanoes where magma forms are too deep in the mantle to be observed directly. Mathematical models based on fluid mechanics and thermodynamics that can simulate the conditions at depth are thus a crucial tool for investigating the processes of differentiation of the silicate Earth. My work involves the development and use of mathematical models and large-scale computation for studying the processes of mantle melting and melt transport. These models are based on a theory that invokes flow of magma through the pores of the crystalline mantle to explain melt transport. Indirect geological and geochemical evidence suggests that melt transport is rapid, with vertical velocities of 10s or 100s of meters per year. The research proposed here attempts to reconcile these and other indirect observations with the theory of porous melt transport. Rapid porous velocities are expected when magmatic flow is localized into high-permeability channels---such localization is a consequence of reactive flow where the fluid is dissolving solid mantle matrix as it flows. This condition is expected to be met by magma in the mantle. Hence one of the aims of the proposed research is to incorporate reactive flow and its attendant channelization of fluid flux into computational models, as a test of the porous flow theory of melt transport. Another important but indirect observation that bears on the dynamics of magma within the mantle is the position of volcanoes in subduction zones, where the oceanic crust and lithosphere founder and sink into the mantle. Subduction invariably leads to volcanism, with its attendant hazards to human populations. Recent work has shown that the depth from the volcano to the top of the subducting crust correlates with the descent rate of the sinking slab. New models suggest that melt transport processes in the mantle control the position of subduction zones volcanoes, although these models do not explicitly calculate melt transport. To make progress on this fundamental problem, I propose to extend current simulations to handle the thermodynamic complexity of subduction-zone melting: the presence of water in the melting region. Models such as those proposed here tend to be complicated: they must consistently include the fluid mechanics of mantle convection and magma transport, the thermodynamics of melting and freezing, as well as the processes of heat and chemical transport. My previous work has demonstrated a capability for the development, validation and interpretation of such models. The University of Oxford has supercomputing facilities that, with the requested support, will provide an excellent resource for the proposed work. By continuing to advance the theory of melt transport in the mantle, and by continuing to deploy and interpret large-scale simulations, the proposed work will generate new insight about the inaccessible source regions of volcanoes and hence about the chemical differentiation of the Earth.