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

在漫长的地质年代里，地球已经分化成铁核、硅酸盐地幔和气体大气层。地幔进一步分化形成地壳，这是一层薄薄的硅酸盐岩石，支撑着地球上的大多数生命。地幔的分异发生在它部分融化的时候。更多易熔的成分被转移到岩浆中，岩浆是浮力的，上升到地表，从火山喷发出来。火山喷发的产物进入大气和地壳，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，无法直接观察到。因此，基于流体力学和热力学的数学模型可以模拟深度条件，是研究硅酸盐地球分化过程的关键工具。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融化和熔体运输的过程。这些模型基于一种理论，该理论利用岩浆通过结晶地幔的孔隙流动来解释熔体的运输。间接的地质和地球化学证据表明，熔体的移动是迅速的，垂直速度为每年10到100米。本文提出的研究试图用多孔熔体输运理论来调和这些和其他间接观测结果。当岩浆流被定位到高渗透率通道时，预计会有快速的孔隙速度——这种定位是流体在流动过程中溶解固体地幔基质的反应性流动的结果。这一条件预计将由地幔中的岩浆满足。因此，提出的研究目标之一是将反应流动及其伴随的流体通量通道化纳入计算模型，作为熔体输送多孔流动理论的测试。另一个与地幔内岩浆动力学有关的重要但间接的观测是火山在俯冲带的位置，在那里海洋地壳和岩石圈形成并下沉到地幔中。俯冲不可避免地导致火山活动，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，地幔中的熔体运输过程控制着俯冲带火山的位置，尽管这些模型没有明确地计算熔体运输。为了在这个基本问题上取得进展，我建议扩展目前的模拟，以处理俯冲带融化的热力学复杂性：在融化区域存在水。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，以及热量和化学运输的过程。我以前的工作已经证明了开发、验证和解释这些模型的能力。牛津大学拥有超级计算设施，如果得到要求的支持，将为所提议的工作提供极好的资源。通过继续推进地幔中熔体运输理论，通过继续部署和解释大规模模拟，拟议的工作将对难以到达的火山源区产生新的见解，从而对地球的化学分化产生新的见解。

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.