Collaborative Research: Rheology of the Earth's Transition Zone - An Integrated Approach

地球中的热对流是由热的、但固体的地幔流控制的。这种对流驱动了板块构造，产生了重大的社会危害（例如，地震、火山爆发、海啸等）控制着行星的成分和热演化自20世纪60年代初以来，量化地幔岩石的变形特性一直是实验地球科学的主要目标。技术的进步受到限制，因为在地球内部深处的极端压力和温度下进行矿物变形实验存在严重困难。 上地幔（顶部410公里）由橄榄岩组成，岩石主要由橄榄石组成，即，这种半宝石叫橄榄岩。在压力超过140，000大气压的过渡区（410-670公里深度），橄榄石不再稳定，并转化为高压矿物，即华德士利石和灵伍德石，它们具有类似的成分，但结构更致密。 几十年的实验工作提供了很强的限制橄榄石的可塑性，但很少有人知道的粘度的矿物在过渡区。本项目的目的是为地球过渡区的粘度提供准确的计算模型，其中将整合在同步加速器设施的最先进的高压变形装置中获得的关于wadsleyite和ringwoodite的新实验数据。这些实验涉及新开发的设备和分析技术，处于高压下材料力学行为研究的最前沿。除了促进我们对地幔对流的理解，该计划还将为一名研究生和本科生提供现代实验科学的支持和培训。所有新的实验和分析工具将提供给其他科学家，推进我们在高压研究方面的一般知识。该团队的研究结果将在地球物理学和地震学中得到直接应用，并在材料科学中得到更广泛的应用。地球物质的流动定律为地幔动力学提供了重要的约束，而原子尺度上的变形机制知识则为地震各向异性等关键观测量提供了见解。变形岩石中应力场的复杂性，由于塑性特性是各向异性的，因此随颗粒的不同而变化，现在可以使用新的高压设备与X射线同步辐射耦合在原位观察，并通过自洽平均场建模来解决。在这个项目中，研究人员将利用这些最新的发展来解决过渡区的塑性。具体来说，他们将研究wadsleyite和ringwoodite的流动特性作为铁和水含量的函数，并使用变形DIA设备和新开发的D-TCup和DT-25与橄榄石进行约束强度对比。 原位X射线照相术和衍射将用于测量应变、应力和纹理（即，晶格择优取向）。 新的流动定律将被纳入过渡区的有效粘度和地震各向异性模型。建模工作将受益于二阶（SO）方法，这是平均场方案的最新改进，可准确描述高度非牛顿材料，如硅酸盐。这些模型将考虑到由于晶体取向、复杂变形机制（位错滑移和扩散）引起的应力场不均匀性，纳入几种矿物，并提高将结果外推到地质应变率的置信度。模型结构将是灵活的，允许在将来可用时集成额外的相和流动定律参数。结果将提供关键的见解过渡带粘度，晶体的优先取向，产生地震各向异性。

Heat convection in the Earth is controlled by flow of the hot, but solid mantle. This convection drives plate tectonics, generating major societal hazards (e.g., earthquakes, volcanic eruptions, tsunamis, etc.) and controls the compositional and thermal evolution of the planet. Since the early 1960s, quantifying the deformation properties of mantle rocks has been a major goal in experimental Earth Sciences. Advancement has been limited by technology, owing to serious difficulties in conducting experiments involving deformation of minerals at the extreme pressures and temperatures prevailing in Earth's deep interior. The upper-mantle (top 410 km) consists of peridotites, rocks comprised dominantly of olivine, i.e., the semi-precious gem known as peridot. In the transition zone (410-670 km depth) at pressure in excess of 140,000 atm, olivine is no longer stable and transforms into high-pressure minerals, wadsleyite and ringwoodite, which have comparable compositions but denser structures. Decades of experimental work have provided strong constraints on olivine plasticity, yet little is known about the viscosity of minerals in the transition-zone. The aim of the present project is to provide accurate computational models for the viscosity of Earth's transition zone, which will integrate new experimental data on wadsleyite and ringwoodite obtained in state-of-the art high-pressure deformation devices set at synchrotron facilities. These experiments, involving newly developed devices and analytical techniques, are at the forefront of research on the mechanical behavior of materials at high pressure. Besides advancing our understanding of mantle convection, this program will provide support and training in modern experimental science to one graduate student as well as undergraduate students. All the new experimental and analytical tools will become available to other scientists, advancing our general knowledge in high-pressure research. The team's results will find direct applications in Geophysics and Seismology, and broader applications in Materials Science. Flow laws for Earth materials provide vital constraints on mantle dynamics, while knowledge of deformation mechanisms at the atomic scale provides insights into crucial observables such as seismic anisotropy. The complexity of the stress field within deforming rocks, which varies from grain to grain as plastic properties are anisotropic, can now be observed in situ using new high-pressure devices coupled with X-ray synchrotron radiation, and addressed by self-consistent mean-field modeling. In this project, the investigators will take advantage of these recent developments to address the plasticity of the transition zone. Specifically, they will study the flow properties of wadsleyite and ringwoodite as a function of iron and water contents, and constrain strength contrasts with olivine, using the Deformation-DIA apparatus and the newly developed D-TCup and DT-25. In situ X-ray radiography and diffraction will be used to measure strain, stress and texture (i.e., lattice preferred orientation). The new flow laws will be integrated into models for the effective viscosity and seismic anisotropy of the transition-zone. Modeling efforts will benefit from the second-order (SO) method, a recent improvement in mean-field schemes which describes accurately highly non-Newtonian materials, such as silicates. The models will account for stress-field heterogeneities due to crystal orientations, complex deformations mechanisms (dislocation glide and diffusion), incorporate several minerals, and improve confidence for extrapolation of results to geologic strain rates. The model construction will be flexible, allowing integration of additional phases and flow law parameters as they become available in the future. The outcome will give crucial insights on transition-zone viscosity, and the crystal preferred orientations that produce seismic anisotropy.