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

地球上的热对流是由热而坚固的地幔流动控制的。这种对流驱动板块构造，产生重大的社会危害（如地震、火山爆发、海啸等），并控制着地球的成分和热演化。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。由于在地球深处普遍存在的极端压力和温度下进行涉及矿物变形的实验存在严重困难，技术的进步受到限制。上地幔（顶部410公里）由橄榄岩组成，岩石主要由橄榄石组成，即一种被称为橄榄石的半宝石。在过渡带（410-670 km深度），压力超过140,000 atm时，橄榄石不再稳定，转变为高压矿物，瓦德利石和环伍德石，它们具有相似的成分，但结构更致密。几十年的实验工作对橄榄石的可塑性提供了强有力的限制，但对过渡带中矿物的粘度知之甚少。本项目的目的是为地球过渡带的粘度提供精确的计算模型，该模型将整合在同步加速器设施中最先进的高压变形装置中获得的关于瓦德斯莱岩和环伍德岩的新实验数据。这些实验涉及新开发的设备和分析技术，是高压下材料力学行为研究的前沿。本项目将为一名研究生和本科生提供现代实验科学方面的支持和培训。所有新的实验和分析工具将可供其他科学家使用，从而提高我们在高压研究方面的一般知识。该团队的研究结果将直接应用于地球物理学和地震学，并在材料科学中得到更广泛的应用。地球物质的流动规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的知识为地震各向异性等关键观测提供了见解。由于塑性特性各向异性，变形岩石内部应力场的复杂性随颗粒的不同而不同，现在可以使用新的高压设备与x射线同步辐射耦合在一起，并通过自一致平均场建模来解决。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可塑性问题。具体来说，他们将使用Deformation-DIA仪器和新开发的D-TCup和DT-25研究wadsleyite和ringwoodite的流动特性作为铁和水含量的函数，并与橄榄石进行约束强度对比。原位x射线照相和衍射将用于测量应变、应力和织构（即晶格优选取向）。新的流动规律将被整合到过渡区的有效粘度和地震各向异性模型中。建模工作将受益于二阶（SO）方法，这是平均场格式的最新改进，可以准确地描述高度非牛顿材料，如硅酸盐。该模型将考虑由于晶体取向、复杂变形机制（位错滑动和扩散）引起的应力场不均匀性，纳入几种矿物，并提高对地质应变率结果外推的信心。模型结构将是灵活的，允许在未来可用时集成其他阶段和流动规律参数。该结果将为过渡带粘度和产生地震各向异性的晶体优选方向提供重要见解。

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.