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

地球上的热对流是由热的但固体的地幔的流动控制的。这种对流驱动板块构造，产生重大社会危害(如地震、火山喷发、海啸等)。并控制着行星的成分和热演化。自20世纪60年代初以来，地幔岩石变形性质的量化一直是实验地球科学的主要目标。进展受到技术的限制，因为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。上地幔(顶部410公里)由橄榄岩组成，岩石主要由橄榄石组成，即被称为橄榄石的半珍贵宝石。在压力超过14万大气压的过渡带(深度410-670公里)，橄榄石不再稳定，转化为组成相似但结构更致密的高压矿物--瓦兹利石和环木石。几十年的实验工作对橄榄石的可塑性提供了很强的限制，但对过渡带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准确的计算模型，该模型将整合在同步加速器设施中安装的最先进的高压变形设备中获得的关于黄斯利石和环杉石的新实验数据。这些实验涉及新开发的设备和分析技术，是研究材料在高压下的力学行为的前沿。除了加深我们对地幔对流的理解外，该项目还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都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。该团队的成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对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则提供了对地震各向异性等关键观测数据的洞察。由于塑性性质是各向异性的，变形岩石内的应力场的复杂性因颗粒而异，现在可以使用新的高压设备与X射线同步辐射相结合进行现场观测，并通过自洽平均场模拟来解决。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。具体地说，他们将使用形变-DIA仪器和新开发的D-TCUP和DT-25，研究水滑石和环木石随铁和水含量变化的流动特性，并将约束强度与橄榄石进行对比。将使用原位X射线照相和衍射来测量应变、应力和织构(即晶格择优取向)。新的流动定律将被整合到过渡带的有效粘性和地震各向异性的模型中。建模工作将受益于二阶(SO)方法，这是平均场方案中的一种最新改进，它准确地描述了高度非牛顿的材料，如硅酸盐。这些模型将考虑由于晶体取向、复杂的变形机制(位错滑动和扩散)而产生的应力场的不均一性，合并了几种矿物，并提高了将结果外推到地质应变率的置信度。模型结构将是灵活的，允许在未来可用时集成额外的相和流动规律参数。这一结果将给出对过渡区粘度和产生地震各向异性的晶体择优取向的关键见解。

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.