Rheology of Multi-Phase Mantle Rocks to 800 km Depth

长期以来，对地幔条件下地幔岩石塑性特性的定量描述一直是地球科学家的追求，因为这些特性控制着板块的运动，定义了地震的性质，并限制了地球的演化。在过去的十年里，一系列实验技术的突破使该团队能够在与地球内部相关的压力和温度下，以可控的流速变形岩石样本。然而，即使对诸如橄榄石、辉石、石榴石、方长石、钙钛矿等矿物的流变性能的了解有所增加，真实岩石样品的流动规律仍然是未知的。复合岩石的流动规律的建立不仅需要其组成部分的性质，而且需要了解它们之间的相互作用。同步加速器x射线辐射与多砧高压变形装置相结合，是流变学实验的最新技术，是该团队在地幔条件下探测岩石内部这些相互作用的工具。在本项目中，目标是(1)以橄榄岩为样本，确定从上地幔到下地幔顶部跨越区域的岩石流动规律。(2)追求特定的混合情景，这将对积极经历相变的区域的流变特性提供更多的见解，并更好地理解区域，如下地幔，在那里存在非常强的相和非常弱的相的混合物。该项目将支持一名研究生的培训，并在向科学界开放的国家x射线同步加速器光束线设施上开发和扩展新的实验工具。该项目的主题是为地幔粘度随深度至800 km的函数提供一个基于实验的参考。这些数据自然会包括由压力引起的相变引起的矿物学和成分随深度变化的影响。通过测量模型地幔岩石的流变规律，研究人员希望看到深度对流变特性的总体影响。通过对比岩石资料和橄榄石资料，可以确定多相骨料性质对有效流动规律的贡献。混合的影响不仅可以通过力学变形数据来解决，还可以通过对最终样品的实验后研究在微观结构水平上解决。本研究将使用同步加速器设施的多砧高压变形设备，这些设备已经提供了相当于600公里深度压力的变形条件。该项目将进一步发展实现800公里条件下最高压力测量的能力。同步x射线提供应力和应变数据，允许在变形过程中记录样品的演变。此外，通过对每个衍射峰的分析来确定应力，从而得出矿物之间的应力分配信息，以及对于每种矿物，相对于应力场具有共同晶体取向的颗粒亚群之间的应力分配信息。所有这些信息限制了每种矿物的变形过程和每种矿物的相对强度。该项目将开发适用于同步加速器高压研究的技术。通过结合不同的技术并将其应用于高压原位研究，拟议的研究有可能开辟新的领域。特别是，目前正在开发能够使这些实验达到400公里深度的实验方案。该研究计划将改进实验工具，使实验能够达到相当于800公里深度的压力。这将扩大国家高压变形研究基地。从这个项目中发展出来的技术将通过一个提案系统向所有对进行这些高压实验感兴趣的科学家开放给光束线的所有用户。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A quantitative description of the plastic properties of mantle rocks at mantle conditions has long been the pursuit of geoscientists, as these properties control the motion of plates, define the nature of earthquakes, and constrain the evolution of the Earth. Over the last decade, a series of experimental technique breakthroughs have enabled the team to deform rock samples with controlled flow rates and at the pressure and temperature relevant to the Earth’s interior. However, even with increased knowledge on the rheological properties of minerals such as olivine, pyroxene, garnet, periclase, perovskite, etc., the flow law for a real rock sample is still unknown. An establishment of flow laws for composite rock requires not only the properties of its constituents, but also the understanding of interactions among them. Synchrotron X-ray radiation coupled with multi-anvil high pressure deformation apparatus, the current state of the art technique for rheological experiments, is the team's tool to probe these interactions inside the rock under mantle conditions. In this project, the goal is to (1) define the flow law of the rocks at conditions spanning the region from the upper mantle through the top of the lower mantle using peridotite as the sample. (2) Pursue specific mixing scenarios that will give added insights into the rheological properties of regions actively undergoing phase transitions and to better understand regions, such as the lower mantle, where there is a mixture of a very strong phase and a very weak phase. This project will support the training of a graduate student, and develop and extend novel experimental tools on national X-ray synchrotron beamline facilities that are open to the scientific community.The theme of this project is to provide an experimentally based reference for the viscosity of the mantle as a function of depth to 800 km. The data will naturally include the effect of mineralogical and compositional changes with depth due to pressure-induced phase transitions. By measuring rheological flow laws of a model mantle rock, the investigators expect to see the total effect of depth on the rheological properties. By comparing the rock data with olivine data, one can identify the contribution of the multi-phase aggregate properties to the effective flow law. The effect of mixing will be addressed not only with the mechanical deformation data but also on a microstructure level with post-experiment study of the final sample. This study will use multi-anvil high pressure deformation equipment at synchrotron facilities that have already provided deformation conditions to pressures equivalent to 600 km depth. This project will further develop the capabilities to achieve 800 km conditions for the highest-pressure measurements. The synchrotron X-ray provides stress and strain data in situ allowing documentation of sample evolution during deformation. In addition, stress is determined from the analysis of each diffraction peak yielding information about the stress partitioning between the minerals, and for each mineral, between sub-populations of grains that share a common crystallographic orientation relative to the stress field. All of this information constrains the deformation process in each mineral and the relative strengths of each mineral. This project will develop techniques suitable for synchrotron high-pressure research. The proposed research has potential to open new frontiers by combining different techniques and applying them to high pressure in situ studies. In particular, the experimental protocol that enables these experiments to 400 km depth is currently developed. This research program will refine the experimental tools to enable the experiments to pressures equivalent to 800 km depth. This will expand the national research base for high pressure deformation studies. The techniques that evolve from this project will be open to all users of the beamline through a proposal system to all scientists interested in carrying out these high-pressure experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.