The Microphysics of Plate Boundary Formation: Dynamic Recrystallization and Phase Mixing

现代板块构造理论解释了地球的构造和运作，从海底扩张到俯冲和造山。尽管这一强有力的理论告诉了我们很多关于地球表面活动的信息，但对于板块构造是如何开始的，以及允许它随时间持续下去的潜在物理原理，我们所知相对较少。毫无疑问，地幔的对流驱动了这些板块的运动。然而，这个过程是如何发生的以及它是如何起源的细节还远未完成。板块构造是由特殊的边界界定的，即大陆边缘的俯冲带、山脊和走滑断层边缘。这些边界是在地幔的顶部形成的，这需要一些特殊的物理性质，通过某些机制使变形局部化。值得庆幸的是，地球提供了一条重要的线索，在板块边界发现了大量被称为糜棱岩的岩石。这些细粒岩石提供的物理证据可能会告诉我们板块构造是如何开始的，以及它是如何持续到今天的。研究者的研究抓住了导致糜棱岩发育的基本物理机制，糜棱岩作为弱带持续存在，最终发展为板块边界。这项工作是PI实验室一名研究生博士论文项目的主要重点，除了为我们对构造板块生成的理解的未来进展提供必要的基础研究外，还将有助于培养学生作为科学家的技能。在本研究中开发的实验技术将应用于新的本科实验室练习，以展示涉及微观结构发展和材料机械性能之间反馈的复杂过程。地球在太阳系中是独一无二的，因为它表现出板块构造。对这种行为的物理解释仍然是一个重大挑战。本研究的重点是(i)二次固相和（ii）局部变形对岩石圈强度的影响及其对板块构造的影响。该项目强调实验室实验的变革方法，旨在研究橄榄石变形的第二阶段的作用，橄榄石是地球上地幔的主要阶段。为了获得稳态微观结构和稳态蠕变速率，该团队将制造晶粒尺寸明显大于稳态晶粒尺寸的两相样品。然后，他们将这些样品在扭转变形到大剪切应变，这样动态再结晶产生稳态晶粒尺寸。应变速率对应力的依赖是通过速率或加载步骤确定的。一些样品将在大应变后退火，以测量完全混合的动态再结晶样品中的晶粒生长。粗粒岩石动态再结晶引起的晶粒缩小现象在实验和自然剪切带观测中都得到了很好的证实。这一过程导致了晶粒尺寸敏感过程和相应的应变弱化。然而，如果没有持续的变形和动态再结晶，晶粒就会长大并最终导致硬化，从而使剪切区难以再激活。在多相材料中，二次相对晶界的固定抑制了晶粒的生长，使局部化得以持续或更容易被重新激活。为了使这一过程有效，各阶段必须彻底混合。了解导致相混合和局部化的化学和机械过程是本提案的主要目标。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。

The modern theory of plate tectonics explains much about the structure and workings of our planet from sea-floor spreading to subduction and mountain building. Although this powerful theory tells us so much about activity on the surface of our planet, relatively little is known about how plate tectonics starts and the underlying physics that allows it to continue over time. There is little doubt that convection of the mantle drives the motion of these plates. However, the details of how this process occurs and how it originated are far from complete. Plate tectonics is defined by special boundaries, namely subduction zones, ridges, and strike-slip fault margins at the edges of continents. These boundaries are formed at the top of the mantle, which requires some special physical properties that localize deformation through certain mechanisms. Thankfully, the Earth has provided an important clue through the abundance of rocks called mylonites that are found at plate boundaries. These fine-grain rocks provide evidence of the physics that might tell us how plate tectonics started and how it persists today. The investigator's research captures the essential physics of the mechanisms that lead to development of mylonites that persist as weak zones and ultimately develop as plate boundaries. This work is the primary focus of the doctoral dissertation project of a graduate student in the PI's lab and will help develop the student's skills as a scientist in addition to providing fundamental research necessary for the future progress of our understanding of the generation of tectonic plates. The experimental techniques developed in this research will be applied to new undergraduate lab exercises that demonstrate complex processes involving feedback between microstructural development and mechanical properties of the material. The Earth is unique in the solar system in that it displays plate tectonics. The physical explanation for this behavior remains a major challenge. This research focuses on the influence of (i) a secondary solid phase and (ii) localized deformation on lithospheric strength with implications for plate tectonics. The project emphasizes a transformative approach to laboratory experiments designed to investigate the role of a second phase in the deformation of olivine, the primary phase in Earth's upper mantle. To achieve steady-state microstructures and thus steady state creep rates, the team will fabricate two-phase samples with grain sizes significantly larger than the steady-state grain size. They will then deform these samples in torsion to large shear strains, such that dynamic recrystallization produces a steady-state grain size. The dependence of strain rate on stress is determined through rate or load steps. Some samples will be annealed after large strain to measure grain growth in thoroughly mixed, dynamically recrystallized samples. Grain-size reduction due to dynamic recrystallization of coarse-grained rocks is well established both experimentally and in observations of natural shear zones. This process leads to a grain-size sensitive process and associated strain weakening. However, without continued deformation and dynamic recrystallization, grains will grow and ultimately lead to hardening that makes reactivation of a shear zone difficult. In polyphase materials, pinning of grain boundaries by a secondary phase inhibits grain growth, allowing localization to persist or to be more easily reactivated. For this process to be effective, the phases must be thoroughly mixed. Understanding both the chemical and mechanical processes that lead to phase mixing and localization is the primary goal of this proposal.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.