Synchrotron deformation experiments of olivine under the deep upper mantle conditions: Transient creep, plastic anisotropy, and the role of grain-boundary sliding.

地幔动力学是由其组成材料的力学特性决定的。这些动力学包括人类影响的地质过程，如地震和冰河期后发生的地表变形，导致海平面上升。橄榄石是上地幔中最丰富的矿物，但它可能是最弱的矿物。因此，正是橄榄石的力学特性控制了上一个冰河期后冰盖融化导致的地壳和地幔的微小垂直位移。这种垂直位移称为冰后反弹，是由地幔瞬态蠕变引起的随时间变化的变形，用于估计地幔粘度。相反，地幔的对流是一种稳态现象，经常导致不同的有效粘度。因此，确定橄榄石的瞬变蠕变可以在时间尺度上精确地约束地幔粘度，但对橄榄石瞬变蠕变的研究是有限的。本项目将在地球上地幔条件下对橄榄石进行有和无溶解水的变形实验。这些结果将从材料科学的角度进行解释，以解释矿物在瞬态和稳态蠕变下如何变形，以便更完整地应用于冰川后反弹和地震后松弛等地球物理过程。这是NSF为PI提出的第一个建议，PI也是耶鲁地球材料表征中心（EMC2）的主任；该项目将扩大和支持EMC2的使命，为学生和博士后学者提供进一步的外展、培训和教育机会。pi积极参与STEM的各级培训；具体而言，本项目将培养一名同步加速器高压变形实验本科生和一名博士后研究员，为他们提供最新技术的宝贵经验。在这个项目中，PI将解决地球物理学上重要的问题，如冰川后反弹和地震后松弛，这些问题需要了解小应变下的瞬态蠕变机制。瞬变蠕变过程中的变形受变形过程中线状晶体缺陷的形成和运动速率的控制。压力和含水量在稳定状态下对线性缺陷的运动有不同的影响，因此很可能也会影响瞬态蠕变。由于蠕变涉及多个微观过程，从短期变形推断出的流变特性可能与长期变形推断出的流变特性不同。对瞬态蠕变的研究有限，特别是在阐明晶间和晶内变形机制的相对作用方面。该研究将为橄榄石的瞬态和稳态蠕变过渡提供新的变形实验。这项工作将包括分析颗粒间和颗粒内变形机制的作用，以及它们如何受到压力和含水量变化的影响。该项目还包括一组实验，其中将使用双晶来研究晶界滑动，以估计晶界滑动对粘度的影响。所有橄榄石集子和单晶的结果将使用材料物理学和微观结构表征进行解释，并确定对冰川后反弹和地震后松弛等地球物理过程的影响。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 Earth’s mantle dynamics are governed by the mechanical properties of its constituent materials. These dynamics include geological processes with human impacts, such as surface deformations that occur after earthquakes and ice ages, with consequences on sea-level rise. Olivine is the most abundant mineral in the upper mantle, and it is likely the weakest mineral. Therefore it is olivine’s mechanical properties that control small vertical displacements of the crust and mantle due to the melting of ice caps after the last ice age. This vertical displacement, called post-glacial rebound, is a time dependent deformation arising from the transient creep of the mantle, and is used to estimate mantle viscosity. In contrast, the convection of the mantle is a steady-state phenomenon, frequently resulting in a different effective viscosity. Determining transient creep of olivine will therefore allow accurate constraint of mantle viscosity across time scales, but studies on olivine transient creep are limited. This project will perform deformation experiments of olivine with and without dissolved water under the conditions of the Earth’s upper mantle. The results will be interpreted from a materials science point of view to interpret how the mineral deforms under transient and steady-state creep for a more complete application to geophysical processes such as post-glacial rebound and post-seismic relaxation. This is the first NSF proposal for the PI, who is also the Director of the Yale Earth Materials Characterization Center (EMC2); this project will broaden and support the mission of EMC2 with further outreach, training, and educational opportunities for students and postdoctoral scholars. The PIs actively participate in all levels of STEM training; specifically, this project will train an undergraduate student and a post-doctoral researcher on synchrotron high-pressure deformation experiments, giving them valuable experience in state-of-the-art techniques. In this project, the PI will address geophysically important questions such as post-glacial rebound and post-seismic relaxation which require understanding of transient creep mechanisms at small strains. Deformation in the transient creep regime is controlled by the rate of formation and motion of linear crystal defects produced during deformation. Pressure and water content has been shown to affect the motion of linear defects differently in the steady state, and therefore will most likely also affect transient creep. As creep involves multiple microscopic processes, the rheological properties inferred from short-term deformation may differ from those relevant to long-term deformation. Studies on transient creep have been limited, especially in elucidating the relative roles of inter-granular versus intra-granular deformation mechanisms. This study will provide new deformation experiments on olivine to bridge transient and steady-state creep regimes. This work will include analysis of the roles of inter- and intra-granular deformation mechanisms, and how they are affected by the variations in pressure and water content. The project further includes a set of experiments where grain-boundary sliding will be studied using a bi-crystal to estimate the effect of grain-boundary sliding on viscosity. All results from olivine aggregate and single crystals will be interpreted using materials physics and microstructural characterization, and implications on geophysical processes such as post-glacial rebound and post-seismic relaxation will be determined.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.