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

地幔的动力学是由其组成材料的力学性质决定的。这些动态包括人类影响的地质过程，如地震和冰河时期后发生的地表变形，其后果是海平面上升。橄榄石是上地幔中含量最丰富的矿物，也可能是最脆弱的矿物，因此正是橄榄石的力学性质控制了末次冰期后冰盖融化所引起的地壳和地幔的微小垂直位移。这种垂直位移称为冰后期回弹，是地幔瞬时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，确定橄榄石的瞬态蠕变将允许跨时间尺度的地幔粘度的准确约束，但橄榄石瞬态蠕变的研究是有限的。该项目将在地球上地幔条件下进行橄榄石有无溶解水的变形实验。将从材料科学的角度解释结果，以解释矿物如何在瞬态和稳态蠕变下变形，以便更完整地应用于地球物理过程，如冰后反弹和地震后松弛。这是美国国家科学基金会对PI的第一个建议，PI也是耶鲁大学地球材料表征中心（EMC 2）的主任;该项目将扩大和支持EMC 2的使命，为学生和博士后学者提供进一步的推广，培训和教育机会。PI积极参与各级STEM培训;具体而言，该项目将培训一名本科生和一名博士后研究员进行同步加速器高压变形实验，为他们提供最先进技术的宝贵经验。在这个项目中，PI将解决诸如冰后回弹和地震后松弛等重要问题，这些问题需要了解小应变下的瞬态蠕变机制。瞬态蠕变状态下的变形受变形过程中产生的线性晶体缺陷的形成和运动速率控制。压力和含水量已被证明会影响线性缺陷的运动不同的稳定状态，因此也很可能会影响瞬态蠕变。由于蠕变涉及多个微观过程，从短期变形推断的流变特性可能与长期变形相关的流变特性不同。瞬态蠕变的研究一直是有限的，特别是在阐明晶间与晶内变形机制的相对作用。这项研究将提供新的变形实验橄榄石桥梁瞬态和稳态蠕变制度。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，其中晶界滑动将使用双晶体进行研究，以估计晶界滑动对粘度的影响。所有橄榄石聚集体和单晶体的结果将使用材料物理学和微观结构特征进行解释，并对地球物理过程的影响，如冰后反弹和地震后松弛将被确定。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.