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物质的机械性质决定的。这些动态包括受人类影响的地质过程，如地震和冰河时代后发生的地表变形，从而导致海平面上升。橄榄石是上地幔中最丰富的矿物，也可能是最弱的矿物。因此，正是橄榄石的机械性能控制着地壳和地幔的微小垂直位移，这是由于上一个冰期之后冰盖融化造成的。这种垂直位移称为冰后反弹，是地幔瞬间蠕变引起的随时间变化的变形，用于估计地幔的粘度。相反，地幔的对流是一种稳定的现象，经常导致不同的有效粘度。因此，确定橄榄石的瞬时蠕变将使地幔粘度在不同时间尺度上得到准确的约束，但对橄榄石瞬时蠕变的研究有限。该项目将在地球上地幔条件下进行有溶解水和无溶解水的橄榄石变形实验。这些结果将从材料科学的角度进行解释，以解释矿物在瞬时和稳态蠕变下是如何变形的，以便更完整地应用于地球物理过程，如冰川后反弹和地震后松弛。这是美国国家科学基金会对兼任耶鲁大学地球材料表征中心(EMC2)主任的PI提出的第一项建议；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。PIs积极参与各级STEM培训；具体地说，该项目将培训一名本科生和一名博士后研究人员进行同步加速器高压变形实验，使他们获得最先进技术的宝贵经验。在这个项目中，PI将解决重要的地球物理问题，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时蠕变机制。瞬时蠕变区的变形由变形过程中产生的线状晶体缺陷的形成和运动速度控制。在稳态下，压力和水含量对线性缺陷的运动有不同的影响，因此很可能也会影响瞬时蠕变。由于蠕变涉及多个微观过程，由短期变形推断的流变特性可能不同于与长期变形相关的流变特性。对瞬时蠕变的研究一直是有限的，特别是在阐明晶间和晶内变形机制的相对作用方面。这项研究将提供新的橄榄石变形实验，以弥合暂态和稳态蠕变状态。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水分变化的影响。该项目还包括一组实验，其中将使用双晶体来研究晶界滑动，以估计晶界滑动对粘度的影响。所有来自橄榄石集合体和单晶的结果将使用材料物理和微观结构表征进行解释，并将确定对地球物理过程的影响，如冰川后反弹和地震后松弛。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.