Rheology of Orthopyroxene

地幔对流的热耗散驱动了板块构造和地壳的变形。这些变形导致地震和富饶的地幔，导致融化，从而导致火山爆发，这两种情况都对社会产生了重大影响。因此，重要的是要了解使地幔散热的机制。地球上地幔是一个多相系统，由大约55%的橄榄石、25%的斜方辉石、15%的单斜辉石和5%的铝相组成(Ringwood，1975)。然而，在岩石圈地幔部分熔融过程中，次生相(主要是单斜辉石和铝相)优先被去除，留下橄榄石和斜方辉石(例如英安岩和方辉橄榄岩)的残留物。因此，了解最主要的矿物橄榄石的变形机制，以及其流变性如何受到各种环境条件(压力、温度和化学环境)和物理因素(粒度和分布)的影响，一直是现场、实验和模拟研究的重点。然而，到目前为止，确定斜方辉石的强度的工作还很少，如果斜辉石的强度明显强于或弱于橄榄石，将影响地幔的粘度，从而影响对流和板块位移的速率。该项目将使用一种实验方法来研究应变速率、水含量、温度和压力如何影响斜方辉石聚集体的强度。这些实验将在两个不同的高压岩石变形设备中进行，一个位于德克萨斯农工大学，另一个位于布鲁克海文国家实验室的国家同步加速器光源，能够承受更高的围压。这些实验的数据可用于对板块边界地幔粘性的现场和数值模拟，并可能有助于我们理解引起地幔变形的过程，这些过程导致地震和火山活动。这项研究还将为职业生涯早期的科学家提供工资和指导，并为至少一名本科生提供实践研究经验。该项目的总体目标是使用实验技术研究由位错蠕变变形的斜方辉石聚集体的流变性，以及在轴向压缩变形几何形状中，这种流变性如何受到化学环境(水逸度或二氧化硅的羟基含量和活性)、压力和温度的影响。我们还将通过使用剪切变形几何进行实验，研究微结构如何作为应变的函数演变。这些目标将通过在广泛的温度、压力和应变率范围内在有水和干燥条件下进行实验，在格里格斯型活塞-圆柱体岩石变形仪器中进行，使用熔融盐池来精确确定力学数据，并由位于布鲁克海文国家实验室的国家同步加速器光源的COMPRES操作的变形DIA。

Heat dissipation by convection of the Earth's mantle drives plate tectonics and deformations in the Earth's crust. These deformations cause earthquakes and fertile mantle to conditions which cause melting and, therefore, volcanoes, both of which have a significant impact on society. Therefore, it is important to understand the mechanisms that enable heat dissipation of the Earth's mantle. The Earth's upper mantle is a polyphase system, composed in bulk of approximately 55% olivine, 25% orthopyroxene, 15% clinopyroxene and 5% aluminous phases (Ringwood, 1975). However, secondary phases (primarily clinopyroxene and aluminous phases) are preferentially removed during partial melting of the lithospheric mantle, leaving a residue of olivine and orthopyroxene (e.g. dunites and harzburgites). As a result, understanding the deformation mechanisms of the most dominant mineral, olivine, and how its rheology is affected by various environmental conditions (pressure, temperature and chemical environment) and physical factors (grain size and distribution) has been a priority of field-based, experimental and modeling studies. However, to date, little work has been performed to determine the strength of orthopyroxene, which if significantly stronger or weaker than olivine, would impact the viscosity of the mantle, and thereby affect rates of convection and plate displacement. This project will use an experimental approach to investigate how strain rate, water content, temperature and pressure affect the strength of orthopyroxene aggregates. The experiments will be performed in two different high pressure rock deformation apparatus, one located at Texas A&M University and another capable of higher confining pressures located at the National Synchrotron Light Source at Brookhaven National Laboratory. Data from these experiments can be used by field-based and numerical simulations of the viscosity of the mantle at plate boundaries and may aid in our understanding of the processes that cause the deformations in the mantle that causes earthquakes and volcanic activity. This study will also provide salary and mentoring for an early-career scientist and provide at least one undergraduate student with a hands-on research experience.The overall goals of this project are to use experimental techniques to investigate the rheology of orthopyroxene aggregates deforming by dislocation creep and how this rheology is affected by the chemical environment (water fugacity or hydroxyl content and activity of silica), pressure and temperature in an axial compression deformation geometry. We will also investigate how microstructures evolve as a function of strain by performing experiments using a shear deformation geometry. These goals will be accomplished by performing experiments in both water-present and dry conditions over a wide range of temperatures, pressures and strain rates in a Griggs-type piston-cylinder rock deformation apparatus using a molten salt cell for precise determination of mechanical data and the deformation DIA operated by COMPRES located at the National Synchrotron Light Source at Brookhaven National Laboratory.