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2 Experimental Deformation of Lower Mantle Rocks and Minerals
Lowell Miyagi
Geology and Geophysics, University of Utah, Salt Lake City, UT, USA
ABSTRACT
Plastic deformation of rocks and mineral phases in the Earth’s interior plays a primary role in controlling large‐scale dynamic processes such as mantle convection and associated plate tectonics. Volumetrically, the lower mantle is the largest region of the Earth, and thus great effort has been made to study the deformation behavior of the mineral phases that comprise the lower mantle. Plastic deformation of these rocks and mineral phases can also lead to preferred orientation development (texture), which, in turn, can result in anisotropic physical properties. Many regions of the Earth’s interior exhibit seismic anisotropy, and thus, considerable effort has been made to understand the processes leading to texture and anisotropy development in deep Earth phases. Studying deformation of lower mantle materials is technically challenging due to the extreme pressures and temperatures experienced by materials in the deep Earth. However, recent technological advances have allowed significant progress to be made toward understanding the deformation behavior of lower mantle phases. This chapter provides an overview of deformation mechanisms in lower mantle materials and the current state of experimental deformation studies on lower mantle mineral phases and polyphase aggregates of materials relevant to the lower mantle.
2.1 INTRODUCTION
Large‐scale geodynamic phenomena such as post‐glacial rebound, mantle convection, slab subduction, and plate motion are intimately linked to plastic deformation of minerals and rocks in the Earth’s deep interior. The lower mantle, which ranges from the 660 km seismic discontinuity and continues to the core mantle boundary (CMB) at ~2900 km depth, constitutes ~55% of the Earth by volume, and thus lower mantle properties play a fundamental role in controlling much of Earth’s dynamic behavior. As such, considerable effort has been directed at parameterizing rheological behavior of mantle rocks and minerals at high pressure and temperature conditions. For studying geodynamic behavior, the goal is generally to understand the stress–strain rate response of rocks and minerals in order to characterize viscosity of mantle materials. This behavior can change dramatically with changes in pressure, temperature, composition, and grain size (Cordier & Goryaeva, 2018; Frost & Ashby, 1982; Karato, 2010; Marquardt & Miyagi, 2015). Thus, a major area of research is to determine and quantify variations in stress–strain rate response of these materials as a function of these conditions. In addition to flow laws, great interest has been generated in understanding the relationship between deformation and anisotropy in mantle rocks. Seismic anisotropy is observed in many regions of the Earth’s interior and is widely believed to be due to deformation‐induced texture (crystallographic preferred orientation) of constituent mineral phases in the Earth’s interior (Karato, 1998; Karato et al., 2007; Long & Becker, 2010; Romanowicz & Wenk, 2017). If one has a detailed understanding of the mechanisms of texture development and the relationship between deformation, texture, and anisotropy, then one can use observations of seismic anisotropy to infer mantle flow patterns in the deep Earth (Chandler