Читать книгу Mantle Convection and Surface Expressions - Группа авторов - Страница 31

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 et al., 2018; Cottaar et al., 2014; Long & Becker, 2010; Nowacki et al., 2010, 2011; Tommasi et al., 2018; Walker et al., 2011, 2018).

Although the exact mineralogy and chemistry of the lower mantle is still debated, seismic velocities and densities of the lower mantle are consistent with a mineralogy composed predominantly (65% to 85% by volume) of (Mg,Fe)(Si,Al,Fe)O₃ bridgmanite (Brg) with (Mg,Fe)O ferropericlase (Fp) as the second‐most abundant phase, and a few percent of CaSiO₃ perovskite (Ca‐Pv) (e.g., Kurnosov et al., 2017; Matas et al., 2007; Mattern et al., 2005). At conditions similar to those of the D” (~2700 km), Brg undergoes a solid‐solid phase transformation to a post‐perovskite structured phase (pPv) (Murakami et al., 2004; Oganov & Ono, 2004; Shim et al., 2004). The stability and depth of this phase transition varies considerably depending on chemical composition (Grocholski et al., 2012), but pPv is likely to be a major phase at least in localized regions above the CMB. Iron content in the lower mantle is likely to be in the 10 mole % range (Matas et al., 2007) but the partitioning of iron between Fp, Brg, and pPv is not fully constrained (e.g., Piet et al., 2016). This chapter will focus on reviewing the current state of deformation experiments on the major lower mantle phases (Brg, Ca‐Pv, pPv, and Fp) with a discussion of the current knowledge of deformation mechanisms in these phases and the potential for interactions between phases when deformed in a polyphase rock.