Mantle Convection and Surface Expressions. Группа авторов

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СКАЧАТЬ earth science, crustal and upper mantle materials can exhibit similar behaviors. Handy (1994) notes in his discussion of IWL behavior that there is large stress, strain, and strain rate heterogeneity in regions surrounding hard feldspar grains in naturally deformed quartz feldspar mylonites, but texture measurements were not reported. In quartz + mica aggregates, the addition of rigid micas has been shown to reduce texture strength in the quartz phase both in simulations and experiments (Canova et al., 1992; J. Tullis & Wenk, 1994). Bystricky et al. (2006) studied deformation of olivine + (Mg_0.7Fe_0.3)O as an analog for polyphase mantle rock. This sample contained 20% olivine and was deformed in dislocation creep to very high shear strains of γ ~15 in a gas media apparatus. In these experiments, the majority of the strain was partitioned into the Fp matrix but olivine grains did deform and developed texture. Although the Fp phase did texture, the texture patterns of the Fp differed significantly from previous single phase experiments (Heidelbach et al., 2003; Yamazaki & Karato, 2002). These anomalous textures were attributed to local heterogeneity in flow geometry adjacent to the olivine clasts (Bystricky et al., 2006). Similar to the behavior of metal composites, these studies show that large differences in rheological properties between two phases in an aggregate can result in heterogeneous deformation even with relatively small amounts of the harder phase. Likewise, in earth materials heterogeneous deformation of the softer phase can result in significantly modified textures and/or randomization of the softer phase.

      2.5.4 Texture Development in High Pressure Studies of Polyphase Aggregates

Similar trends in polyphase texture development are also observed in high‐pressure studies. When Brg + Fp are deformed together in the DAC, Fp does not develop significant texture (Miyagi & Wenk, 2016; Wenk et al., 2004; Wenk, Lonardelli, et al., 2006) (Figure 2.7). In these materials, weaker Fp is expected to accommodate the majority of strain (Girard et al., 2016), but in contrast to metal matrix composites and studies on quartz + mica and olivine + Fp aggregates the harder bridgmanite phase is volumetrically dominant. This behavior is similar however to Al+Pb where textures in Pb become nearly random at larger volume fractions of Al (Brokmeier et al., 1988). Similarly, in two‐phase aggregates of NaMgF₃ Pv + NaCl deformed in the D‐DIA at a range of strain rates, temperatures and phase proportions, NaCl does not develop significant texture while the stronger NaMgF₃ phase does (Kaercher et al., 2016). These materials have large strength contrasts of ~10 and this behavior is observed over a wide range of phase proportions (NaCl = 15–70%). Microstructural analysis using synchrotron tomography and scanning electron microscopy shows that although texture development in this phase is weak, the majority of the strain partitions into NaCl. Plasticity modeling indicates that local stress–strain heterogeneities caused by grain‐to‐grain interactions between the Pv phase and the NaCl phase result in a randomization of the NaCl phase (Kaercher et al., 2016), consistent with polyphase texture studies at lower pressures (Brokmeier et al., 1988; Canova et al., 1992; Garcés et al., 2006; Poudens et al., 1995; J. Tullis & Wenk, 1994).

      In contrast to experiments on Brg + Fp in the DAC and NaMgF₃ + NaCl aggregates in the D‐DIA, D‐DIA deformation experiments on CaGeO₃ + MgO do observe texture development in MgO (Y. Wang et al., 2013). Notably the strength contrast between CaGeO₃ and MgO (~2) is significantly lower than NaMgF₃ and NaCl (~10) and similar or lower than Brg and Fp (~1.4‐4). This discrepancy may due strength contrast but also to the degree of anisotropy of the Pv phase. Both Brg and NaMgF₃ are orthorhombic and exhibit a dominant slip plane that has no additional symmetric variants (of the plane). In Brg the dominant slip plane at room temperature appears to be (001) (e.g., Cordier et al., 2004; Miyagi & Wenk, 2016; Miyajima et al., 2009) or (100) at higher temperatures (Tsujino et al., 2016). In NaMgF₃ Pv the dominant slip plane appears to be (100) (Kaercher et al., 2016). Thus, plastic anisotropy of these phases is relatively high. In contrast, CaGeO₃ is close to cubic (pseudo‐cubic) and deform predominantly on the {110}〈10〉 slip system. This slip system has six symmetric variants and so plastic anisotropy is much lower than for Brg and NaMgF₃ Pv. Thus, it is likely that the degree of plastic anisotropy of the harder phase plays an important role in controlling texture development in the softer phase. The modeling of Kaercher et al. (2016) shows that highly anisotropic NaMgF₃ Pv grains impinge on NaCl grains and cause heterogeneous local stress‐strain fields around the soft NaCl grains. This causes the local stress–strain field to deviate from the macroscopic field, and since this varies from grain to grain, the NaCl phase does not develop “coherent” texture.

Figure 2.7 Texture development during diamond anvil cell deformation of bridgmanite + ferropericlase synthesized from San Carlos olivine. Inverse pole figures for bridgmanite are shown in (a) and inverse pole figures for ferropericlase are shown in (b). Surprisingly, ferropericlase, which is the weaker phase, does not develop texture while bridgmanite does.

      Source: Miyagi & Wenk (2016).

In contrast to Brg + Fp and their analogs, deformation studies on pPv + Fp have not been performed either on the silicate system or in analog materials. Since Mg‐pPv is orthorhombic and appears to exhibit dominant slip on the (001) plane (Miyagi et al., 2010) one might expect that Fp deformed with Mg‐pPv may not develop texture due to high plastic anisotropy of the Mg‐pPv. However, a theoretical study by Ammann et al. (2010) suggests that Mg‐pPv may be considerably weaker than Brg. Experimental work on CaIrO₃ Pv and pPv found that the pPv phase is ~5 time weaker than the Pv phase (Hunt et al., 2009), and in NaCoF₃ the pPv phase is 5‐10 time weaker than the Pv phase (Dobson et al., 2012). This strength difference is similar to or larger than proposed strength differences between Brg and Fp (Girard et al., 2016; Kraych et al., 2016; Miyagi & Wenk, 2016) and it is possible that MgSiO₃ pPv is similar in strength or even weaker than Fp. If MgSiO₃ pPv is significantly weaker than Brg, then the strength contrast between MgSiO₃ pPv and Fp may be low enough that the polyphase behavior is significantly different than in Brg + Fp aggregates and it is not clear how texture may develop in the individual phases. Additionally experiments suggest coexistence of Brg and MgSiO₃ pPv in many regions above the CMB (Andrault et al., 2010; Grocholski et al., 2012). Thus the deformation behavior of MgSiO₃ pPv + Brg and MgSiO₃ pPv + Brg + Fp aggregates will also be important in the lowermost mantle.

      Clearly, large differences in rheological properties between two phases in an aggregate can result in heterogeneous deformation, even with relatively small amounts of the harder phase. Heterogeneous deformation of the softer phase can result in randomization of the softer phase. While many examples on metals and crustal rocks are aggregates where the soft phase is volumetrically dominant, in high‐pressure phases it appears that low‐symmetry hard phases that are volumetrically dominant can disrupt texture formation in the soft phase. However, systematic study of this phenomenon as a function of plastic anisotropy, microstructure, phase fractions, and strength contrast has not yet been undertaken.

2.6 IMPLICATIONS

      In polyphase mantle rock, complex interactions with respect to bulk strength are likely. At this point, it seems well established that Fp is weaker than Brg but the degree of this strength contrast is not fully constrained (Girard et al., 2016; Kraych et al., 2016; Miyagi & Wenk, 2016; Yamazaki & Karato, 2001b). Depending on the amount of Fp and the microstructure of the mantle and IWL type, microstructure may be relevant and Fp would control the strength of the aggregate. Given that estimates of the volume fraction СКАЧАТЬ