Читать книгу Mantle Convection and Surface Expressions - Группа авторов - Страница 37

Caution should be exercised when extrapolating laboratory results to the deep Earth. First and foremost, laboratory strain rates are many orders of magnitude faster than natural strain rates. Laboratory strain rates for deformation of lower mantle phases are typically on the order of 10⁻⁴ s⁻¹ to 10⁻⁷ s⁻¹. In contrast, strain rates in the Earth are likely on the order of 10⁻¹² s⁻¹ to 10⁻¹⁶ s⁻¹. Given the large difference in the human time scale and geologic time this discrepancy cannot be addressed directly by laboratory studies and systematic trends from laboratory experiments must be extrapolated to conditions of the Earth. One potential pitfall with this is the underlying assumption that no unknown deformation mechanism exists that operates at mantle conditions but is not operative in the laboratory (Boioli et al., 2017). Unlike the crust and upper mantle, natural samples that can provide evidence of operative deformation mechanisms do not exist and thus one is left to use the best data from the laboratory and from theory.

Total strain achieved in experiments can also be quite different than those in nature. While some devices can achieve large strains, for example the RDA has been used to achieve shear strains of ~2 (200%) (Y. Xu et al., 2005) and the rotational DAC has been used to deform samples to shear strains of ~4 (400%) (Nomura et al., 2017), compression studies typically attain strains of ~0.3 strain (30%) (e.g., Marquardt & Miyagi, 2015; Y. Wang et al., 2003). In contrast, slabs subducted into the lowermost mantle likely experience much larger shear strains on the order of 6–10 (e.g., Allègre & Turcotte, 1986; Tommasi et al., 2018; Wenk et al., 2011).

High pressures and temperatures of the full range of the lower mantle are also difficult to achieve in deformation experiments, and, thus, deformation experiments have been limited to lower pressures and high temperatures or high pressures and low temperatures. Unfortunately, deformation mechanisms in lower mantle phases can change with temperature (e.g., Immoor et al., 2018) and also with pressure (Marquardt & Miyagi, 2015), and thus direct extrapolation of lower‐pressure, high‐temperature studies or high‐pressure, low‐temperature studies may not represent the behavior in the deep Earth. However, with appropriate caution, some constraints on the deformation mechanisms and general rheology of the lower mantle can be obtained.