Effects of the post-perovskite phase transition properties on the stability and structure of primordial reservoirs in the lower mantle of the Earth
Introduction
One of the most important discoveries of the past decade in mineral physics is the phase change from perovskite (pv) to post-perovskite (pPv), which may occur under the conditions of the lowermost mantle of the Earth (Murakami et al., 2004, Oganov and Ono, 2004, Tsuchiya et al., 2004). This phase transition was predicted by Sidorin et al. (1999), who noted that an exothermic phase change above the core–mantle boundary (CMB) would explain the discontinuity observed by seismologists better than a chemically distinct layer. Since its discovery, post-perovskite was found to bear properties compatible with the properties of the region. In particular, the shear modulus of pPv is larger than that of pv, implying that shear-waves travel faster in pPv regions (e.g., Caracas and Cohen, 2005, Mao et al., 2007, Stackhouse and Brodholt, 2007). Recent seismic observations (Cobden and Thomas, 2013) suggest that the discontinuity may however have different origins depending on the polarities of P- and S-waves, and that the pPv phase transition may be a good candidate for regions where these polarities are opposite. A key property of the pPv phase transition is its large Clapeyron slope, around 8–10 MPa/K (Oganov and Ono, 2004), or even more according to recent estimates (Tateno et al., 2009, Hernlund, 2010), implying that pPv should not be stable in hot regions, which would explain why the seismic discontinuity atop is not ubiquitous. Interestingly, pPv is a strongly anisotropic mineral, and its presence may thus explain the anisotropy observed in the region (Wookey et al., 2005).
Seismic tomography models further reveal two Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle beneath Africa and the Pacific (e.g., Masters et al., 2000, Trampert et al., 2004, Ni et al., 2002, He and Wen, 2012). Cluster analysis (Lekic et al., 2012) and the fact that they are also observed by normal mode tomography (Ishii and Tromp, 1999, Trampert et al., 2004, Mosca et al., 2012) indicate that LLSVPs are robust features, not artifacts. Furthermore, probabilistic tomography (Trampert et al., 2004, Mosca et al., 2012) suggest that they are hotter and chemically distinct compared to the ambient mantle. Since the phase change from perovskite (pv) to post-perovskite (pPv) is mainly expected to occur in cold regions of the lowermost mantle, pPv may not be found within LLSVPs. This is in agreement with the most recent thermo-chemical distributions deduced from probabilistic tomography (Mosca et al., 2012). If it is present outside LLSVPs, pPv may explain the anti-correlation between shear-wave and bulk sound velocity anomalies (Hutko et al., 2008, Davies et al., 2012).
The presence of post-perovskite may have some substantial influences on mantle dynamics. It has been pointed out, for instance, that the distributions of dense material and post-perovskite are anti-correlated (Nakagawa and Tackley, 2005, Nakagawa and Tackley, 2006), and the spectra of chemical anomalies are strongly influenced by the topography of the post-perovskite phase transition (Nakagawa and Tackley, 2006). Furthermore, due to its large Clapeyron slope, the post-perovskite phase transition may be responsible for specific structures such as double-crossings in warm regions (Hernlund et al., 2005). It is therefore important to properly describe the interactions between the pPv phase transition and the LLSVPs. This, in turn, requires a good knowledge of the properties of pPv and of the conditions under which it appears, including the temperature at the CMB, the Clapeyron slope of the pPv phase transition, and the viscosity of the pPv relative to that of pv. These parameters, however, remain poorly constrained.
The CMB temperature provides important constrains on the thermal structure of Earth's mantle. Previous laser-heated diamond-anvil cell (DAC) experiments indicated that the solidus temperature of primitive mantle is about 4200 K at the CMB (e.g., Zerr et al., 1998, Fiquet et al., 2010, Andrault et al., 2011). This high CMB temperature indicates that part of the CMB region is in the perovskite stability field, thus preventing a global layer of pPv covering the CMB. However, a recent study by Nomura et al. (2014) suggests that a natural primitive mantle (pyrolite) with 400 ppm H2O could result in a much lower CMB temperature () compared to the previously assumed range of values.
The viscosity contrast between pv and pPv remains a matter of debate. Some experimental studies (e.g., Yoshino and Yamazaki, 2007, Hunt et al., 2009), as well as theoretical calculations by Ammann et al. (2010) based on the first-principle methods, reported a weak pPv viscosity, by a factor of O(103) to O(104) lower than that of pv. A low viscosity of pPv is consistent with recent geoid modelling, which requires colder regions of the deepest lower mantle to be weaker than hotter regions (Cadek and Fleitout, 2006). Meanwhile, some other studies give opposite results favouring more viscous pPv (Karato, 2011).
The Clapeyron slope of the phase transition from pv to pPv also varies in different experimental studies from early measured values of 8–10 MPa/K (e.g., Oganov and Ono, 2004) to the current preferred value of 13 MPa/K or higher (e.g., Tateno et al., 2009, Hernlund, 2010).
In this study, we perform a series of numerical experiments of thermo-chemical convection to investigate the influence of each of the three parameters discussed above on mantle convection. We focus in particular on their effects on the stability and structure of the primordial reservoirs in the lower mantle.
Section snippets
Numerical experiments set up
The numerical experiments are performed with StagYY (Tackley, 2008), which solves the conservation equations of mass, momentum, energy, and composition for an anelastic compressible fluid with infinite Prandtl number. Calculations are performed in 2-D spherical annulus geometry (Hernlund and Tackley, 2008) with a ratio between inner and outer radii of , matching the Earth's mantle.
The viscosity is assumed to depend on temperature, depth, phase, and yield stress. A viscosity jump of 30 is
Results
Using the setup outlined in Section 2, we performed a series of numerical experiments in which we varied the temperature at the CMB (), the viscosity contrast between pv and pPv (), and the Clapeyron slope of the pPv phase transition (). Model parameters are listed in Table 1, and properties of the runs discussed in this section are detailed in Table 2.
Implications for the lower mantle
Several interesting conclusions may be drawn from the numerical experiments discussed in Section 3.
First, increasing the CMB temperature favours large scale reservoirs of dense primordial material. Larger CMB temperature also reduces the stability field of pPv, which becomes thinner and more discontinuous as the CMB temperature increases. On the other hand, increasing CMB temperature promotes the growth of instabilities in the bottom TBL, allowing plumes to rise from regions outside the
Conclusions and perspectives
In this study, we have investigated the effects of the CMB temperature, viscosity contrast between pv and pPv, and Clapeyron slope of the pPv phase change on the distribution and size of the pPv stability field, and on the structure and stability of primordial reservoirs in the lower mantle. The numerical experiments we performed showed that:
1. The CMB temperature has a strong influence on the number and size of reservoirs of primordial material because it controls the mantle temperature, which
Acknowledgments
We are grateful to the Editor, Bruce Buffet, and the reviewer, Mingming Li, for the constructive review. This work was supported by Swiss National Science Foundation Grants SNF 200021-129510, 200020-149625, Academia Sinica (Taipei, Taiwan) Grant AS-102-CDA-M02, and National Science Council of Taiwan (NSC) Grant 101-2116-M-001-001-MY3. Calculations were run on ETH's brutus cluster. All the data for this study are available upon request to the corresponding author.
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