Layered convection and the impacts of the perovskite‐postperovskite phase transition on mantle dynamics under isochemical conditions

Abstract

[1] The issue of the style of the mantle convection process remains important to the understanding of Earth's deep interior and surface processes. While results from structural seismology may be interpreted to support the existence of a whole mantle convection regime, in several geographic regions high-resolution reconstructions of Benioff zone body wave heterogeneity demonstrate that the downgoing slab appears to be "trapped" in the transition zone rather than continuing to penetrate unimpeded into the lower mantle. The presence of the recently discovered exothermic perovskite-postperovskite (Pv-pPv) phase transition that appears to define the top of the D″ layer adjacent to the core-mantle boundary may be expected to exert considerable impact on the mixing process. On the basis of the use of the most recent mantle parameters derived on the basis of mineral physics and a viscosity model inferred from glacial isostatic adjustment and Earth rotation observables, we reinvestigate the impact of the Pv-pPv transition on mantle mixing. Our analyses are based on a newly constructed axisymmetric control volume model, which is described in detail. Analyses with this model demonstrate that the action of the Pv-pPv transitions slightly decreases the tendency to layered flow due to the endothermic transitions that occur at 660 km depth and enhances the absolute radial mass flux especially in the lower mantle. However, the results also demonstrate that the episodically layered style of mantle mixing persists in models in which the strength of the Pv-pPv transition is significant.

The History of the Earth's Rotation: Impacts of Deep Earth Physics and Surface Climate Variability

“…More recently, further analyses have been performed in Liu and Peltier (2013a,b) to directly investigate the variations in global sea level that would have accompanied these severe glaciation events. This is the so-called avalanche effect discussed at length by Peltier and Solheim (1992), by Solheim and Peltier (1994a,b), and more recently in Shahnas and Peltier (2010), an effect upon the temporal variability of the convection process that is controlled by the action of the phase transition from spinel to a mixture of perovskite and magnesiowü stite that exists at 660 km depth and which separates the so-called transition zone of the mantle above this level from the lower mantle beneath. For the purpose of these analyses, the version of the SLE formalism described in Section 9.09.3.5 was employed, which enables a direct assessment of the extent to which such a surface ice-sheet loading event might be expected to produce a significant wander of the rotation pole.…”

The impacts of mantle phase transitions and the iron spin crossover in ferropericlase on convective mixing—is the evidence for compositional convection definitive? New results from a Yin‐Yang overset grid‐based control volume model

“…Beginning with the work of Richter [], the influence of mantle phase transitions upon the convection process has been extensively studied. In particular, the role of mantle phase transitions on the degree of mantle layering has been tested in a wide range of numerical models [ Christensen , ; Christensen and Yuen , , ; Machetel and Weber , ; Peltier and Solheim , ; Solheim and Peltier , ; Tackley et al ., ; Christensen , ; Brunet and Machetel , ; Shahnas and Peltier , ]. At Earth‐relevant conditions, an exothermic phase transition (in which the latent heat is released in transition from the low‐pressure to high‐pressure forms) energizes convective mixing, whereas an endothermic phase transition diminishes its strength.…”

“…However, the degree of mantle layering that is expected to be due to the endothermic phase transition at 660 km depth has yet to be established definitively. A detailed recent review of the implications of high‐resolution seismic tomographic imaging studies of several subduction zones has recently been provided by Shahnas and Peltier [], and no purpose will be served by repeating it here. Suffice it to say that in many such regions the downgoing slab is inferred to be either entirely arrested in its descent and lying flat across the endothermic horizon or accumulating within the transition zone between 420 and 660 km depth.…”

“…This mechanism has been suggested to be the primary candidate for the explanation of the Wilson cycle of supercontinent creation and destruction [ Peltier et al ., ] and the generation of superplumes. The issue of the (effective) Clapeyron slope of the endothermic transition at 660 km depth continues to be an issue regarding the effectiveness of its impact upon convective mixing, the literature on which was also reviewed by Shahnas and Peltier []. It is important to understand in this context that there in fact exists a series of phase transitions near the 660 km depth horizon involving the transformation of spinel + stishovite to ilmenite at ~20 GPa and from ilmenite to perovskite at ~24 GPa.…”

“…The nondimensional momentum equation is then given by where the Rayleigh number of the system is Dropping the primes in order to simplify the notation then reduces equation to The components of the momentum equations and the stress tensors in spherical geometry are given in Appendix A. The nondimensional form of the energy equation may be written [ Shahnas and Peltier , ] as where the further nondimensional parameters appear as …”