Topology of the postperovskite phase transition and mantle dynamics

Abstract

The postperovskite (ppv) phase transition occurs in the deep mantle close to the core-mantle boundary (CMB). For this reason, we must include in the dynamical considerations both the Clapeyron slope and the temperature intercept, T int, which is the temperature of the phase transition at the CMB pressure. For a CMB temperature greater than T int, there is a double crossing of the phase boundary by the geotherms associated with the descending flow. We have found a great sensitivity of the shape of the ppv surface due to the CMB from variations of various parameters such as the amount of internal heating, the Clapeyron slope, and the temperature intercept. Three-dimensional spherical models of mantle convection that can satisfy the seismological constraints depend on the Clapeyron slope. At moderate value, 8 MPa/K, the best fit is found with a core heat flow amounting for 40% of the total heat budget (Ϸ15 TW), whereas for 10 MPa/K the agreement is for a lower core heat flow (20%, Ϸ7.5 TW). In all cases, these solutions correspond to a temperature intercept 200 K lower than the CMB temperature. These models have holes of perovskite adjacent to ppv in regions of hot upwellings. has provoked an immense interest in the earth sciences because of the close proximity of the phase change to the core-mantle boundary (CMB), unveiling a large part of the enigmatic DЉ layer. The DЉ is a seismic structure whose complexity has remained puzzling for a long time. Interpreted in terms of phase boundary undulations, it would offer the chance to provide an absolute constraint on the temperature within the thermal boundary layer of the mantle. This would have great impact on our understanding of the dynamical state and thermal history of the earth. Concerted efforts recently have been devoted to sharpening the seismic imaging of DЉ (4-6), but results remain interpreted in the framework of 1D thermal models. On the other hand, previous work (7-11) investigating mantle convection with a ppv transition has focused more on dynamical effects and the magnitude of the Clapeyron slope than on the topology of the ppv surface itself. Thus, these investigators have neglected to consider the temperature intercept of the phase transition, which matters a lot in the case of a ppv transition because it is situated so near to the lower boundary of mantle convection.This delicate location makes it necessary for one to take into account the relative magnitude between the temperature at the CMB, T CMB , and the temperature of the ppv transition at the CMB pressure of 135 GPa. This latter temperature is called the temperature intercept, T int (see Fig. 1). In Fig. 1, we show the relationship between various geotherms (cold, warm, and hot) and the phase boundary, commonly known as the Clapeyron curve (red color), which is given by the equation cast in red. We note that in the case when T CMB is greater than T int , the geotherm may cross the phase boundary at two places (known as ''double-crossing'') (4). This feature is necessarily related to the existe...

Deep Earth Structure: Lower Mantle and D″

“…Because the Clapeyron slope for the deep mantle transition is about twice that of the upper mantle transition, the effect is enhanced, and convection models that include the postperovskite transition have quite unstable lower thermal boundary layers that tend to generate vigorous deep mantle flow (e.g., Cizkova et al, 2010;Kameyama and Yuen, 2006;Yuen, 2004, 2006;Monnereau and Yuen, 2007;Nakagawa and Tackley, 2004Tackley et al, 2007;Tosi et al, 2010;van den Berg et al, 2010;Yuen et al, 2007). Warmer regions of the boundary layer will have a thinner layer of dense postperovskite mineralogy, while colder regions of the boundary layer will have a thicker layer of the denser material.…”

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

“…At the present stage of the cooling history, the Pv‐pPv transition is expected to occur only in the lowest temperature regions within D ″. This could explain the characteristics of the observed shear velocity discontinuity, which are such that the high‐velocity lenses are found only in regions that are expected to be anomalously cold [e.g., Monnereau and Yuen , 2007, 2010]. The presence of a very low‐viscosity post‐perovskite lens or layer above the CMB would be expected to enhance the CMB heat flux in the cold slab regions and may cause deflection of the cold subducting material in D ″ [ Cizkova et al , 2010].…”

“…Similarly, in anomalously warm regions adjacent to the CMB, the postperovskite phase is expected to be absent. For a CMB temperature greater than the intercept temperature, which is the temperature of the phase transition at CMB pressure, there is a “double crossing” of the phase boundary by the geotherms associated with the cold descending flow [ Hernlund et al , 2005; Monnereau and Yuen , 2007]. Employing the seismic images of the D ″ layer and double crossing depths, Monnereau and Yuen [2010] have attempted to put constraints on the CMB heat flux.…”

Statistical study of seismic heterogeneities at the base of the mantle from PKP differential traveltimes

“…The transformation of perovskite to post‐perovskite is now the favoured explanation for a discontinuity at the top of the D″ layer. Due to the large clapeyron slope of this phase change, the topography of the D″ discontinuity is expected to be quite large, of the order of a few hundred kilometres over scales that are related to the lateral variations of temperature in D″ (Monnereau & Yuen 2007). However, the P ‐wave velocity contrast of this phase transition at constant chemistry is small (Oganov & Ono 2004; Stackhouse et al 2006).…”