Deep mantle structure and the postperovskite phase transition

Helmberger, Donald V.; Lay, Thorne; Ni, Sidao; Gurnis, Michael

doi:10.1073/pnas.0502504102

Cited by 46 publications

(24 citation statements)

References 54 publications

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“…These velocity jumps range from 0.48% (in mod7) to 2.52% (in mod11). The strength of these velocity discontinuities is similar to that reported for downwelling regions (e.g., Helmberger et al, 2005; Sun et al, 2016; Yao et al, 2015; Young & Lay, 1987a, 1987b). …”

Section: Apparent Splitting For a Deep Earthquake Sourcesupporting

confidence: 87%

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D ′′ layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D ′′, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.

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Section: Apparent Splitting For a Deep Earthquake Sourcesupporting

confidence: 87%

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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“…3, we have used two different Clapeyron slopes: ␥ ϭ 11.5 MPa/K, recently determined by high-pressure experiments (27), and ␥ ϭ 6 MPa/K, proposed earlier to fit the global DЉ features from tomography models and seismic forward modeling (28). Precisely, the latter has been used here for a Clapeyron curve that just crosses the average temperature profile 250 km above the CMB, the average depth of DЉ.…”

Section: Topology Of the Postperovskite Phase Transitionmentioning

confidence: 99%

Topology of the postperovskite phase transition and mantle dynamics

Monnereau

Yuen

2007

Proc. Natl. Acad. Sci. U.S.A.

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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...

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“…1A), but one must focus on the scattered wave field to image interfaces associated with transitions in mineralogy or composition. Scattering of PKP (the main P wave propagating through the core) in D′′, first recognized in the early 1970s (10), has been used to constrain stochastic models of deep mantle structure [e.g., (11)], but the most detailed and accurate constraints on D′′ structure to date have come from forward modeling of shear waves reflected at or near the CMB (12,13). This approach has its drawbacks, however.…”

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Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

Hilst

Hoop

Wang

et al. 2007

Science

246

172

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We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earth's deep interior (D′′) and detected multiple, piecewise continuous interfaces in the lowermost layer (D′′) beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D′′ stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 ± 200 kelvin. Beneath Central America, a site of deep subduction, the D′′ is relatively cold (DT = 700 ± 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m A t a depth of~2890 km, the core-mantle boundary (CMB) separates turbulent flow of liquid metals in the outer core from slowly convecting, highly viscous mantle silicates. The 200-to 300-km-thick thermochemical boundary layer on the mantle sidethe so-called D′′ layer-is enigmatic (1, 2), but a recently discovered phase transition from perovskite (pv) to postperovskite (ppv) in (Mg,Fe)SiO 3 (3-5) begins to explain seismologically observed complexity [e.g., (6)]. If the ppv transition occurs, one can, in principle, estimate in situ variations in temperature from the pressure-temperature dependence (that is, the Clapeyron slope) and the seismologically inferred location of the associated interface (7). Steep (conductive) thermal gradients in D′′ can produce multiple crossings of the phase boundary, and identification of associated seismic signals offers new opportunities for constraining (local) core heat flux (8, 9).Seismic (transmission) tomography delineates smooth changes in wave speed associated with mantle convection (Fig. 1A), but one must focus on the scattered wave field to image interfaces associated with transitions in mineralogy or composition. Scattering of PKP (the main P wave propagating through the core) in D′′, first recognized in the early 1970s (10), has been used to constrain stochastic models of deep mantle structure [e.g., (11)], but the most detailed and accurate constraints on D′′ structure to date have come from forward modeling of shear waves reflected at or near the CMB (12, 13). This approach has its drawbacks, however. First, it requires prior knowledge about the target structure and often assumes relatively simple geometries, the uniqueness of which is not easily established. Second, it relies on signal associated with near-and postcritical incidence, which limits radial resolution and the CMB regions that can be studied (14). The small distance window can also reduce the available source-receiver azimuths, which can degrade imaging in dir...

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Deep mantle structure and the postperovskite phase transition

Cited by 46 publications

References 54 publications

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Topology of the postperovskite phase transition and mantle dynamics

Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

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