Seismic Detection of Post-perovskite Inside the Earth

Cobden, Laura; Thomas, Christine; Trampert, Jeannot

doi:10.1007/978-3-319-15627-9_13

Cited by 54 publications

(79 citation statements)

References 201 publications

(308 reference statements)

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“…Furthermore, only thermo-chemical models produce an anti-correlation between dlnV s and density (the purely thermal model predicts radially averaged correlations of 1, even within the post-perovskite stability field) R (e.g. Wookey et al 2005;Ammann et al 2010;Stixrude and Lithgow-Bertelloni 2011;Cobden 2015). It remains to be tested how such R values, which also display strong lateral variations , would be resolved in long-wavelength seismic images.…”

Section: High ∂Ln V S /∂Ln V P and Anti-correlated ∂Ln V S And ∂Ln V φmentioning

confidence: 99%

Thermally Dominated Deep Mantle LLSVPs: A Review

Davies

Goes

Lau

2015

The Earth's Heterogeneous Mantle

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The two large low shear-wave velocity provinces (LLSVPs) that dominate lower-mantle structure may hold key information on Earth's thermal and chemical evolution. It is generally accepted that these provinces are hotter than background mantle and are likely the main source of mantle plumes. Increasingly, it is also proposed that they hold a dense (primitive and/or recycled) compositional component. The principle evidence that LLSVPs may represent thermo-chemical 'piles' comes from seismic constraints, including the following: (i) their longwavelength nature; (ii) sharp gradients in shear-wave velocity at their margins; (iii) non-Gaussian distributions of deep mantle shear-wave velocity anomalies; (iv) anti-correlated shear-wave and bulk-sound velocity anomalies (and elevated ratios between shear-and compressional-wave velocity anomalies); (v) anti-correlated shear-wave and density anomalies; and (vi) 1-D/radial profiles of seismic velocity that deviate from those expected for an isochemical, well-mixed mantle. In addition, it has been proposed that hotspots and the reconstructed eruption sites of large igneous provinces correlate in location with LLSVP margins. In this paper, we review recent results which indicate that the majority of these constraints do not require thermo-chemical piles: they are equally well (or poorly) explained by thermal heterogeneity alone. Our analyses and conclusions are largely based on comparisons between imaged seismic structure and synthetic seismic structures from a set of thermal and thermo-chemical mantle convection models, which are constrained by ~300 Myr of plate motion histories. Modelled physical structure (temperature, pressure and composition) is converted into seismic velocities via a thermodynamic approach that accounts for elastic, anelastic and phase contributions and, subsequently, a tomographic resolution filter is applied to account for the damping and geographic bias inherent to seismic imaging. Our results indicate that, in terms of large-scale seismic structure and dynamics, these two provinces are predominantly thermal features and, accordingly, that chemical heterogeneity is largely a passive component of lowermost mantle dynamics.

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Section: High ∂Ln V S /∂Ln V P and Anti-correlated ∂Ln V S And ∂Ln V φmentioning

confidence: 99%

Thermally Dominated Deep Mantle LLSVPs: A Review

Davies

Goes

Lau

2015

The Earth's Heterogeneous Mantle

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“…It also causes scattering of seismic energy (e.g., Bataille et al, ; Brana & Helffrich, ; Mancinelli et al, ; Thomas et al, ; Vidale & Hedlin, ) and displays anisotropy (e.g., see Nowacki et al, ; Romanowicz & Wenk, , for reviews). A phase transition from a magnesium‐silicate bridgmanite to postperovskite (ppv) structure (Murakami et al, ; Oganov & Ono, ) has opened the possibility to explain some of these seismic features and reflections from the D″ discontinuity (Hirose, ; Lay & Garnero, ; Thomas et al, ; Wookey, Stackhouse, et al, ); however, not all deep mantle observations are easily reconciled with the ppv phase transition (Cobden et al, ).…”

Section: Introductionmentioning

confidence: 99%

Discriminating Between Causes of D″ Anisotropy Using Reflections and Splitting Measurements for a Single Path

2019

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Knowledge of deep mantle deformation is based on seismic anisotropy: the variation of seismic wave speed and polarization with direction. Measuring this directional dependency requires azimuthal seismic coverage at D″ depth-the bottom few hundred kilometers of the mantle-which is often a limit in retrieving the style of anisotropy. Shear wave splitting is the standard technique for probing mantle anisotropy, and recently, reflections from the D″ region have been used to infer azimuthal anisotropy. Here we combine observations and modeling of D″ reflections with shear wave splitting along a given raypath direction in order to constrain mineralogy and dynamics of the lower mantle. From our modeling, a clear distinction between different anisotropic media is possible by using both types of observations together but only one directional path. We focus on the lowermost mantle beneath the central Atlantic Ocean by using South-Central American earthquakes recorded in Morocco. We find complex azimuthal and distance variation for both polarities of D″ reflections and shear wave splitting parameters, which rules out a simple style of anisotropy-such as vertical transverse isotropy-for the region. Our preferred model consists of a phase transition from a randomly oriented bridgmanite to lattice-preferred orientation fabric in postperovskite, developed in a tilted plane sheared along a roughly SW-NE deformation direction.

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“…One possible explanation of such reflections off the top of the D″ region is a phase transition from perovskite to postperovskite located from tens of kilometers up to approximately 400 km above the core‐mantle boundary (CMB) [e.g., Murakami et al ., ; Tsuchiya et al ., ]. This phase transition can cause a small velocity jump and a density jump of around 1% [e.g., Murakami et al ., ; Wookey et al ., ; Lay and Garnero , ; Cobden et al ., ; Irifune and Tsuchiya , ], and it might also explain variable P wave contrasts due to anisotropic behavior [ Thomas et al ., ]. This phase transition also elegantly explains the diversity of depth observations, as the phase transition depth is temperature dependent [ Hirose , ].…”

Section: Introductionmentioning

confidence: 99%

“…Finally, the earthquake signals are often polluted by upgoing depth phases, such as pP or sP, and Moho multiples [e.g., Chaloner et al, 2009]. Therefore, only very few source receiver combinations are available at present [Lay, 2007;Lay and Garnero, 2007;Cobden et al, 2014] to illuminate this region near the CMB, which effectively prevents good coverage of the globe.…”

Section: Introductionmentioning

confidence: 99%

Imaging the D″ reflector with noise correlations

Poli

Thomas

Campillo

et al. 2015

Geophys. Res. Lett.

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The lowermost mantle of the Earth is characterized by seismic structures that range from a few tens to thousands of kilometers. At present, it is difficult to test hypotheses put forward to explain seismic observations due to poor seismic coverage, as particular earthquake-station geometries are needed. We demonstrate here that seismic noise correlations can be used to robustly image deep-mantle reflections with larger stacked amplitudes of reflected waves compared with earthquake data. In a comparison between noise and earthquake data, we find that the arrival times and the slowness of reflected waves, both sampling a region beneath Siberia, agree with those for a reflector at 2530 km depth, and the small amplitude reflections are sufficiently clear in the noise correlations to compare them reliably with synthetic data. Our data open exciting prospects for illuminating new target zones in the deep mantle to further constrain the dynamics and mineralogy of the deep Earth.

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Seismic Detection of Post-perovskite Inside the Earth

Cited by 54 publications

References 201 publications

Thermally Dominated Deep Mantle LLSVPs: A Review

Thermally Dominated Deep Mantle LLSVPs: A Review

Discriminating Between Causes of D″ Anisotropy Using Reflections and Splitting Measurements for a Single Path

Imaging the D″ reflector with noise correlations

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