The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3

Stackhouse, Stephen; Brodholt, John P.; Wookey, James; Kendall, J. M.; Price, G. D.

doi:10.1016/j.epsl.2004.11.021

Cited by 142 publications

(127 citation statements)

References 51 publications

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“…(Mg,Fe)O ferropericlase, although it makes up perhaps 20-25% of the lower mantle by volume, has a very large intrinsic shear wave anisotropy (*50% or more) at depth; perovskite is considerably less anisotropic (e.g., Karki et al 1999;Wentzcovitch et al 2006;Mainprice 2007;Marquardt et al 2009). The single-crystal elasticity of postperovskite is not yet well understood; several different sets of elastic constants obtained from first-principles calculations have been published (Iitaka et al 2004;Stackhouse et al 2005;Wentzcovitch et al 2006) but discrepancies among studies exist.…”

Section: The D 00 Regionmentioning

confidence: 99%

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

2009

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Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth's mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D 00 layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting-including new measurement methodologies and forward and inverse modeling techniques,-provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.

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Section: The D 00 Regionmentioning

confidence: 99%

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

2009

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“…Wentzcovitch et al 1995;Karki et al 1999;Stackhouse et al 2005;159 Perger et al 2009). For monoclinic crystals, symmetry constraints reduce the number of 160 independent elastic constants to just 13, rather than the full set of 21 for a triclinic crystal.…”

Section: Ab Initio Calculationsmentioning

confidence: 99%

The effect of cation order on the elasticity of omphacite from atomistic calculations

Skelton

Walker

2015

Phys Chem Minerals

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9Omphacite, a clinopyroxene mineral with two distinct crystallographic sites, M1 and M2, and 10 composition intermediate between diopside and jadeite, is abundant throughout the Earth's upper 11 mantle, and is the dominant mineral in subducted oceanic crust. Unlike the end-members, 12 omphacite exists in two distinct phases, a P2/n ordered phase at low temperature and a high-13 temperature C2/c disordered phase. The crystal structure and full elastic constants tensor of ordered 14 P2/n omphacite have been calculated to 15 GPa using plane-wave density functional theory. Our 15 results show that several of the elastic constants, notably C 11 , C 12 , and C 13 deviate from linear-16 mixing between diopside and jadeite. The anisotropy of omphacite decreases with increasing 17 pressure and, at 10 GPa, is lower than that of either diopside or jadeite. The effect of cation disorder 18 is investigated through force-field calculations of the elastic constants of Special Quasirandom 19 2 Structures supercells with simulated disorder over the M2 sites only, and over both cation sites. 20These show that cation order influences the elasticity, with some components displaying particular 21 sensitivity to order on a specific cation site. C 11 , C 12 , and C 66 are sensitive to disorder on M1, while 22 C 22 is softened substantially by disorder on M2, but insensitive to disorder on M1. This shows that 23 the elasticity of omphacite is sensitive to the degree of disorder, and hence the temperature. We 24 expect these results to be relevant to other minerals with order-disorder phase transitions, implying 25 that care must be taken when considering the effects of composition on seismic anisotropy. 26

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“…Although the latter are a source of error due to uncertainties in mantle rheology at appropriate pressures and temperatures (Mitrovica and Forte, 2004), molecular dynamics simulations of the newly discovered post-perovskite phase (Oganov and Ono, 2004;Tsuchiya et al, 2004;Stackhouse et al, 2005) enable estimates of some key material properties of a ppvdominated lower mantle, specifically the bulk and shear moduli as a function of temperature, to be made with some degree of confidence. The dilatancy instability which drives fluid flow by reducing the local intergranular pressure, requires a porosity field as an initial condition.…”

Section: Discussionmentioning

confidence: 99%

“…Details of the model, including full analytical solutions to the governing differential equations, are given in Koenders and Petford (2000) and Petford and Koenders (2003) and not repeated in detail here. A key feature of this analysis is the coupling of pure shearing stress to volumetric strain, so that the modified Biot equation captures many important parameters relevant to deforming porous layers including the average melt displacement, melt (fluid) pressure, permeability and the set temperature corresponding to a pressure of 136 GPa, calculated using a molecular dynamics ab-initio approach (Stackhouse et al, 2005). The estimated temperature interval in the region of the CMB is shown (shaded) for reference.…”

Section: Discussionmentioning

confidence: 99%

“…2), whose material properties are likely to govern the rheology of the lowermost mantle. At conditions relevant to the CMB (P = 136 GPa) the calculated ppv shear modulus ranges from 285 GPa (T = 4000 K) to 300 GPa at T = 3000 K (Stackhouse et al, 2005). These values are used to inform the analytical calculations that follow.…”

Section: Time Constants and Flow Ratesmentioning

confidence: 99%

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Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition

et al. 2005

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We present a novel mechanical model for the extraction of outer core material upwards across the CMB into the mantle side region of D and subsequent interaction with the post-perovskite (ppv) phase transition. A strong requirement of the model is that the D region behaves as a poro-viscoelastic granular material with dilatant properties. Using new ab-initio estimates of the ppv shear modulus, we show how shear-enhanced dilation promoted by downwelling mantle sets up an instability that drives local fluid flow. If loading rates locally exceed c. 10 −12 s −1 , calculated core metal upwelling rates are >10 −4 m/s, far in excess of previous estimates based on static percolation or capillary flow. Associated mass flux rates are sufficient to deliver 0.5% outer core mass to D in < 10 6 yr, provided the minimum required loading rate is maintained. Core metal transported upwards into D may cause local rapid changes in electrical and thermal conductivity and rheology that if preserved, may account for some of the observed small wavelength heterogeneties (e.g. PKP scattering) there.

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The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3

Cited by 142 publications

References 51 publications

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

The effect of cation order on the elasticity of omphacite from atomistic calculations

Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition

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