The Nature of the 660-Kilometer Discontinuity in Earth's Mantle from Global Seismic Observations of
            <i>PP</i>
            Precursors

Deuss, Arwen; Redfern, Simon A. T.; Chambers, Kit; Woodhouse, John

doi:10.1126/science.1120020

Cited by 154 publications

(141 citation statements)

References 17 publications

Supporting

Mentioning

123

Contrasting

Unclassified

Order By: Relevance

“…In an earlier study (Jacobs and de Jong 2007) we have shown that multiple-phase transitions in the system MgOSiO 2 , at pressure-temperature conditions prevailing in the transition zone of Earth's mantle, are visible in sound wave velocities commensurate with a recent study of Deuss et al (2006) based on global seismic observations. However, sound wave velocities and densities calculated along plausible adiabatic paths do not match those of PREM (Dziewonski and Anderson 1981) within tomographic accuracy, indicating that at least iron is lacking in the model description.…”

Section: Introductionsupporting

confidence: 73%

Thermodynamic mixing properties of olivine derived from lattice vibrations

Jong

2009

Phys Chem Minerals

View full text Add to dashboard Cite

We use a lattice vibrational technique to derive thermophysical and thermochemical properties of fayalite, Fe 2 SiO 4 . This semi-empirical technique is based on an extension of Kieffer's model to incorporate details of the phonon spectrum. It includes treatment of intrinsic anharmonicity and electronic effects based on crystal field theory. We extend it to predict thermodynamic mixing properties of olivine (Mg,Fe) 2 SiO 4 solid solutions by using results of our previous work on the system MgO-SiO 2 . Achieving this requires a relation between phonon frequency and composition and a composition relation for the energy of the static lattice. Directed by experimental Raman spectroscopic data for specific optic modes in magnesium-iron solid solutions of olivine and pyroxene we use an empirical relation for the composition dependence for phonon frequencies. We show that lattice vibrations have a large effect on the excess entropy and that the static lattice contribution and lattice vibrations have a large impact on excess enthalpy and excess Gibbs energy. Our model indicates that compositional effects in electronic and magnetic properties are negligible. The compositional variation the Néel temperature has a large impact on excess heat capacity for temperatures below 100 K.

show abstract

Section: Introductionsupporting

confidence: 73%

Thermodynamic mixing properties of olivine derived from lattice vibrations

Jong

2009

Phys Chem Minerals

View full text Add to dashboard Cite

show abstract

“…Alternatively, multiple reactions compatible with P-T conditions realized at the base of mantle transition zone (such as majorite garnet to bridgmanite phase transition, besides the post-spinel transformation) could occur simultaneously and could jointly contribute to determine the 660-km seismic discontinuity. This second scenario is nowadays supported by several evidences coming from seismology, geochemistry and mineral physics [122,[143][144][145][146][147][148]. Table 6.…”

Section: Post-spinel Reactionmentioning

confidence: 83%

“…It is widely accepted that the disproportionation reaction of olivine with spinel-type structure (ringwoodite) into an assemblage of bridgmanite and ferropericlase (the so called "post-spinel phase transformation") is an appropriate candidate to interpret the 660-km depth global seismic discontinuity in the Earth's mantle [13,121,122]. For the magnesium end-member phases this reaction can be written as: γ-Mg 2 SiO 4 (ringwoodite) = MgO (periclase) + MgSiO 3 (bridgmanite)…”

Section: Phase Equilibrium Calculation At Hp-ht: the Post-spinel Phasmentioning

confidence: 99%

First Principles Thermodynamics of Minerals at HP–HT Conditions: MgO as a Prototypical Material

Belmonte

2017

Minerals

View full text Add to dashboard Cite

Ab initio thermodynamic properties, equation of state and phase stability of periclase (MgO, B1-type structure) have been investigated in a broad P-T range (0-160 GPa; 0-3000 K) in order to set a model reference system for phase equilibria simulations under deep Earth conditions. Phonon dispersion calculations performed on large supercells using the finite displacement method and in the framework of quasi-harmonic approximation highlight the performance of the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid density functional in predicting accurate thermodynamic functions (heat capacity, entropy, thermal expansivity, isothermal bulk modulus) and phase reaction boundaries at high pressure and temperature. A first principles Mie-Grüneisen equation of state based on lattice vibrations directly provides a physically-consistent description of thermal pressure and P-V-T relations without any need to rely on empirical parameters or other phenomenological formalisms that could give spurious anomalies or uncontrolled extrapolations at HP-HT. The post-spinel phase transformation, Mg 2 SiO 4 (ringwoodite) = MgO (periclase) + MgSiO 3 (bridgmanite), is taken as a computational example to illustrate how first principles theory combined with the use of hybrid functionals is able to provide sound results on the Clapeyron slope, density change and P-T location of equilibrium mineral reactions relevant to mantle dynamics.

show abstract

“…Despite the common observations of P 660P at short periods (Benz & Vidale 1993;LeStunff et al 1995), longer period P660P signals are rarely seen (Estabrook & Kind 1996;Shearer & Flanagan 1999). Studies that do record a P660P signal detect an intermittent and complicated discontinuity (Deuss et al 2006;Thomas & Billen 2009;Schmerr & Thomas 2011). Observed P 660P signals, on the other hand, indicate that the '660' is sharp and narrow, with a thickness of less than 4 km (Xu et al 2003).…”

Section: Introductionmentioning

confidence: 99%

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Day

Deuss

2013

Geophysical Journal International

Self Cite

View full text Add to dashboard Cite

S U M M A R YHigh frequency precursors to P P almost invariably observe a narrow 660 km discontinuity, whereas PP precursor studies at long periods struggle to detect a reflection from the '660' despite its apparent sharpness to P P . To investigate these contradictory observations we compare PP and P P precursors in the same region. Using short period P P precursors we observe a sharp 660 km discontinuity, which appears to vary in depth substantially. The apparent topography on the '660' is too large to originate solely from thermal variations, regardless of its cause, therefore indicating chemical variations at the base of the mantle transition zone. Long period P P precursors show no '660' as they are sensitive to a larger area and thus average out the apparent topography, in agreement with long period PP precursors. Instead, we see some evidence in both long period data types for a reflection from 720 km depth, which is likely to correspond to a phase change in the garnet system. Key words: Mantle processes; Composition of the mantle; Body waves . I N T RO D U C T I O NThe 660 km seismic discontinuity (the '660') at the base of the mantle transition zone (MTZ) marks the boundary between the upper and lower mantle. The seismic signature of the '660' is attributed to the post-spinel phase transition in olivine, in which ringwoodite transforms into perovskite and periclase (Ita & Stixrude 1992). The nature of the 660 km seismic discontinuity governs the behaviour of the mantle as a whole, with its properties as a chemical and mechanical boundary controlling mantle convection and the fate of subducting slabs (Schubert et al. 2001).Observations of the 660 km seismic discontinuity in precursors to body waves have painted a complex and sometimes even contradictory picture. Despite the common observations of P 660P at short periods (Benz & Vidale 1993;LeStunff et al. 1995), longer period P660P signals are rarely seen (Estabrook & Kind 1996;Shearer & Flanagan 1999). Studies that do record a P660P signal detect an intermittent and complicated discontinuity (Deuss et al. Here, we explore the complex and potentially contradictory observations of the '660' in compressional body waves by directly comparing P P and long period PP precursor observations in the same region under South East Africa. We investigate the topography of the 660 km discontinuity and the frequency dependence of the seismic signals to reconcile the PP and P P observations. DATA A N D M E T H O D SSurface reflected body waves such as PP and P P are preceded by smaller amplitude arrivals, known as precursors. These arrivals have been reflected at discontinuities below the Earth's surface and are therefore sensitive to the impedance change at the discontinuity. P P df (also known as PKIKPPKIKP and hereafter as P P ) is a seismic phase which traverses the entire Earth before reflecting off the Earth's surface, as illustrated in Fig. 1. P P precursors are referred to as P dP , where 'd' denotes the depth of the interface at which they have been reflected. ...

show abstract

The Nature of the 660-Kilometer Discontinuity in Earth's Mantle from Global Seismic Observations of PP Precursors

Cited by 154 publications

References 17 publications

Thermodynamic mixing properties of olivine derived from lattice vibrations

Thermodynamic mixing properties of olivine derived from lattice vibrations

First Principles Thermodynamics of Minerals at HP–HT Conditions: MgO as a Prototypical Material

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Contact Info

Product

Resources

About