Post-perovskite Phase Transition Research Articles

How lowermost mantle viscosity controls the chemical structure of Earth’s deep interior

Determining the fate of subducted oceanic crust is critical for understanding material cycling through Earth’s deep interior and sources of mantle heterogeneity. A key control on the distribution of subducted slabs over long timescales is the bridgmanite to post-perovskite phase transition in the lowermost mantle, thought to cause rheological weakening. Using high-resolution computational models, we show that the ubiquitous presence of weak post-perovskite at the core-mantle boundary can facilitate or prevent the accumulation of basaltic oceanic crust, depending on the amount of weakening and the crustal thickness. Moderately weak post-perovskite ( ~ 10–100× weaker) facilitates segregation of crust from subducted slabs, increasing basalt accumulation in dense piles. Conversely, very weak post-perovskite (more than 100× weaker) promotes vigorous plumes that entrain more crustal material, decreasing basalt accumulation. Our results reconcile the contradicting conclusions of previous studies and provide insights into the accumulation of subducted crust in the lowermost mantle throughout Earth’s history.

Effects of Fe and Al distributions on the Pv–PPv phase transition pressure of MgSiO3

SUMMARY Based on the first-principles method combined with Special Quasi-random Structures package, we investigated the effects of impurities distributions on the perovskite (Pv)–post-perovskite (PPv) phase transition pressures of Fe- and Al-bearing MgSiO3. 61 different distribution configurations were calculated. The results show that the distribution has marginal effect on the Pv–PPv phase transition pressure of Fe2+-bearing MgSiO3. However, the separate of Al3+Al3+ ions can significantly increase the Pv–PPv phase transition pressure to that higher than the core–mantle boundary (CMB) pressure. In the most disordered structures of Al3+Al3+-bearing case constructed by SQS, the Pv–PPv phase transition pressure is higher than 400 GPa. Based on our previous result, we proposed that only Fe2+-bearing MgSiO3 can induce the discontinuous increase in shear wave velocity at the CMB, while Al3+Al3+-, Fe3+Al3+- and Fe3+Fe3+-bearing MgSiO3 cannot.

Imaging paleoslabs and inferring the Clapeyron slope in D″ beneath the northern Pacific based on high-resolution inversion of seismic waveforms for 3-D transversely isotropic structure

We use waveform inversion to infer high-resolution 3-D models of the S-velocity perturbation δVS and the anisotropy parameter δξ in the lowermost 400 km of the mantle beneath the northern Pacific. Our inferred models show three prominent features: (i) a widespread positive δVS layer with ~100 km thickness that exists ~250 km above the core-mantle boundary (CMB), which could be due to a bridgmanite to post-perovskite phase transition related to the D″ discontinuity; (ii) distinct positive -δVS anomalies with positive δξ resolved from 100 km to more than 400 km above the CMB, which could be the subducted Izanagi, Farallon, and Telkhinia slabs; (iii) a vertically continuous low-δVS anomaly with negative δξ at the edge of the subducted slab, which we interpret as an upwelling plume induced by slab sinking. We infer the Clapeyron slope of the post-perovskite phase transition in the lowermost mantle beneath the northern Pacific to be 10.2 ± 1.3 MPa/K, based on the δVS gradient as a function of depth for the inferred 3-D VS structure. Our estimated large positive Clapeyron slope at the CMB suggests vigorous convection in the lowermost mantle.

Absolute Primary Pressure Scale to 120 GPa: Toward a Pressure Benchmark for Earth's Lower Mantle

AbstractSelf‐consistent absolute primary pressure scale available to whole Earth's lower mantle pressure regime was established by the simultaneous measurement of densities and acoustic wave velocities of NaCl‐B2 phase up to ~120 GPa without any use of prior pressure standards. While our pressure scale shows excellent agreement with the theoretical predictions, some of the previous secondary pressure scales constructed based on the prior pressure standard by the shock wave data significantly deviate from our primary scale reaching around 10 GPa at most within lower mantle pressure regime, which corresponds to ~170‐km difference in depth. Our newly established pressure scale would thus be a new benchmark to deal with the controversy on the depth of the boundaries of Earth's layered structure, such as magnesium‐silicate postperovskite phase transition boundary at the bottom of the Earth's mantle which might represent the D″ seismic discontinuity.

Effects of Fe and Al incorporations on the bridgmanite–postperovskite coexistence domain

The postperovskite phase transition of Fe and Al-bearing MgSiO3 bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe2+SiO3, Fe3+Fe3+O3, Fe3+Al3+O3, and Al3+Al3+O3 derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA+U method and a lattice dynamics approach. We found that at ∼2500K, incorporations accompanied by Fe3+ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe2+-bearing bridgmanite.

Effects of the post-perovskite phase transition properties on the stability and structure of primordial reservoirs in the lower mantle of the Earth

Two key features of the lowermost Earth's mantle are the presence of Large Low Shear Velocity Provinces (LLSVPs), which may be reservoirs of primordial, chemically distinct material, and the phase change from perovskite (pv) to post-perovskite (pPv), which may occur at lowermost mantle conditions. However, the influence of this phase change on the shape, dynamics, and stability of chemically distinct reservoirs is not well constrained. Here, we performed numerical experiments of thermo-chemical convection in 2-D spherical annulus geometry to investigate the effects on thermo-chemical structure in the lower mantle of three parameters affecting the pPv phase change: the core–mantle boundary (CMB) temperature (TCMB), the viscosity ratio between pv and pPv (ΔηpPv), and the Clapeyron slope of the pPv phase transition (ΓpPv). Our results indicate that increasing CMB temperature increases the wavelength of the primordial reservoirs. Furthermore, a high CMB temperature promotes the development of plumes outside the reservoir of primordial material. High CMB temperature and large Clapeyron slope both favour the formation of pPv patches and of a double-crossing of the phase boundary, thus preventing the formation of continuous layer of pPv above the CMB. Combined with a low CMB temperature and/or a low Clapeyron slope of the pPv phase transition, a full layer of weak pPv above the CMB strongly enhances the mixing efficiency of primordial material with ambient regular mantle material, which may not allow the generation of large reservoirs. Based on our experiments, we conclude that the models of convection best describing the Earth's mantle dynamics include a large pPv Clapeyron slope (typically in the range of 13–16 MPa/K), and a moderate CMB temperature (around 3750 K). Our models do not provide further constraints on the value of the pPv viscosity, both regular and low values giving similar results on stability and structure of large primordial reservoirs for models with a moderate CMB temperature and large Clapeyron slope, but more plumes can be observed outside these large reservoirs in the cases with regular pPv than those in the cases with weak pPv.

Effects of the postperovskite phase change on the observed geoid

In the lowermost mantle, seismic velocity variations beneath Pacific margins have been related to the perovskite to postperovskite (pPv) phase transition. We investigate the influence of this phase transformation on the geoid using 3-D spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle. We demonstrate that the geoid anomalies are strongly affected by the presence of pPv because of phase-dependent viscosity changes relative to the surrounding mantle. Whereas geoid heights above subduction zones are increased for high-viscosity pPv, the presence of weak pPv reduces them, thereby improving the fit to the observed geoid. An investigation using two different tomography models, different pPv density contrasts, and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrates the various effects of weak pPv on the geoid.

Pressure-induced volume collapse and structural phase transitions in SrRuO3

We report on the low temperature (6 K) structural properties of SrRuO 3 under quasi-hydrostatic pressure studied by synchrotron X-ray powder diffraction in a diamond anvil cell. First principle calculations predict a first-order perovskite (Pv) to post-perovskite (pPv) phase transition at ∼40 GPa accompanied by a 1.9% volume collapse. Our results rule out the occurrence of a pPv phase to 54 GPa. Instead, we find a Pv to monoclinic to triclinic sequence of phase transitions. The monoclinic to triclinic phase transition at ∼38 GPa is accompanied by a 3.5% volume collapse. X-ray absorption spectroscopy indicates that this volume collapse is not accompanied by a change in Ru valence state. Our results should help guide improvements to theoretical treatments of this and other correlated d -electron systems based on density functional theory. Unit cell volume as a function of pressure ( T =6 K). Black squares and red circles correspond to Ne and He pressure media, respectively. Blue dashed lines are fit to the data before the volume collapse using a second-order Birch–Murnaghan equation of state excluding (lower curve) and including (upper curve) neutron diffraction data for the low temperature, ambient pressure volume. Dashed lines denote structural phase boundaries. • Crystal structure of SrRuO3 was studied under high pressure up to 54 GPa. • Pv to monoclinic to triclinic sequence of phase transitions was observed. • Transition to triclinic phase at ∼38 GPa is accompanied by a 3.5% volume collapse. • Volume collapse in SrRuO3 is not driven by a change in Ru valence state. • Pv-to-pPv phase transition predicted by DFT at 40 Gpa is not observed up to 54 GPa.

High-Pressure, High-Temperature X-ray Diffraction Measurements and the Discovery of Post-Perovskite Phase Transition

With recent developments in x-ray diffraction (XRD) measurements at synchrotron radiation facilities, we can examine stable crystal structure and phase transition in-situ at high-pressure and -temperature (\(P\)–\(T\)) conditions corresponding to the deep Earth. MgSiO3 perovskite is a principal mineral in most parts of the lower mantle, but its stability and possible phase transition in the lowermost mantle have long been unclear. Recent synchrotron XRD experiments at SPring-8 revealed the phase transition from perovskite to post-perovskite above 120 GPa and 2400 K, the conditions at a depth of 2600-km, a few hundred kilometers above the bottom of the mantle. It is now widely accepted that post-perovskite is a primary mineral in the D” layer. The layered crystal structure of post-perovskite causes unique physical properties such as enhanced electrical and thermal conductivity. More recent progress of synchrotron XRD measurements at ultrahigh \(P\)–\(T\) conditions is also briefly reviewed.

Dynamic origins of seismic wavespeed variation in [formula omitted

The D″ discontinuity is defined by a seismic velocity increase of 1–3% about 250km above the core–mantle boundary (CMB), and is mainly detected beneath locations of inferred paleosubduction. A phase change origin for the interface can explain a triplicated arrival observed in seismic waveform data and is supported by the recent discovery of a post-perovskite phase transition. We investigate the interaction of slabs, plumes, and the phase change within D″ in 2-D compressible convection calculations, and predict waveform complexity in synthetic seismic data. The dynamic models produce significant thermal and phase heterogeneity in D″ over small distances and reveal a variety of behaviors including: (1) distinct pPv blocks separated by upwellings, (2) notches at the top of a pPv layer caused by plume heads, (3) regions of Pv embedded within a pPv layer due to upwellings. Advected isotherms produce complicated thermal structure that enable multiple crossings of the phase boundary. Perturbations to S, SdS, and ScS arrivals (distances <84 degrees) are linked to the evolutionary stage of slabs and plumes, and can be used to determine phase boundary height and velocity increase, volumetric wavespeed anomaly beneath the discontinuity, and possibly the lengthscale of slab folding near the CMB. Resolving fine-scale structure beneath the interface requires additional seismic phases (e.g., Sd, SKS) and larger distances (>80 degrees).

Waveform inversion of broad-band body wave data for the S-velocity structure in the lowermost mantle beneath the Indian subcontinent and Tibetan Plateau

SUMMARY We conduct waveform inversion for the radial shear velocity structure of the lowermost mantle beneath the Indian subcontinent and Tibetan Plateau using transverse component body wave waveform data obtained from the ORFEUS (Observatories and Research Facilities for European Seismology) and IRIS (Incorporated Research Institutions for Seismology) arrays in thepassband8–200s.Asthesearehigherfrequencydatathanusedbymostpreviouswaveform inversion studies, several improvements in terms of techniques were necessary. We conduct tests to confirm the robustness of the results. The average S velocity of this region is almost the same as Preliminary Reference Earth Model (PREM), but we find that the velocity is faster than PREM in the depth range 2400–2750 km and slower than PREM in the depth range 2750 km—core–mantle boundary (CMB). This suggests that the perovskite to post-perovskite phase transition occurs in the depth range 2400–2600 km. If we assume the composition of D �� is pyrolitic, the thickness of the thermal boundary layer at the CMB is about 200–300 km.

Insitu observation of shear stress-induced perovskite to post-perovskite phase transition in CaIrO3 and the development of its deformation texture in a diamond-anvil cell up to 30 GPa

The perovskite (Pv) to post-perovskite (PPv) phase transition and the deformation texture of the PPv phase were investigated on the basis of a high-pressure X-ray diffraction (XRD) study of CaIrO3 using a diamond-anvil cell in a pressure range up to 31GPa. The development of a crystallographic preferred orientation (CPO) in the PPv phase was observed after the plastic deformation from 8 or 9GPa to 31GPa at both room and high (∼1500K) temperatures. The observed CPOs in the present study indicate that the (010) plane worked as an active slip plane in the PPv phase over the entire pressure and temperature range of the present experiment. We also confirmed that the Pv to PPv phase transition proceeds at room temperature under high-stress conditions. The phase transition under high-stress condition is in stark contrast to the results of a previous hydrostatic experiment in which the Pv-CaIrO3 remained stable in a helium media at 31GPa and room temperature. This indicates that shear stress plays an important role in the Pv to PPv phase transition, and this effect should be taken into account when the thermal structure at the D″ layer is discussed on the basis of the high-pressure experiments.

On the increase in thermal diffusivity caused by the perovskite to post-perovskite phase transition and its implications for mantle dynamics

The thermal diffusivity (κ) of perovskite and post-perovskite CaIrO3 has been measured, at elevated pressure and temperatures up to 600°C, using the X-radiographic Ångström method. At high temperatures we find that the thermal diffusivity of post-perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated. Assuming a similar effect occurs in MgSiO3 post-perovskite, the effect of the contrasting thermal transport properties between perovskite and post-perovskite on mantle dynamics has been investigated using simple two-dimensional convection models. These show a reduction in extent and increase in depth of post-perovskite lenses, as well as increased core–mantle-boundary heat-flux, broader upwellings and more vigorous downwellings when compared to the reference, constant a, case.

Raman spectroscopy of CaSnO3 at high temperature: a highly quasi-harmonic perovskite

Calcium stannate perovskite (CaSnO3) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8 nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Grüneisen parameters and thermal line broadening following Γ = Acothθ/T, where the quantum temperature θ is determined by the phonon branch without further coupling with other degrees of freedom. The geometrical nature of phonon lines has been identified. High-temperature powder x-ray diffraction measurements provide thermal expansion coefficients of αx = 13.9 × 10−6 K−1, αy = 2.7 × 10−6 K−1, αz = 14.3 × 10−6 K−1. The strongly quasi-harmonic behaviour observed and the lack of any indication of instability with respect to the post-perovskite structure points to the strongly first-order character of the reported perovskite to post-perovskite phase transition in this material, which appears to behave as a very good analogue to MgSiO3 in the Earth’s interior.

Influences of lower-mantle properties on the formation of asthenosphere in oceanic upper mantle

Asthenosphere is a venerable concept based on geological intuition of Reginald Daly nearly 100 years ago. There have been various explanations for the existence of the asthenosphere. The concept of a plume-fed asthenosphere has been around for a few years due to the ideas put forth by Yamamoto et al.. Using a two-dimensional Cartesian code based on finite-volume method, we have investigated the influences of lower-mantle physical properties on the formation of a low-viscosity zone in the oceanic upper mantle in regions close to a large mantle upwelling. The rheological law is Newtonian and depends on both temperature and depth. An extended-Boussinesq model is assumed for the energetics and the olivine to spinel, the spinel to perovskite and perovskite to post-perovskite (ppv) phase transitions are considered. We have compared the differences in the behavior of hot upwellings passing through the transition zone in the mid-mantle for a variety of models, starting with constant physical properties in the lower-mantle and culminating with complex models which have the post-perovskite phase transition and depth-dependent coefficient of thermal expansion and thermal conductivity. We found that the formation of the asthenosphere in the upper mantle in the vicinity of large upwellings is facilitated in models where both depth-dependent thermal expansivity and conductivity are included. Models with constant thermal expansivity and thermal conductivity do not produce a hot low-viscosity zone, resembling the asthenosphere. We have also studied the influences of a cylindrical model and found similar results as the Cartesian model with the important difference that upper-mantle temperatures were much cooler than the Cartesian model by about 600 to 700 K. Our findings argue for the potentially important role played by lower-mantle material properties on the development of a plume-fed asthenosphere in the oceanic upper mantle.

Prediction of the phase transition from ferromagnetic perovskite to non-magnetic post-perovskite in SrRuO 3 : A first-principles study

The phase transition in the perovskite (Pv) SrRuO 3 under pressure has been studied by using first-principles calculations based on density functional theory. The post-perovskite (Ppv) phase transition of SrRuO 3 will take place under hydrostatic pressure of about 40 GPa. The stability of Ppv- SrRuO 3 is justified by the enthalpy calculations, and this phase transition accompanies volume disconnection and magnetic moment collapses. The crystallographic data and the electronic structure of Ppv- SrRuO 3 are also predicted. The crystal structure of Ppv- SrRuO 3 is similar to that of Ppv- CaRuO 3. A non-magnetic ground state is found in Ppv- SrRuO 3. The strong hybridization of Ru and O is evident in the electronic structures of both phases. We expect that these results will help further understanding of SrRuO 3 under high pressure.

Spin and structural transitions in AlFeO 3 and FeAlO 3 perovskite and post-perovskite

We extend the field of investigation of the perovskite to post-perovskite phase transition from the Earth's lowermost mantle with a study of the double substitution Mg + Si = Al + Fe. For this we use an advanced formalism within the density functional theory, the planar augmented wavefunction method, to investigate the perovskite and post-perovskite structures containing only Al and Fe 3+ as cations. We look in particular at their relative stability and at their magnetic properties. We distinguish two crystallographic cases: AlFeO 3 and FeAlO 3, corresponding respectively to two ordered cases: one with Fe and then one with Al in octahedral coordination. For each case we investigate several spin configurations. We observe that up to 90 GPa the FeAlO 3 perovskite structure, i.e. with Fe 3+ in the interoctahedral space, with antiferromagnetic configuration and large local magnetic moment is the most stable one. Beyond 90 GPa the post-perovskite structure of AlFeO 3, i.e. with Fe 3+ in octahedral coordination, with antiferromagnetic configuration and small local magnetic moment is the most stable one. The perovskite to post-perovskite phase transition at 90 GPa is associated with a site exchange that triggers a partial collapse of the magnetic moment. The local magnetic moment vanishes beyond 150 GPa in post-perovskite. Our calculations suggest that the presence of Al + Fe 3+ in perovskite/post-perovskite renders the phase transition sluggish, induces a large density jump at the transition and contributes into maintaining a residual magnetic spin down to the base of the mantle.

Structural distortion of CaSnO 3 perovskite under pressure and the quenchable post-perovskite phase as a low-pressure analogue to MgSiO 3

Structural distortion and the phase transition of CaSnO 3 perovskite were examined in situ at high-pressure and -temperature on the basis of synchrotron X-ray diffraction measurement in a laser-heated diamond-anvil cell. The results show that CaSnO 3 perovskite transforms into a CaIrO 3-type post-perovskite phase above 40 GPa and 2000 K with a high positive Clapeyron slope. It is noted that CaSnO 3 post-perovskite is quenchable to ambient condition. Combined with available compression data on other perovskite-type materials, we discuss a mechanism of perovskite to post-perovskite phase transition under pressure. The perovskites, which are distorted largely from the ideal cubic perovskite structure already at ambient condition, become more deformed with increasing pressure. This eventually results in the post-perovskite phase transition at a critical angle of octahedral rotation.

Large angle reflection from a dipping structure recorded as a PKIKP precursor: Evidence for a low velocity zone at the core–mantle boundary beneath the Gulf of Mexico

We observed a clear phase-like arrival prior to the PKIKP wave at a broadband seismic array in eastern Tibet from an intermediate depth earthquake occurring in Guatemala. The measured incident angle and back azimuth of this phase indicate that it originates from scattering near the core–mantle boundary (CMB) of the source side. This phase, however, was not observed from another earthquake that is only 60 km away, suggesting that scattering depends strongly on the angles of the incident waves. Ray tracing and diffraction migration indicates that the precursor is a large-angle reflection from a dipping structure in the lowermost mantle east of Mexico. The seismic reflector dips northward by ∼ 52º and is centered at ∼ 93.31ºW and 23.89ºN with an east–west extension of ∼ 100 km. A decrease of P-wave velocity by a few to 10% is required to explain the amplitude and polarity of the phase. It is unlikely to explain the large P-wave velocity contrast and the large dipping feature with the post-perovskite phase transition. The reflector is located in a region where the lowermost mantle is marked by a high velocity anomaly related to the subducted Farallon slab. Previous numerical modeling suggested that a substantial amount of hot mantle could be trapped beneath a slab over long periods of time, leading to the formation of a mega-plume. Thus, the observed sharp dipping boundary here might correspond to the edge of a low velocity zone below the subducted Farallon slab. Mixed results on the presence of an ultra low velocity zone in the region have been reported by previous seismic studies. We speculated that the low velocity zone below the cold slab was developed as a consequence of long-term heating from core.

On the interaction of the geotherm with a post-perovskite phase transition in the deep mantle

The energy balance in the presence of a perovskite (Pv) to post-perovskite (pPv) transition within Earth’s D” layer is examined in order to explore the relationship between changes in seismic velocity associated with this phase change, the extent of the two-phase Pv–pPv coexistence region, and the thermal structure of the deep mantle. This is motivated in part by the fact that discontinuities attributed to the Pv–pPv transition are inferred to be seismically sharper than permitted by some recent estimates of the pressure increment across the two-phase co-existence region. Here it is shown that sharp gradients in phase abundance may arise even when the two-phase loop is very broad, and therefore the pressure increment determined from thermodynamic stability alone is a poor proxy for predicting the sharpness of Pv–pPv related seismic discontinuities. The change in pPv fraction over the steepest gradients in phase can also be highly variable, which would lead to potentially complex variations in the total strength of seismic discontinuities. Latent heat plays an important role in the structure of the pPv phase change and its influence upon the geotherm. For the double-crossing scenario – in which a deeper reverse transformation from pPv to Pv occurs in a steep thermal boundary layer – latent heat release from the shallower Pv–pPv transition moderates the effects of latent heat absorption at the deeper reverse transition.