Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO 3 orthopyroxene

178

107

The postperovskite transition in MgSiO3 at conditions similar to those expected at the D؆ discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D؆ region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are (i) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D؆ discontinuity and (ii) the anticorrelation between shear and bulk velocity anomalies as detected within the D؆ region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.Earth's mantle ͉ elasticity ͉ DЉ layer ͉ core-mantle boundary T he recent discovery of the postperovskite transition in MgSiO 3 (1) was followed by a series of new findings that may shed light on the enigmatic properties of the DЉ layer, the lowest 250 km of Earth's lower mantle (LM), just above the coremantle boundary. Pressures in this region vary from Ϸ125 to Ϸ135 GPa, while temperatures should vary from Ϸ2,500 to Ϸ4,000 K. So far, two first-principles quasiharmonic calculations (2, 3) have offered consistent thermodynamic phase boundaries compatible with experimental findings (1, 3-6) and with seismologic͞geodynamic inferences of this region (7). Static first principles elasticity calculations (3, 8-10) have indicated that postperovskite's elasticity may cause some of the puzzling observations of this region, such as discontinuity jumps, anisotropy, and lateral heterogeneities. Because of the static nature of these calculations, these inferences were not definite.Here we present the thermoelastic properties of the postperovskite phase at relevant conditions obtained by first-principles computations within the quasiharmonic approximation. We compute shear, longitudinal, and bulk velocities of the postperovskite phase, compare with those of the perovskite phase (11), and, by invoking our previous thermodynamic phase boundary (2), we predict the seismic signature of its presence in the LM, i.e., density and velocity jumps across this transformation. We also predict in a continuum P,T space various ratios of relative changes in velocities and density caused by temperature and phase change in pure aggregates. They prove to be fundamental for interpreting the unexplained origin of lateral heterogeneities in DЉ. Results and DiscussionThe elastic constants of MgSiO 3 postperovskite are remarkably different from those of perovskite (11). Postperovskite is a layered structure, expands anisotropically, and has complex pressure-and temperature-dependent elastic behavior (see ref.8 and Fig. 4, which is published as supporting in...

Section: Resultssupporting

confidence: 56%

MgSiO ₃ postperovskite at D″ conditions

Wentzcovitch

Tsuchiya

2006

178

107

“…This may also affect the high-P,T pPv stability, but the effects in a multicomponent system are poorly constrained (17,18,24,33). The solution mechanisms of iron under the ultrahigh-pressure condition, where the low-spin state with smaller ionic radius is predominant, are also unclear (34), although some low-pressure experiments report that iron may also influence the site preference of Al 3ϩ in Pv (35).…”

Section: Resultsmentioning

confidence: 99%

Postperovskite phase equilibria in the MgSiO ₃ –Al ₂ O ₃ system

Tsuchiya

2008

We investigate high-P,T phase equilibria of the MgSiO3-Al2O3 system by means of the density functional ab initio computation methods with multiconfiguration sampling. Being different from earlier studies based on the static substitution properties with no consideration of Rh2O3(II) phase, present calculations demonstrate that (i) dissolving Al2O3 tends to decrease the postperovskite transition pressure of MgSiO3 but the effect is not significant (Ϸ-0.2 GPa/mol% Al2O3); (ii) Al2O3 produces the narrow perovskite؉postperovskite coexisting P,T area (Ϸ1 GPa) for the pyrolitic concentration (xAl2O3 Ϸ6 mol%), which is sufficiently responsible to the deep-mantle D؆ seismic discontinuity; (iii) the transition would be smeared (Ϸ4 GPa) for the basaltic Al-rich composition (xAl2O3 Ϸ20 mol%), which is still seismically visible unless iron has significant effects; and last (iv) the perovskite structure spontaneously changes to the Rh2O3(II) with increasing the Al concentration involving small displacements of the Mg-site cations.ab initio density functional method ͉ Earth's lower mantle ͉ DЉ seismic discontinuity ͉ solid-solution thermodynamics ͉ Rh2O3(II) structure A lthough the postperovskite (pPv) phase transition in MgSiO 3 (1-3) is suggested to be strongly related to the deep-mantle DЉ seismic discontinuity (4-6), phase relations in more realistic chemical compositions, containing particularly aluminum and iron, are needed for further detailed investigations of this region. High-pressure experiments (7, 8) demonstrated that magnesium silicate perovskite (Pv) is the major host of aluminum over the entire pressure (P) and temperature (T) range of the lower mantle, possessing Ϸ5 mol% of Al 2 O 3 in pyrolite and Ϸ20 mol% in MORB. The pPv phase transition of aluminous silicate has therefore been studied extensively both theoretically and experimentally (9-13) along with the effects of Al on the elastic properties of ). An ab initio study (9) showed that the Al incorporation into Mg-Pv drastically increases the pPv transition pressure and also enhances the PvϩpPv coexistence region, which reaches Ͼ10 GPa even for 5 mol% Al 2 O 3 . Also, a laser-heated diamond-anvil-cell experiment of pyrope composition (25 mol% Al 2 O 3 ) (11) proposed a similar phase diagram with wide PvϩpPv coexisting pressure ranges of 20-30 GPa, although their estimations of transition width were quite rough. Similar broadening of the transition width is also reported for the iron-bearing systems (17, 18), although their transition pressures are still highly contradictory. These gradual phase changes through such huge divariant loops suggest the pPv transition fails to explain the sharp seismic discontinuity as observed at the top of DЉ (9, 11). However, the ultrahigh-P,T experiments of multicomponent phase equilibria currently usually involve significant uncertainties in controlling homogeneous P,T conditions and reaction kinetics. In this study, we reinvestigate the aluminum-bearing Pv system theoretically for the first step of better understanding the ...

“…Iron is likely to be an important component in ppv, and its incorporation in silicate ppv has been found to have a considerable influence on the properties of the phase. Experimental studies have demonstrated that ppv can take up much more iron than silicate perovskite (pv) (14,15), and the equation of state (EOS) and phase boundaries have been determined as a function of Fe 2þ content (16)(17)(18)(19). In Al-bearing ppv, the presence of Fe 3þ may also have a significant effect on the physical properties, phase boundaries and iron partitioning (20).…”

mentioning

confidence: 99%

Crystal structures of (Mg _{1-
x} ,Fe _x )SiO ₃ postperovskite at high pressures

Yamanaka

Hirose

Mao

et al. 2012

Self Cite

X-ray diffraction experiments on postperovskite (ppv) with compositions (Mg 0.9 Fe 0.1 )SiO 3 and (Mg 0.6 Fe 0.4 )SiO 3 at Earth core-mantle boundary pressures reveal different crystal structures. The former adopts the CaIrO 3 -type structure with space group Cmcm , whereas the latter crystallizes in a structure with the Pmcm ( Pmma ) space group. The latter has a significantly higher density ( ρ = 6.119(1) g/cm 3 ) than the former ( ρ = 5.694(8) g/cm 3 ) due to both the larger amount of iron and the smaller ionic radius of Fe 2+ as a result of an electronic spin transition observed by X-ray emission spectroscopy (XES). The smaller ionic radius for low-spin compared to high-spin Fe 2+ also leads to an ordered cation distribution in the M1 and M2 crystallographic sites of the higher density ppv structure. Rietveld structure refinement indicates that approximately 70% of the total Fe 2+ in that phase occupies the M2 site. XES results indicate a loss of 70% of the unpaired electronic spins consistent with a low spin M2 site and high spin M1 site. First-principles calculations of the magnetic ordering confirm that Pmcm with a two-site model is energetically more favorable at high pressure, and predict that the ordered structure is anisotropic in its electrical and elastic properties. These results suggest that interpretations of seismic structure in the deep mantle need to treat a broader range of mineral structures than previously considered.