Effect of Al on the sharpness of the MgSiO<sub>3</sub> perovskite to post‐perovskite phase transition

Akber-Knutson, S.; Steinle‐Neumann, Gerd; Asimow, Paul D.

doi:10.1029/2005gl023192

Cited by 75 publications

(68 citation statements)

References 25 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In addition, Al tends to increase and decrease the volumes of Pv and pPv, respectively, as reported in ref. 9. The present pPv's data are in agreement with recent Rietveld refinements (24).…”

Section: Resultssupporting

confidence: 80%

“…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.…”

mentioning

confidence: 99%

“…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)(10)(11)(12)(13) along with the effects of Al on the elastic properties of Pv and pPv (14)(15)(16)). 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 .…”

mentioning

confidence: 99%

“…The pPv phase transition of aluminous silicate has therefore been studied extensively both theoretically and experimentally (9)(10)(11)(12)(13) along with the effects of Al on the elastic properties of Pv and pPv (14)(15)(16)). 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.…”

mentioning

confidence: 99%

“…Early theoretical calculations investigated the stable Al substitution mechanisms in Pv and pPv at static 0 K condition (9,13,19), and finite temperature free energies were evaluated quite approximately in particular with no consideration of phonon contributions (9). If, however, comparing the transition pressures of end-member compositions, the pPv transition in MgSiO 3 is expected to occur at much lower pressure than in Al 2 O 3 at low temperatures, but it is comparable or rather higher in MgSiO 3 than in Al 2 O 3 at temperatures Ͼ3,000 K, because the Pv-pPv phase boundary in MgSiO 3 has a largely positive Clapeyron slope (2, 3), whereas the Rh 2 O 3 (II) (Rh)-pPv boundary in Al 2 O 3 has a largely negative one (12,20).…”

mentioning

confidence: 99%

See 4 more Smart Citations

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

Tsuchiya

2008

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

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 ...

show abstract

Section: Resultssupporting

confidence: 80%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 3 more Smart Citations

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

Tsuchiya

2008

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

show abstract

Major disruption of D″ beneath Alaska

et al. 2016

View full text Add to dashboard Cite

D″ represents one of the most dramatic thermal and compositional layers within our planet. In particular, global tomographic models display relatively fast patches at the base of the mantle along the circum‐Pacific which are generally attributed to slab debris. Such distinct patches interact with the bridgmanite (Br) to post‐bridgmanite (PBr) phase boundary to generate particularly strong heterogeneity at their edges. Most seismic observations for the D″ come from the lower mantle S wave triplication (Scd). Here we exploit the USArray waveform data to examine one of these sharp transitions in structure beneath Alaska. From west to east beneath Alaska, we observed three different characteristics in D″: (1) the western region with a strong Scd, requiring a sharp δVs = 2.5% increase; (2) the middle region with no clear Scd phases, indicating a lack of D″ (or thin Br‐PBr layer); and (3) the eastern region with strong Scd phase, requiring a gradient increase in δVs. To explain such strong lateral variation in the velocity structure, chemical variations must be involved. We suggest that the western region represents relatively normal mantle. In contrast, the eastern region is influenced by a relic slab that has subducted down to the lowermost mantle. In the middle region, we infer an upwelling structure that disrupts the Br‐PBr phase boundary. Such an interpretation is based upon a distinct pattern of travel time delays, waveform distortions, and amplitude patterns that reveal a circular‐shaped anomaly about 5° across which can be modeled synthetically as a plume‐like structure rising about 400 km high with a shear velocity reduction of ~5%, similar to geodynamic modeling predictions of upwellings.

show abstract