Stability and structure of MgSiO3 perovskite to 2300-kilometer depth in Earth's mantle

Incorporation mechanism of Fe and Al into bridgmanite in a subducting mid-ocean ridge basalt and its crystal chemistry

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg_0.662Fe_0.338Si_0.662Al_0.338O₃ bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron ⁵⁷Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution ^AMg²⁺  + ^BSi⁴⁺  ↔ ^AFe³⁺(high-spin) + ^BAl³⁺ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO₆ octahedra, yielding the unusual expansion of mean <A–O> bond-length due to flexibility of A–O bonds for the structural distortion, in contrast to mean <B–O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO₁₂ (practically AO₈) and BO₆ polyhedra.

Stability of Fe,Al-bearing bridgmanite in the lower mantle and synthesis of pure Fe-bridgmanite

The physical and chemical properties of Earth's mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa. The compressibility of ferric iron-bearing bridgmanite is significantly different from any known bridgmanite, which has direct implications for the interpretation of seismic tomography data.

Photocatalytic degradation of endocrine disruptor Bisphenol-A in the presence of prepared CexZn1-xO nanocomposites under irradiation of sunlight

Photocatalytic degradation of Bisphenol A (BPA), a representative endocrine disruptor chemical, was carried out under irradiation of sunlight in the presence of CexZn1-xO nanophotocatalyst. Cerium (Ce) ions were successfully incorporated into the bulk lattice of ZnO by simple co-precipitation process. The CexZn1-xO composite nanostructures exhibited higher photocatalytic efficiency than pure ZnO in the degradation of BPA under sunlight irradiation and nearly complete mineralization of BPA was achieved. The degradation rate was strongly dependent on factors such as the size and structure of catalyst, doping material concentration, BPA concentration, catalyst load, irradiation time and pH levels. This work suggested that the CexZn1-xO assisted photocatalytic degradation is a versatile, economic, environmentally benign and efficient method for BPA removal in the aqueous environment.

Demixing instability in dense molten MgSiO3 and the phase diagram of MgO

The phase diagrams of MgSiO3 and MgO are studied from first-principles theory for pressures and temperatures up to 600 GPa and 20,000 K. Through the evaluation of finite-temperature Gibbs free energies, using density-functional theory within the generalized gradient approximation as well as with hybrid exchange-correlation functionals, we find evidence for a vast pressure-temperature regime where molten MgSiO3 decomposes into liquid SiO2 and solid MgO, with a volume change of approximately 1.2%. The demixing transition is driven by the crystallization of MgO--the reaction only occurs below the high-pressure MgO melting curve. The predicted transition pressure at 10,000 K is in close proximity to an anomaly reported in recent laser-driven shock experiments of MgSiO3. We also present new results for the high-pressure melting curve of MgO and its B1-B2 solid phase transition, with a triple point at 364 GPa and 12,000 K.

Ab initio thermodynamics of MgSiO3 perovskite at high pressures and temperatures

Using quantum-mechanical simulations based on density-functional perturbation theory, we address the problem of stability of MgSiO3 perovskite to decomposition into MgO and SiO2 at pressures and temperatures of the Earth's lower mantle. We show that MgSiO3 perovskite (and its post-perovskite phase) is more stable than the mixture of oxides throughout the pressure-temperature regime of the Earth's mantle. Structural stability and lattice dynamics of phases in the system MgO-SiO2 are discussed.

Pressure-induced magnetization in FeO: evidence from elasticity and Mössbauer spectroscopy

The complete elastic tensor of Fe0.94O (wüstite) has been determined to 10 GPa using acoustic interferometry at GHz frequencies inside a diamond-anvil cell. The soft mode (C44) elastic constant of FeO is reduced by 20% over the experimental pressure range. An unusual discontinuity in the pressure derivatives of C11 and C12 at 4.7+/-0.2 GPa corresponds to the pressure at which the onset of a magnetic ordering transition is observed by high-pressure Mössbauer spectroscopy and neutron powder diffraction. Our new results combined with literature structural high P-T data suggest that there is a magnetic, although still cubic, phase of FeO between approximately 5 and approximately 17 GPa.

Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D" layer

The Earth's lower mantle is believed to be composed mainly of (Mg,Fe)SiO3 perovskite, with lesser amounts of (Mg,Fe)O and CaSiO3 (ref. 1). But it has not been possible to explain many unusual properties of the lowermost approximately 150 km of the mantle (the D" layer) with this mineralogy. Here, using ab initio simulations and high-pressure experiments, we show that at pressures and temperatures of the D" layer, MgSiO3 transforms from perovskite into a layered CaIrO3-type post-perovskite phase. The elastic properties of the post-perovskite phase and its stability field explain several observed puzzling properties of the D" layer: its seismic anisotropy, the strongly undulating shear-wave discontinuity at its top and possibly the anticorrelation between shear and bulk sound velocities.

Polymorphism in NaSbO3: structure and bonding in metal oxides

A new polymorph of NaSbO(3) has been synthesized at 10.5 GPa and 1150 degrees C in a uniaxial split sphere anvil type press (USSA-2000) and recovered back to ambient conditions. The high-pressure form of NaSbO(3) adopts an orthorhombically distorted perovskite structure, isostructural with CaTiO(3), GdFeO(3), and NaTaO(3). The space group is Pnma, and the unit cell dimensions are a = 5.43835(6) A, b = 7.66195(8) A, c = 5.38201(5) A. It is a white insulator with an optical band gap of 3.4 eV. This compound represents the first ternary perovskite prepared containing Sb(5+) on the octahedral site. The octahedral tilting distortion in this compound is much larger than expected from ionic radii considerations. The distortion is driven by a second-order Jahn-Teller distortion originating on oxygen that can be traced back to strong Sb-O covalent bonding. A conflict arises between the strong covalent bonding interactions at oxygen that favor a large octahedral tilting distortion and the repulsive Na-O interactions that oppose excessive octahedral tilting. This conflict destabilizes the perovskite topology, thereby stabilizing the ilmenite polymorph under ambient conditions. Analysis of ionic and covalent bonding explains why ASbO(3) and ABiO(3) compositions frequently adopt structures that violate Pauling's rules.

Post-Perovskite Phase Transition in MgSiO 3

In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO 6 -octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

Post-Perovskite Phase Transition in MgSiO3

In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D" seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D" discontinuity.

Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O

The emerging picture of Earth's deep interior from seismic tomography indicates more complexity than previously thought. The presence of lateral anisotropy and heterogeneity in Earth's mantle highlights the need for fully anisotropic elasticity data from mineral physics. A breakthrough in high-frequency (gigahertz) ultrasound has resulted in transmission of pure-mode elastic shear waves into a high-pressure diamond-anvil cell using a P-to-S elastic-wave conversion. The full elastic tensor (c(ij)) of high-pressure minerals or metals can be measured at extreme conditions without optical constraints. Here we report the effects of pressure and composition on shear-wave velocities in the major lower-mantle oxide, magnesiowüstite-(Mg,Fe)O. Magnesiowüstite containing more than approximately 50% iron exhibits pressure-induced c(44) shear-mode softening, indicating an instability in the rocksalt structure. The oxide closer to expected lower-mantle compositions ( approximately 20% iron) shows increasing shear velocities more similar to MgO, indicating that it also should have a wide pressure-stability field. A complete sign reversal in the c(44) pressure derivative points to a change in the topology of the (Mg,Fe)O phase diagram at approximately 50-60% iron. The relative stability of Mg-rich (Mg,Fe)O and the strong compositional dependence of shear-wave velocities (and partial differential c(44)/ partial differential P) in (Mg,Fe)O implies that seismic heterogeneity in Earth's lower mantle may result from compositional variations rather than phase changes in (Mg,Fe)O.

Zoned mantle convection

We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced negative buoyancy and can more easily founder to the very base of the mantle. Plateau segregation remains statistical and no sharp compositional interface is expected from the multiple fate of the plates. We show that the variable depth subduction of heavily laden plates can prevent full vertical mixing and preserve a vertical concentration gradient in the mantle. In addition, it can account for the preservation of scattered remnants of primitive material in the deep mantle and therefore for the Ar and (3)He observations in ocean-island basalts.