Fe 3+ and Al solubilities in MgSiO 3 perovskite: implication of the Fe 3+ AlO 3 substitution in MgSiO 3 perovskite at the lower mantle condition (original) (raw)
Related papers
Effect of the incorporation of FeAlO 3 into MgSiO 3 perovskite on the post-perovskite transition
Geophysical Research Letters, 2007
Effect of the incorporation of FeAlO 3 into MgSiO 3 perovskite on post-perovskite transition was investigated in Mg 0.85 Fe 0.15 Al 0.15 Si 0.85 O 3 on the basis of high pressure and temperature in-situ X-ray diffraction experiments using a laser heated diamond anvil cell. Results demonstrate that single perovskite is stable up to 143 GPa and 2500 K and perovskite and post-perovskite coexist at 157-162 GPa and 1600-2500 K for the pressure scales by Tsuchiya (2003). Post-perovskite formed as single phase at 176-178 GPa and 1600-2600 K. The post-perovskite transition pressure in Mg 0.85 Fe 0.15 Al 0.15 Si 0.85 O 3 was much higher than that in MgSiO 3. The present experimental study indicates that the incorporation of FeAlO 3 component expands the stability region of perovskite toward high pressure. The FeAlO 3 concentration may strongly influence the thickness of the D 00 layer at the lowermost of the lower mantle.
Al, Fe substitution in the MgSiO3 perovskite structure: A single-crystal X-ray diffraction study
Physics of the Earth and Planetary Interiors, 2006
We have determined by single-crystal X-ray diffraction the crystal structure of three Fe-Al-MgSiO 3 perovksite samples containing up to 9.5 wt% of Al 2 O 3 and 19 wt% of FeO. We find that there is no evidence for Fe (Fe 3+ or Fe 2+) on the octahedral site. Therefore, we deduce that the two dominant substitution mechanisms for the combined substitution of Al and Fe into the perovskite structure are: (i) Mg A 2+ + Si B 4+ ⇔ Fe A 3+ + Al B 3+ , where the excess of Fe is accommodated by (ii) Mg A 2+ ⇔ Fe A 2+. This is in agreement with all past theoretical and experimental work and solves the long-debated issue of Fe 3+ occupancy in the perovskite structure.
P-V-T equation of state of (Mg,Fe)SiO3 perovskite: constraints on composition of the lower mantle
Physics of The Earth and Planetary Interiors, 1994
Unit-cell volumes (V) of Mg11Fe~SiO3 perovskite (x = 0.0 and 0.1) have been measured along several isobaric paths up to P = 11 GPa and T= 1300 K using a DIA-type, cubic anvil high-pressure apparatus (SAM-85). With a combination of X-ray diffraction during heating cycles and Raman spectroscopy on recovered samples, pressure and temperature conditions were determined under which the P-V-T behavior of the perovskite remains reversible. At 1 bar, perovskites of both compositions begin to transform to amorphous phases at T 400 K, accompanied by an irreversible cell volume contraction. Electron microprobe and analytical electron microscopy studies revealed that the iron-rich perovskite decomposed into at least two phases, which were Fe and Si enriched, respectively. At pressures above 4 GPa, the P-V-T behavior of MgSiO3 perovskite remained reversible up to about 1200 K, whereas the Mg09Fe015i03 exhibited an irreversible behavior on heating. Such irreversible behavior makes equation-of-state data on Fe-rich samples dubious. Thermal expansivities (ay) of MgSiO3 perovskite were measured directly as a function of pressure. Overall, our results indicate a weak pressure dependence in a~,for MgSiO3. Analyses on the P-V-T data using various thermal equations of state yielded consistent results on thermoelastic properties. The temperature derivative of the bulk modulus, (oK~/~T)~, is -0.023(±0.011)GPa K 1 for MgSiO 3 perovskite. Using these new results, we examine the constraints imposed by av and (8K/OT)~on the Fe/(Mg + Fe) and (Mg + Fe)/Si ratios for the lower mantle. For a temperature of 1800 K at the foot of an adiabat (zero depth), these results indicate an overall iron content of Fe/(Mg + Fe) = 0.12(1) for a lower mantle composed of perovskite and magnesiowüstite. Although the (Mg + Fe)/Si ratio is very sensitive to the thermoelastic parameters of the perovskite and it is tentatively constrained between 1.4 and 2.0, these results indicate that it is unlikely for the Jower mantle to have a perovskite stoichiometry.
Formation of a solid solution in the MgSiO3–MnSiO3 perovskite system
Physics and Chemistry of Minerals, 2013
Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM-EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO 3-MnSiO 3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO 3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO 3 components incorporated seems to obey Vegard's rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and ffiffi ffi 2 p a=c;monotonically decreases from MgSiO 3 to MnSiO 3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg 0.5 , Mn 0.5)SiO 3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg 0.9 , Mn 0.1)SiO 3 perovskite increases with pressure. (Mg, Mn)SiO 3 perovskite incorporating less than 10 mol% of MnSiO 3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg 0.9 , Mn 0.1)SiO 3. MnSiO 3 is the first chemical component confirmed to form a complete solid solution with MgSiO 3 perovskite at the P-T conditions present in the lower mantle.
Earth and Planetary Science Letters, 2004
We have investigated the effect of Al 3 + on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO 3 -AlO 1.5 join with up to 25 mol% AlO 1.5 . Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of f 30 -100 GPa at temperatures of f 2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al 3 + into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V 0 ). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al 3 + causes a mild increase in compressibility, with the bulk modulus (K 0 ) decreasing at a rate of À67 F 35 GPa/X Al . This decrease in K 0 is consistent with recent theoretical calculations if essentially all Al 3 + substitutes equally into the six-and eight-fold sites by charge-coupled substitution with Mg 2 + and Si 4 + . In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al 3 + into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO 3 -AlO 1.5 -SiO 2 indicate that a stability field of ternary defect Mgperovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO 3 -AlO 1.5 -FeO 1.5 -SiO 2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mgperovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al 3 + and Fe 3 + substitutions. Primitive mantle with about 5 mol% AlO 1.5 and an Fe 3+ /(Fe 3+ +Fe 2+ ) ratio of f 0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al 3 + and Fe 3 + defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, 0012-821X/$ -see front matter D address: M.J.Walter@bristol.ac.uk (M.J. Walter). www.elsevier.com/locate/epsl Earth and Planetary Science Letters 222 (2004) 501 -516 defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth. D
The energetics of aluminum solubility into MgSiO3 perovskite at lower mantle conditions
Earth and Planetary Science Letters, 2004
SiO 3 perovskite, commonly believed to be the most abundant mineral in the Earth, is the preferred host phase of Al 2 O 3 in the Earth's lower mantle. Aiming to better understand the effects of Al 2 O 3 on the thermoelastic properties of the lower mantle, we use atomistic models to examine the chemistry and elasticity of solid solutions within the MgSiO 3 (perovskite)^Al 2 O 3 (corundum)M gO(periclase) mineral assemblage under conditions pertinent to the lower mantle: low Al cation concentrations, P = 25^100 GPa, and T = 1000^2000 K. We assess the relative stabilities of two likely substitution mechanisms of Al into MgSiO 3 perovskite in terms of reactions involving MgSiO 3 , MgO, and Al 2 O 3 , in a manner similar to the 0 Kelvin calculations of Brodholt [J.P. Brodholt (2000) Nature 407, 620^622] and Yamamoto et al. [T. Yamamoto et al. (2003) Earth Planet. Sci. Lett. 206, 617^625]. We determine the equilibrium composition of the assemblage by examining the chemical potentials of the Al 2 O 3 and MgO components in solid solution with MgSiO 3 , as functions of concentration. We find that charge coupled substitution dominates at lower mantle pressures and temperatures. Oxygen vacancyforming substitution accounts for 3^4% of Al substitution at shallow lower mantle conditions, and less than 1% in the deep mantle. For these two pressure regimes, the corresponding adiabatic bulk moduli of aluminous perovskite are 2% and 1% lower than that of pure MgSiO 3 perovskite.
Physics of The Earth and Planetary Interiors, 2008
Please cite this article as: Fang, C., Ahuja, R., Local structure and electronic-spin transition of Fe-bearing MgSiO 3 perovskite up to the earth's lower mantle conditions, ABSTRACT: We report first-principles electronic structure calculations on the structural and electronic-spin behaviours of Fe-bearing MgSiO 3 crystals up to the pressure of Earth's mantle. The transition pressure of the Fe-bearing MgSiO 3 from the orthorhombic perovskite (OPv) to the orthorhombic post-perovskite (OPPv) phase decreases with increasing Fe concentration. The lattice distortion has impacts on the electronic-spin behaviour of the Fe ions in the PVs. The spin-polarizations of the Fe ions in the (Fe,Mg)SiO 3 OPvs and OPPvs keep unchanged up to the pressures in the lower mantle. Meanwhile the Fe-bearing MgSiO 3 OPv containing Fe Mg -Fe Si pairs exhibits multiple magnetic moments co-existing in a large pressure range (from about 78 to 110 GPa), and finally becomes non-magnetic at pressure higher than 110 GPa. These results provide a mechanism to understand the recent experimental results about Fe valence states and the electronic transitions of the Fe-bearing MgSiO 3 under high pressure.
Journal of Geophysical Research, 1997
San Carlos olivine and its synthetic ringwoodite polymorph have been transformed to (Mg,Fe)SiO3-perovskite and magnesiowtistite at a pressure of 26 GPa in a 2000-t uniaxial split-sphere apparatus (USSA-2000) for temperatures ranging from 700øC to 1600øC and run durations at peak temperatures of 0 min to 19 hours. The recovered samples were studied by analytical transmission electron microscopy to determine the evolution of the microstructures and the crystallographical relationships and iron partitioning between the coexisting phases in these assemblages. At 700øC, metastable olivine remained untransformed even after 19 hours. In runs performed at 1000øC and 1200øC, ringwoodite, in a topotactic relation with olivine, was identified even though olivine was used as starting material. Our results indicate that ringwoodite is an intermediate phase in the olivine -• (Mg,Fe)SiO3-perovskite + magnesiowtistite transformation in this temperature range. At or above 1300øC the transformation of olivine or ringwoodite (used as the starting material) into (Mg,Fe)SiO3-perovskite + magnesiowtistite was complete in less than 10 min. The first microstructures that appear are eutectoid-like as already described by previous authors. For longer run durations the microstructure consisted mostly of cylindrical magnesiowtistite crystals embedded within large, twinned (Mg,Fe)SiO3-perovskite crystals. These observations suggest that magnesiowtistite grains are very unlikely to be interconnected for a wide range of possible bulk mantle compositions; magnesiowtistite will therefore play a relatively minor role in determining the transport properties of Earth's lower mantle. Analyses of (Mg,Fe)SiO3perovskite and magnesiowtistite crystals formed in the first steps of the transformation show that most of the iron-magnesium partitioning is completed within the first minutes of the reaction and that subsequently only isochemical grain growth of the two phases occurs. The iron-magnesium distribution between (Mg,Fe)SiO3-perovskite pv) and magnesiowtistite (mw), characterized by Kd_Fe = (Fe/Mg)mw/(Fe/Mg)p v was precisely measured by analytical transmission electron microscopy in equilibrated runs and found to be Kd_Fe = 3.8(3) at 1300øC and Kd_Fe = 4.3(4) at 1600øC. Paper number 96JB03188. 0148-0227/97/96JB-03188509.00 reported [e.g., Brearley and Rubie, 1994; Brearley et al., 1992; Guyot et al., 1991; Burnley and Green, 1989], very few studies have been devoted to microstructures developed during the breakdown of (Mg,Fe)2SiO4 into (Mg,Fe)SiO3-perovskite and magnesiowiistite [e.g., Poirier et al., 1986; Madon et al., 1989; Ito and Sato, 1991; Wang, 1991]. Moreover, the previous experimental and theoretical studies did not address the influence of such microstructures on the transport properties of this dominant phase assemblage in the lower mantle. The iron contents of the two main phases in the lower mantle assemblage also strongly influence the transport properties. The partitioning of iron and magnesium between the coexisting (Mg,Fe)SiO3-perovskite and magnesiowiistite is equally important for constraining phase relations in pressuretemperature-composition space. Earlier partitioning studies were mostly based on the specific volumes of (Mg,Fe)SiO3perovskite and magnesiowiistite measured with X ray diffraction [e.g., Yagi et al., 1979; Ito and Yamada, 1982]. Both Bell et al. [1979] and Ito and Yamada [1982] have reported large 5265 5266 MARTINEZ ET AL.: PEROVSKITE-MAGNESIOWUSTITE ASSEMBLAGES