Elasticity of (Mg, Fe)(Si, Al)O₃-perovskite at high pressure     

Li Li  John P. Brodholt  Donald J. Weidner  Maria Alfredsson 《Earth and Planetary Science Letters》2005,240(2):529-536

The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO₃ with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli (c_ij) for (Mg, Fe^3 +)(Si, Al)O₃ perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe³⁺ ions. Considering the favored coupled substitution of Mg²⁺-Si^4 + by Fe³⁺-Al³⁺, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe³⁺ for Al³⁺, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (< 0.5% variation) on both bulk modulus and shear modulus.  相似文献   

Iron oxidation state in lower mantle mineral assemblages: II. Inclusions in diamonds from Kankan, Guinea     

C.A McCammon  T Stachel 《Earth and Planetary Science Letters》2004,222(2):423-434

Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O₃ perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe³⁺/ΣFe. Fe³⁺ concentration in the (Mg,Fe)(Si,Al)O₃ inclusion is consistent with empirical relations relating Fe³⁺/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na_0.5M_0.5³⁺O (M=Fe³⁺, Cr³⁺, Al³⁺). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe-(Mg,Fe)O equilibrium, but more reducing than Re-ReO₂ equilibrium. Fe²⁺/Mg partition coefficients between (Mg,Fe)(Si,Al)O₃ and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe³⁺ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ¹³C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe³⁺ concentration in (Mg,Fe)(Si,Al)O₃ perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.  相似文献   

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle     

Lin-Gun Liu 《Earth and Planetary Science Letters》1977,36(1):237-245

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.  相似文献   

Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties     

Krystle Catalli  Sang-Heon Shim  Vitali B. Prakapenka  Jiyong Zhao  Wolfgang Sturhahn  Paul Chow  Yuming Xiao  Haozhe Liu  Hyunchae Cynn  William J. Evans 《Earth and Planetary Science Letters》2010,289(1-2):68-75

Recent studies have indicated that a significant amount of iron in MgSiO₃ perovskite (Pv) is Fe³⁺ (Fe³⁺/ΣFe = 10–60%) due to crystal chemistry effects at high pressure (P) and that Fe³⁺ is more likely than Fe²⁺ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mössbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe³⁺ in the laser-heated diamond-anvil cell to over 100 GPa. Fe³⁺ increases the anisotropy of the Pv unit cell, whereas Fe²⁺ decreases it. In Pv synthesized above 50 GPa, Fe³⁺ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe³⁺ in the B site becomes LS, while Fe³⁺ in the A site remains HS to at least 136 GPa. Fe³⁺ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe³⁺.  相似文献   

Post-ilmenite phases of silicates and germanates     

Lin-Gun Liu 《Earth and Planetary Science Letters》1977,35(1):161-168

MgSiO₃, ZnSiO₃, MgGeO₃, MnGeO₃, and ZnGeO₃ are the only silicates and germanates known to crystallize in the ilmenite-like structure at high pressures and high temperatures. With the exception of the zinc compounds, the above-mentioned ilmenites have all been found to transform to the orthorhombic modification of the perovskite structure at higher pressures. The ilmenite phase of ZnSiO₃, on the other hand, transforms to its component oxide mixture with the rocksalt and rutile structures, whereas ZnGeO₃ (ilmenite) transforms first to an as yet undetermined orthorhombic phase and then to its component oxide mixture. The direct transformation from the ilmenite to perovskite structures observed in the metasilicates and metagermanates is consistent with all other reported high-pressure post-ilmenite phases (CdTiO₃, CdSnO₃, MnVO₃, and (Fe,Mg)TiO₃). The observation of the ilmenite-perovskite transformation in MgSiO₃ and its solid solutions towards Al₂O₃ suggests that MgO (rocksalt) + SiO₂ (rutile) + Al₂O₃ (corundum) is not a stable mineral assemblage for the earth's lower mantle.  相似文献   

Ferrous iron in post-perovskite from first-principles calculations     

Razvan Caracas  R.E. Cohen 《Physics of the Earth and Planetary Interiors》2008,168(3-4):147-152

We investigate by first-principles calculations the effect of ferrous iron, Fe²⁺, on the structure and the equation of state of MgSiO₃ post-perovskite. We find that ferrous iron is high-spin over the pressure range of the mantle assuming a ferromagnetic structure. The bulk modulus and the specific volume increase with the addition of ferrous iron to MgSiO₃. We find that Fe partitions preferentially to post-perovskite and broadens the two-phase pressure range.  相似文献   

Synthesis and crystal-chemical characterization of MgSiO₃ perovskite     

Eiji Ito  Yoshito Matsui 《Earth and Planetary Science Letters》1978,38(2):443-450

The orthorhombic MgSiO₃ perovskite has been synthesized with the aid of a double-stage split-sphere-type high-pressure apparatus at about 280 kbar and 1000°C. The unit cell dimensions are: a = 4.7754(3)Å, b = 4.9292(4)Å and c = 6.8969(5)Å with the probable space group Pbnm. Calculated density is 4.108 g cm^?3. Crystal structure determination has been carried out by means of both the geometrical simulation (DLS) technique and the ordinary powder X-ray analysis. The results indicate that the MgSiO₃ perovskite is closer to the ideal perovskite than ScAlO₃ perovskite.  相似文献   

Silicate perovskites: A review     

Lin-Gun Liu 《Surveys in Geophysics》1989,10(1):63-81

Since the first discovery of silicate perovskites at high pressures and high temperatures in the laboratory in 1974, silicate perovskites have probably become the most studied materials in the geophysical community during the past decade or so and it is nearly established that these silicates are the most abundant materials making up the bulk of the Earth. There are basically two groups of silicate perovskites. Ferromagnesian silicates with or without Al₂O₃ crystallizing in a common orthorhombic perovskite structure at high pressures and temperatures (HPT) are preservable at ambient conditions. Silicates of large cations such as Ca and Na crystallizing in an ideal cubic perovskite structure at HPT cannot be preserved at ambient conditions. Thus, the lattice parameters, crystal structure, thermal expansion and compressional data have been studied, both experimentally and theoretically, mainly for orthorhombic silicate perovskites, and for MgSiO₃ in particular. For MgSiO₃ perovskite, the recommended lattice parameters area=4.777±0.003,b=4.931±0.003 andc=6.899±0.004 Å; bulk modulusB ₀=2.4±0.2 Mbar; and volume thermal expansivity =(3±1)×10^–5 deg^–1 at ambient conditions. Cubic CaSiO₃ perovskite is probably less compressible than orthorhombic MgSiO₃ perovskite. The lattice parameters of MgSiO₃ perovskite increase linearly with increasing contents of both FeSiO₃ and Al₂O₃, forming limited solid solutions. The degree of distortion of orthorhombic silicate perovskites does not appear to change at HPT.  相似文献   

The high-pressure phases of MgSiO₃     

Lin-Gun Liu 《Earth and Planetary Science Letters》1976,31(2):200-208

The high-pressure and temperature phase transformations of MgSiO₃ have been investigated in a diamond-anvil cell coupled with laser heating from 150 to 300 kbar at 1000–1400°C. X-ray diffraction study of the quenched samples reveals that the sequence of phase transformations for this compound is clinoenstatite → β-Mg₂SiO₄ plus stishovite → Mg₂SiO₄(spinel) plus stishovite → ilmenite phase → perovskite phase with increasing pressure. The hexagonal form of MgSiO₃ observed by Kawai et al. is demonstrated to have the ilmenite structure and the “hexagonal form” of MgSiO₃ observed by Ming and Bassett is shown to be predominantly the orthorhombic perovskite phase plus the ilmenite phase. The mixture of oxides, periclase plus stishovite, reported by Ming and Bassett was not observed in this study. The very wide stability field for the ilmenite phase of MgSiO₃ found in this study suggests that this phase is of importance in connection with the observed rapid increase of velocity in the transition zone of the earth's mantle. On the basis of the extremely dense-packed structure of the perovskite phase of MgSiO₃, this phase should be the most important component for the lower mantle.  相似文献   

Ab initio study of the composition dependence of the pressure-induced spin crossover in perovskite (Mg1 − x,Fex)SiO3     

Amelia Bengtson  Kristin Persson  Dane Morgan 《Earth and Planetary Science Letters》2008,265(3-4):535-545

We present ab initio calculations of the zero-temperature iron high- to low-spin crossover in (Mg_1 ? xFe_x)SiO₃ perovskite at pressures relevant to Earth's lower mantle. Equations of state are fit for a range of compositions and used to predict the Fe spin transition pressure and associated changes in volume and bulk modulus. We predict a dramatic decrease in transition pressure as Fe concentration increases. This trend is contrary to that seen in ferropericlase, and suggests the energetics for spin crossover is highly dependent on the structural environment of Fe. Both Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) exchange-correlation methods are used, and both methods reproduce the same compositional trends. However, GGA gives a significantly higher transition pressure than LDA. The spin transition is made easier by the decreasing spin-flip energy with pressure but is also driven by the change in volume from high to low spin. Volume trends show that high-spin Fe²⁺ is larger than Mg²⁺ even under pressure, but low-spin Fe²⁺ is smaller at ambient conditions and approximately the same size as Mg²⁺ under high pressure, indicating that low-spin Fe²⁺ is less compressible than high-spin Fe²⁺. We find large changes between high- and low-spin in the slope of volume with Fe concentration. Although these changes are small in absolute magnitude for small Fe content, they are still important when measured per Fe and could be relevant for calculating partitioning coefficients in the lower mantle.  相似文献   

Pyroxene-garnet solid-solution equilibria in the systems Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂ and Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ at high pressures and temperatures     

M. Akaogi  S. Akimoto 《Physics of the Earth and Planetary Interiors》1977,15(1):90-106

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.  相似文献   

Minor elements in perovskite from kimberlites and distribution of the rare earth elements: An electron probe study     

Adrian P. Jones  Peter J. Wyllie 《Earth and Planetary Science Letters》1984,69(1):128-140

We report REE and minor element distributions for perovskites from seven kimberlites (South Africa and U.S.A.). The REE (1.6–6.3 oxides wt.%) are always strongly light REE enriched, often with Ce > La (chondrite-normalized), and show an expected close correlation with whole-rock analyses. Where examined, perovskite contains far more REE than coexisting apatite, by about an order of magnitude. Calculations indicate that iron is mostly present as Fe³⁺ and is low (1.0–2.9 wt.% Fe₂O₃) compared with perovskite from carbonatite complexes such as Oka (4.4 wt.% FeO [3]). In addition to established Nb (0.3–1.7 oxide wt.%), geochemically interesting elements encountered include Zr (up to 1.5 oxide wt.%), Ba and Sr (up to 0.2, 0.4 oxide wt.% respectively). Specific geological applications suggest a possible genetic link between Wesselton pipe and Benfontein Sills kimberlites, and that carbonate-rich dikes in the Premier mine were derived from kimberlites. The overall similarities with incompatible element-rich titanates in veined mantle peridotites suggest a more direct link between kimberlite magmatism and mantle metasomatism.  相似文献   

Iron oxidation state in lower mantle mineral assemblages: I. Empirical relations derived from high-pressure experiments     

C.A. McCammon  S. Lauterbach  F. Seifert  P.A. van Aken 《Earth and Planetary Science Letters》2004,222(2):435-449

The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe³⁺/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O₃ perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O₃ majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mössbauer spectra were measured for each run product, and a robust fitting method was developed for Mössbauer spectra using EELS results as a standard that enabled Fe³⁺/∑Fe of (Mg,Fe)(Si,Al)O₃ perovskite to be determined from Mössbauer spectra of multiphase assemblages. There is a close to linear variation between Fe³⁺/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O₃ perovskite, independent of oxygen fugacity. The concentration of Fe³⁺ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe³⁺ concentrations. Fe²⁺/Mg partition coefficients calculated from chemical composition data corrected for measured Fe³⁺/∑Fe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe³⁺/∑Fe in (Mg,Fe)(Si,Al)O₃ perovskite and (Mg,Fe)O ferropericlase samples for which no Mössbauer or EELS data were available, and tested by applying them to calculation of Fe²⁺/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O₃ perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe²⁺/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations.  相似文献   

Phase transformations in a harzburgite composition to 26 GPa: implications for dynamical behaviour of the subducting slab     

T. Irifune  A.E. Ringwood 《Earth and Planetary Science Letters》1987,86(2-4)

The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. The pyroxene-garnet component of the harzburgite composition (harzburgite minus 82 wt.% olivine) transforms to majorite garnet by 18–19 GPa, and further disproportionates to the assemblage of garnet + stishovite + Mg₂SiO₄ spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) and then perovskite MgSiO₃ (24–26 GPa) are found to coexist with garnet. Garnet disappears at 26 GPa and almost complete transition to perovskite is achieved at this pressure. The mineral proportions and density profiles in the subducting oceanic lithosphere, modelled by a combination of 80% harzburgite + 20% primitive MORB compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle, and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km. Under isothermal conditions, the slab has a density similar to surrounding mantle to a depth of 600 km. However, between 600 and 700 km, the slab is up to 0.08 g/cm³ denser than surrounding mantle. This is caused primarily by the higher alumina content in pyrolite as compared to harzburgite, which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite. The presence of alumina also smears out the garnet-perovskite transition in pyrolite over a depth interval of 50 km, whereas this transformation is much sharper in the harzburgite composition. Calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser (by 0.1–0.3 g/cm³) than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth. These results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity.  相似文献   

Viscosity and conductivity of the lower mantle; an experimental study on a MgSiO₃ perovskite analogue,KZnF₃     

J.P. Poirier  J. Peyronneau  J.Y. Gesland  G. Brebec 《Physics of the Earth and Planetary Interiors》1983,32(3):273-287

We have investigated the high-temperature rheological and electrical behaviour of single-crystal KZnF₃ fluoperovskite, an analogue of the MgSiO₃ perovskite in the lower mantle.The crystals flow by Newtonian dislocation creep (Harper-Dorn creep), predominantly on {100} planes. Below the melting point, solid-electrolyte behaviour appears, accompanied by a weakening of the mechanical properties. Geophysical implications are examined: the lower mantle can convect by Newtonian dislocation creep and an asthenosphere may exist at the bottom of the mantle. Electromagnetic interactions between the core and solid-electrolyte lower mantle may also be important.  相似文献   

Melting temperature distribution and fractionation in the lower mantle     

Eiji Ohtani 《Physics of the Earth and Planetary Interiors》1983,33(1):12-25

The melting curve of perovskite MgSiO₃ and the liquidus and solidus curves of the lower mantle were estimated from thermodynamic data and the results of experiments on phase changes and melting in silicates.The initial slope of the melting curve of perovskite MgSiO₃ was obtained as $\text{d}\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{/}\text{d}\text{P?77}\text{K}\text{GPa}{}^{\mathrm{?1}}$ at 23 GPa. The melting curve of perovskite was expressed by the Kraut-Kennedy equation as $\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{(}\text{K}\text{)=917(}\text{1+29.6ΔV}\text{V}{}_0\text{)}$, where T_m?2900 K and P?23 GPa; and by the Simon equation, $\text{P(}\text{GPa}\text{)?23=21.2[}\text{(T}{}_{\mathrm{<mtext>m</mtext>}}\text{(}\text{K}\text{)}\text{2900)}{}^{1.75}\text{?1}\text{]}$.The liquidus curve of the lower mantle was estimated as $\text{T}{}_{\mathrm{<mtext>liq</mtext>}}\text{? 0.9 T}{}_{\mathrm{<mtext>m</mtext>}}$ (perovskite) and this gives the liquidus temperature T_liq=7000 ±500 K at the mantle-core boundary. The solidus curve of the lower mantle was also estimated by extrapolating the solidus curve of dry peridotite using the slope of the solidus curve of magnesiowüstite at high pressures. The solidus temperature is ～ 5000 K at the base of the lower mantle. If the temperature distribution of the mantle was 1.5 times higher than that given by the present geotherm in the early stage of the Earth's history, partial melting would have proceeded into the deep interior of the lower mantle.Estimation of the density of melts in the MgOFeOSiO₂ system for lower mantle conditions indicates that the initial melt formed by partial fusion of the lower mantle would be denser than the residual solid because of high concentration of iron into the melt. Thus, the melt generated in the lower mantle would tend to move downward toward the mantle-core boundary. This downward transportation of the melt in the lower mantle might have affected the chemistry of the lower mantle, such as in the D″ layer, and the distribution of the radioactive elements between mantle and core.  相似文献   

Petrologic properties of the upper 670 km of the earth's mantle; geophysical implications     

Thomas J. Ahrens 《Physics of the Earth and Planetary Interiors》1973,7(2):187-198

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Elasticity of single-crystal SmAlO₃, GdAlO₃ and ScAlO₃ perovskites     

Jay D. Bass 《Physics of the Earth and Planetary Interiors》1984,36(2):145-156

The adiabatic single-crystal elastic moduli of SmAlO₃, GdAlO₃ and ScAlO₃, all with the orthorhombic perovskite structure, have been measured by Brillouin spectroscopy under ambient conditions. These 3 compounds display various degrees of crystallographic distortion from the ideal cubic perovskite structure. We find that longitudinal moduli in directions parallel to the axes of a pseudocubic subcell are nearly equal and insensitive to distortions of the crystal structure from cubic symmetry, whereas, the moduli C₁₁ and C₂₂, parallel to the orthorhombic axes, display pronounced anisotropy with the exception of ScAlO₃. The shear moduli also correlate with distortion from cubic symmetry, as measured by rotation, or tilt angles, of the AlO₆ octahedra. Our data support the observations of Liebermann et al. that perovskite-structure compounds define consistent elasticity trends relating bulk modulus and molar volume, and sound speed and mean atomic weight. These relationships have been used to estimate bulk and shear moduli for the high-pressure polymorphs of CaSiO₃ and MgSiO₃ with the perovskite structure.  相似文献   

Tetrahedral occupancy of ferric iron in (Mg,Fe)O: Implications for point defects in the Earth's lower mantle     

Kazuhiko Otsuka  Catherine A. McCammon  Shun-ichiro Karato 《Physics of the Earth and Planetary Interiors》2010,180(3-4):179-188

We investigated the concentration and site occupation of ferric iron (Fe³⁺) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mössbauer spectroscopy of (Mg_0.8Fe_0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO₂ and Mo–MoO₂ oxygen fugacity buffers. The isomer shift of the Mössbauer spectra suggests that Fe³⁺ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe³⁺ is more stable than unassociated octahedral Fe³⁺ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe³⁺ concentration indicates a change in the dominant site occupancy of Fe³⁺: (1) Fe³⁺ in the tetrahedral site, (2) Fe³⁺ in the octahedral site, and (3) defect clusters of Fe³⁺ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe³⁺ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe³⁺ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe³⁺ and protons (H⁺), and its implications for transport properties in the lower mantle.  相似文献   

Synthesis of a new high-pressure ohase of tin dioxide and some geophysical implications     

Lin-Gun Liu 《Physics of the Earth and Planetary Interiors》1974,9(4):338-343

Tin dioxide (SnO₂) in the rutile structure as starting material has been found to transform to the orthorhombic α-PbO₂ structure (S.G. Pbcn) at about 155 kbar and 1000–1400°C when compressed in a diamond-anvil cell and heated by irradiation with a YAG laser. The lattice parameters at room temperature and 1 bar are $\text{a}{}_{\mathrm{<mtext>o</mtext>}}\text{= 4.719 ± 0.002, b}{}_{\mathrm{<mtext>o</mtext>}}\text{= 5.714 ± 0.002,}\text{and}\text{c}{}_{\mathrm{<mtext>o</mtext>}}\text{= 5.228 ± 0.002}\text{A}\text{?}\text{with}\text{Z = 4}$ for the orthorhombic form of SnO₂, which is 1.5% more dense than the rutile form. Crystal-chemical arguments suggest that stishovite (SiO₂) may also transform to the α-PbO₂ structure at elevated pressure and temperature with an increase in zero-pressure density of about 2–3%. Mineral assemblages containing the orthorhombic SiO₂ are unstable relative to those containing the perovskite MgSiO₃ under lower-mantle conditions.  相似文献