Ultrasonic data for the velocities of the ilmenite and perovskite polymorphs of CdTiO3 have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 25 kbar. This transition is characterized by the following velocity (ν)-density (?) relationships: (1) the changes in compressional (νp) and bulk sound (ν?) velocities are comparable in percentage magnitude to the density jump, while the shear (νs) velocity jump is three times greater than that for ?; (2) (νp/νs) decreases across the transition from the low- to high-pressure phase; and (3) low slopes (linear or logarithmic) on ν-? diagrams. The (νp/νs) behaviour for the ilmenite-perovskite transformation is unusual for the transitions studied in our laboratory. The observed relationships (1) and (2) are typical of the elasticity behaviour across phase transformations which involve increases in cation-anion co-ordination and in nearest-neighbour interatomic distances, such as those exhibited by CdTiO3 in transforming from the ilmenite to the perovskite phase. Elasticity systematics for isostructural sequences are used to estimate the bulk moduli of the perovskite polymorphs of CaSiO3 (2.7 Mbar) and MgSiO3 (2.8 Mbar). 相似文献
Ultrasonic data for the velocities of the ilmenites MgTiO3 and CoTiO3 have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed in a piston-cylinder apparatus at pressures up to 30 kbar. Titanate and germanate ilmenites define divergent isostructural trends on a Birch diagram of bulk sound velocity (υφ) vs. density (ρ). On a υφ vs. mean atomic weight () diagram, however, all of the ilmenite consistent with a single . Elasticity systematics for isostructural sequences are used to e the bulk modulus (2.09 Mbar) and bulk sound velocity (7.4 km/sec) of MgSiO3-elmenite. 相似文献
Ultrasonic data for the velocities of a large number of perovskite-structure compounds have been determined as a a function of pressure to 6 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 100 kbar in solid-media devices: ScAlO3, GdAlO3, SmAlO3, EuAlO3, YAlO3, CdTiO3, CdSnO3, CaSnO3 and CaGeO3. The elasticity data for these orthorhombic and cubic perovskites define systematic patterns on bulk modulus (KS)-volume (VO) and bulk sound velocity (υφ—mean atomic weight (M) diagrams which are insensitiv to the details of cation chemistry and crystallographic structure. These isostructural trends are used to estimate KS = 2.5 ± 0.3 Mbar and υφ = 7.9 ± 0.4 km/s for the perovskite polymorph of MgSiO3. On a Birch diagram of veloc vs. density, the perovskite data define linear trends which lead to erroneous estimates of velocity for MgSiO3 unless specific account is taken of ionic radius effects in isomorphic substitutions. 相似文献
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 Mg4Si4O12Mg3Al2Si3O12, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe4Si4O12Fe3Al2Si3O12. 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(Al0.8Mg0.6Si0.6)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 Fe4Si4O12Fe3Al2Si3O12 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
Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg2+Fe2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg2SiO4Fe2SiO4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe2+/(Mg + Fe2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe2+/(Mg + Fe2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg2+Fe2+ partition relation among coexisting minerals, an increase of about 1% in the Fe2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km. 相似文献
Natural ilmenite (Fe,Mg)TiO3 has been found to transform to the perovskite structure and then to disproportionate into its component oxides, (Fe,Mg)O plus a cubic phase of TiO2, at loading pressures of 140 and 250 kbar respectively, and at temperatures of 1,400 to 1,800°C. Samples were compressed in a diamond-anvil press and heated by irradiation with a YAG laser. The lattice parameters of the perovskite phase of (Fe,Mg)TiO3 at room temperature and 1 bar are with 4 molecules per cell. The zero-pressure volume change is 8.0% for the ilmenite-perovskite transition, 13.3% for the perovskite-mixed-oxides transition, and 20.2% for the ilmenite-mixed-oxides transition. The cubic phase of TiO2 can be indexed on the basis of space group Fm3m with at room temperature and 1 bar, which corresponds to a decrease in zero-pressure volume of 29.2% for the rutile-cubic-phase transition. An isentropic bulk modulus at zero pressure of 5.75 ± 0.30 Mbar and a pressure derivative greater than 8 were calculated for the high-pressure cubic phase. The calculated bulk modulus for the mixture of (Fe,Mg)O and cubic TiO2 is 2.48 ± 0.25 Mbar. All the phase transformations, the calculated lattice parameters, and the bulk moduli observed in this study are in good agreement with published shock-Hugoniot data for ilmenite and rutile. 相似文献
Pressure effects on the lattice parameters of β- and γ-Mg2SiO4 have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V0) at 90 kbar are 5.4 · 10?2 and 4.2 · 10?2 for β- and γ-Mg2SiO4, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg2SiO4, respectively. The α → γ transition obeys Wang's linear Vφ?ρ relation but the α → β transition does not. 相似文献
Phase transformations in baddeleyite (ZrO2) and zircon (ZrSiO4) have been investigated in the pressure range between 100 and 300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Baddeleyite has been found to transform to an orthorhombic cotunnite-type structure at pressures greater than 100 kbar, and is the first oxide known to adopt this structure. The lattice parameters of the cotunnite-type ZrO2 at room temperature and atmospheric pressure area = 3.328 ± 0.001 ,b = 5.565 ± 0.002 , andc = 6.503 ± 0.003A? withZ = 4 , and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO2. It is suggested that all the polymorphic structures of ZrO2 are possible high-pressure models for the post-rutile phase of SiO2. The polyhedral coordination in these model structures varies from 7 to “9”, compared with 6 for stishovite. If SiO2 were to adopt any of these structures in the deep mantle, Birch's hypothesis of a mixed-oxide lower mantle may still be viable, but the primary coordination of silicon would be greater than 6. Zircon has been found to transform to a scheelite-type structure at about 120 kbar as noted earlier. The scheelite-type ZrSiO4 was found to decompose further into a mixture of ZrO2 (cotunnite-type) plus SiO2 (stishovite) in the pressure range 200–250 kbar. As implied by the transitions in zircon, the large cations of U and Th in the earth's deep mantle are most likely to occur in dioxides with structures such as the cotunnite-type, rather than to occur in silicates. 相似文献
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
Using acoustic measurement interfaced with a large volume multi-anvil apparatus in conjunction with in situ X-radiation techniques, we are able to measure the density and elastic wave velocities (VP and VS) for both ortho- and high-pressure clino-MgSiO3 polymorphs in the same experimental run. The elastic bulk and shear moduli of the unquenchable high-pressure clinoenstatite phase were measured within its stability field for the first time. The measured density contrast associated with the phase transition OEN → HP-CEN is 2.6-2.9% in the pressure of 7-9 GPa, and the corresponding velocity jumps are 3-4% for P waves and 5-6% for S waves. The elastic moduli of the HP-CEN phase are KS=156.7(8) GPa, G = 98.5(4) GPa and their pressure derivatives are KS′=5.5(3) and G′ = 1.5(1) at a pressure of 6.5 GPa, room temperature. In addition, we observed anomalous elastic behavior in orthoenstatite at pressure above 9 GPa at room temperature. Both elastic wave velocities exhibited softening between 9 and 13-14 GPa, which we suggest is associated with a transition to a metastable phase intermediate between OEN and HP-CEN. 相似文献
High-pressure phase transformations for three intermediate compositions (including diopside) in the system enstatite (MgSiO3)-wollastonite (CaSiO3) were investigated in the pressure range 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The phase behaviour of the two end components (enstatite and wollastonite) at high pressure has been reported earlier. The results of this study reveal that there is very limited solid solution of diopside (CaMgSi2O6) in the various high-pressure phase assemblages of enstatite. At pressures greater than about 200 kbar, diopside and a composition between diopside and wollastonite were found to transform into non-quenchable phases, as does wollastonite. It is thought probable that diopside and wollastonite form solid solutions with the perovskite structure at high pressure, but that on release of pressure it is not possible to preserve the high-pressure modification. 相似文献
By incorporating the knowledge of the observed high-pressure phase transformations, the measured equilibrium phase boundaries for certain transitions, and the measured thermochemical data for certain phases in the system MgOSiO2, the equilibrium phase boundaries for all the phase transitions in this system at 1000°C have been calculated on the basis of an assumption that volume change across a phase boundary is independent of temperature. Three major seismic velocity discontinuities in the mantle (420, 570, and 650 km) have been chosen for comparison with the measured and calculated equilibrium phase boundaries for the high-pressure phases observed in the system MgOSiO2. Complications of phase changes due to the addition of FeO and Al2O3 to the system have also been accounted for. It is suggested by the results of this study that the 420- and 570-km discontinuities are probably related to phase transitions observed in the system MgOSiO2, but that the 650-km discontinuity is not likely to be associated with any of the equilibrium phase boundaries observed in olivine, pyroxene, and garnet, but may instead be a chemical change. 相似文献
High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China. 相似文献
Phase relations in the system Mg4Si4O12-Mg3Al2Si3O12 were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg4Si4O12-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg3Al2Si3O12-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg4Si4O12. The stability field of Mg3Al2Si3O12 ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al2O3 content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg4Si4O12, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg3Al2Si3O12. Thermodynamic calculations using these data give rise to phase relations in the system Mg4Si4O12-Mg3Al2Si3O12 at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg4Si4O12 to Mg3Al2Si3O12 with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg2SiO4-Fe2SiO4 system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs. 相似文献
The elastic moduli of polycrystalline ringwoodite, (Mg0.91Fe0.09)2SiO4, were measured up to 470 K by means of the resonant sphere technique. The adiabatic bulk (KS) and shear (μ) moduli were found to be 185.1(2) and 118.22(6) GPa at room temperature, and the average slopes of dKS/dT and dμ/dT in the temperature range of the study were determined to be −0.0193(9) and −0.0148(3) GPa/K, respectively. Using these results, we estimate seismic wave velocity jumps for a pure olivine mantle model at 520 km depth. We find that the jump for the S-wave velocity is about 1.5 times larger than that for the P-wave velocity at this depth. This suggests that velocity jumps at the 520 km discontinuity are easier to detect using S-waves than P-waves. 相似文献
Phase assemblages for five selected compositions in the system CaSiO3-Al2O3 have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al2O3. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl2O4-SiO2. Garnet solid solutions are stable between about 10 and 25 mole% Al2O3. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO3 composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier. 相似文献
Phase behaviors of albite, jadeite and nepheline have been studied in the diamond-anvil press employing YAG laser heating from about 100 to 280 kbar and at about 1000°C. Incorporating earlier work, the sequences of phase transformations with increasing pressure are as follows:
where the percentages are the decreases in zero-pressure volume from one to the other. Both NaAlSi3O8 hollandite and NaAlSiO4 (CaFe2O4-type) are new sodium aluminosilicates. The latter is the most likely host for sodium in the earth's lower mantle. High-pressure phases or phase assemblages revealed in albite and jadeite by static experiments are poorly defined by shock-wave studies of albitite and jadeite to about 900 and 1200 kbar respectively. 相似文献
The densities of silicate liquids with basic, picritic, and ultrabasic compositions have been estimated from the melting curves of minerals at high pressures. Silicate liquids generated by partial melting of the upper mantle are denser than olivine and pyroxenes at pressures higher than 70 kbar, and garnet is the only phase which is denser than the liquid at pressures from 70 kbar to at least 170 kbar. In this pressure range, garnet and some fraction of liquid separate from ascending partially molten diapirs. It is therefore suggested that aluminium-depleted komatiite with a high Ca/OAl2O3 ratio may be derived from diapirs which originated in the deep upper mantle at pressures from 70 kbar to at least 140 kbar (200–400 km in depth), where selective separation of pyropic garnet occurs effectively. On the other hand, aluminium-undepleted komatiite is probably derived from diapirs originating at shallower depths (< 200 km). Enrichment of pyropic garnet is expected at depths greater than 200 km by selective separation of garnet from ascending diapirs. The 200-km discontinuity in the seismic wave velocity profile may be explained by a relatively high concentration of pyropic garnet at depths greater than 200 km. 相似文献
A 3D velocity model of the Earth’s crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of parental melts of Klyuchevskoy volcano magnesian basalts have been obtained. The parental melt corresponds to picrite (MgO = 13–14 wt %) with an ultimate saturation of SiO2 (49–50 wt %), a high H2O content (2.2–2.9%), and incompatible elements (Sr, Rb, Ba). This melt is formed at pressures of 15–20 kbar and temperatures of 1280–1320°C. Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1–2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model. 相似文献
Ultrasonic data for the velocities of SiO2-stishovite have been determined as a function of pressure to 10 kbar at room temperature for polycrystalline specimens hot-pressed at pressures P = 120kbar and temperatures T = 900°C. These cylindrical specimens are 2 mm in diameter and 0.9–1.4 mm long and have a grain size less than 10 μm. Compressional and shear wave velocities were measured both parallel and perpendicular to the axis of pressing and were found to be isotropic at 10 kbar with νp = 11.0 ± 0.2km/sec andνs = 6.9 ± 0.3km/sec; this shear velocit is substantially higher than that of Mizutani et al. (1972) perhaps due to the presence of crack orientations in their specimen which affected νs but not νp. The Murnaghan P-V trajectories calculated from the ultrasonic data [bulk modulus Ks = 2.5 ± 0.3Mbar and assuming (?Ks/?P)T = 6 ± 2] are consistent with recent hydrostatic compression data and with the shock wave compression data above 600 kbar. The combined evidence from the data of the ultrasonic and hydrostatic compression techniques suggests that the most probable value of the bulk modulus of stishovite at zero pressure is close to the upper limit of the uncertainty of our ultrasonically determined value, K0 = 2.7?2.8Mbar. Elasticity data for rutile-type oxides are not compatible with normal Ks-V0 systematics perhaps due to the neglect of non-central forces in the lattice model. These new stishovite data would make it impossible to satisfy the elasticity-density data of the lower mantle using an oxide mixture with either olivine or pyroxene stoichiometry. 相似文献
