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1.
Ultrasonic data for the velocities of the low- and high-pressure polymorphs of germanate compounds undergoing the pyroxene-garnet (CaGeO3, CdGeO3) and pyroxene-ilmenite (MgGeO3, MnGeO3) phase transformations have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 60 kbar. These transitions are characterized by the following velocity (υ)-density (ρ) relationships: (1) the velocity jumps are comparable in percentage magnitude to the density jumps, with the exception of shear velocity for the pyroxene-ilmenite transition; (2) the ratio (υp/υs) of the compressional to shear velocity is constant or increases slightly across the transitions; and (3) low slopes (linear or logarithmic) on υ-ρ diagrams. The observed relationships (1) and (2) are similar to those for the coesite-stishovite transition, but are in marked contrast with those from the olivine-spinel and olivine-beta phase transformations. Coordination changes are thus important factors to be considered in establishing velocity-density systematics governing polymorphic transitions. The υ-ρ changes across the pyroxene-garnet and pyroxene-ilmenite transitions are also distinctly smaller than those produced by compression or thermal expansion of a homogeneous material or by varying composition at constant mean atomic weight. Systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues; this scheme is applied to estimate the bulk moduli of the garnet (1.80 Mbar) and ilmenite (2.11 Mbar) polymorphs of MgSiO3.  相似文献   

2.
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   

3.
A polycrystalline specimen of anorthite has been hot-pressed atP = 15kbar andT = 1000°C in a piston-cylinder apparatus. Compressional(νp)and shear(νs) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The specimen is less than 0.5% porous and is elastically isotropic within 1%. The velocities at 7.5 kbar areνp = 7.29km/secandνs = 3.85km/sec. These data are consistent with those for most terrestrial and lunar plagioclase rocks but not for certain anisotropic rocks and single crystals. The measured velocities demonstrate, moreover, that it is impossible to distinguish between rocks of gabbro, anorthositic gabbro, or anorthosite compositions for the 20–55 km layer of the lunar crust on the basis of seismic data alone. The mean composition of the crust could well be that of a gabbro (17% Al2O3) rather than of an anorthositic gabbro(～25%Al2O3) as assumed in some current models.  相似文献   

4.
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   

5.
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 (M) diagram, however, all of the ilmenite consistent with a single υφM12 = constant trend. 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   

6.
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 a0 = 4.471 ± 0.004, b0 = 5.753 ± 0.005, and c0 = 7.429 ± 0.006 A? 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 Z = 4 and a0 = 4.455 ± 0.008 A? 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   

7.
A polycrystalline specimen of clinopyroxene diopside has been hot-pressed at P = 15 kbar and T = 850°C in a piston-cylinder apparatus. Compressional (νP) and shear (νS) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The velocities at 7.5 kbar are νP = 8.06 km/sec and νS = 4.77 km/sec. These data are consistent with velocity-density trends for orthopyroxenes due to the compensating effects of the monoclinic structure (positive) and Ca content (negative). With the addition of the new data for diopside, it is possible to calculate directly the velocities of various upper-mantle mineral assemblages.  相似文献   

8.
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   

9.
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   

10.
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   

11.
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   

12.
Natural marokite (CaMn2O4) has been studied at high pressures and temperatures using a diamond-anvil press coupled with laser heating in the pressure range 100–250 kbar. A mixture of marokite, CaMnO3 (perovskite) and MnO (rocksalt) has been observed in all runs in the above pressure range by X-ray diffraction study of the quenched samples. It was interpreted that marokite disproportionates into the mixture CaMnO3 (perovskite) + MnO (rocksalt) at pressures below 100 kbar. A general comparison of the molar volume for all known compounds having the marokite-related structures (including CaFe2O4 and CaTi2O4) with those for a mixture of perovskite plus rocksalt structures suggested that the mixture is more stable than the marokite-related structures at high pressures, as confirmed by the present experimental result. The CaFe2O4-modification of common nepheline (NaAlSiO4) is also suggested to be unstable relative to the component oxides of α-NaAlO2 + SiO2 (stishovite) at high pressures.  相似文献   

13.
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   

14.
Ferromagnesian silicate olivines, pyroxenes and garnets with Mg/(Mg + Fe)?0.3 (molar) have been found to transform to high-pressure phases characterized by the orthorhombic perovskite structure when compressed to pressures above 250 kbar in a diamond-anvil press and heated to temperatures above 1,000°C with a YAG laser. The zero-pressure density of the perovskite phase of (Mg,Fe)SiO3 is about 3–4% greater than that of the close-packed oxides, rocksalt plus stishovite. For (Mg,Fe)2SiO4 compounds, the perovskite plus rocksalt phase assemblage is 2–3% denser than the mixed oxides. The experimental synthesis of such high-density perovskite phases in olivine, pyroxene and garnet compounds suggests that (Mg,Fe)SiO3-perovskite is the dominant mineral phase in the earth's lower mantle.  相似文献   

15.
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   

16.
Compressional (Vp) and shear (Vs) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both VpandVs show wide ranges (3.70to6.38km/sec forVpand1.77to3.40km/sec forVsat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of ?1.89and?1.35km/sec per100my forVpandVs respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.  相似文献   

17.
The single-crystal elastic moduli of the ilmenite phase of MgSiO3 have been determined from Brillouin spectroscopy. They are: C11 = 472, C12 = 168, C33 = 382, C13 = 70, C44 = 106, C14 = ?27, C66 = 152 and C25 = ?24 in GPa. These elastic properties are consistent with a structural mechanical model where the silicon octahedra are very stiff under compression and shear. This latter property yields an unexpectedly high shear modulus for the magnesium silicate ilmenite as compared with analogue compounds. The further transformation to perovskite will probably be associated with a significant increase in elastic properties since the strong silicon polyhedra form a structural network in this phase. The transformation of spinel and stishovite to ilmenite is associated with a slight density increase and a slight decrease in acoustic velocities. This transformation will probably not produce a seismic discontinuity even if it does occur in the Earth's mantle.  相似文献   

18.
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   

19.
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   

20.
We use the technique of direct minimization of the Gibbs free energy of the 8-component (K2O-Na2O-Fe2O3-FeO-CaO-MgO-Al2O3-SiO2) multiphase system in order to determine the equilibrium mineral assemblages of rocks of different bulk chemical compositions equilibrated at various P-T conditions. The calculated modal compositions of rocks and experimental data on elastic moduli of single crystals are then used to calculate densities and isotropic elastic wave velocities of rocks together with their pressure and temperature derivatives. Sufficient accuracy of the calculations is confirmed by comparison with experimental data on the gabbro-eclogite transformation and precise ultrasonic measurements of elastic wave velocities in a number of magmatic and metamorphic rocks.We present calculated phase diagrams with isolines of density, elastic wave velocities, and their pressure and temperature derivatives for several anhydrous magmatic rocks, from granite to lherzolite. Density and elastic properties of rocks are controlled by their chemical compositions, especially the SiO2 content, and by P-T of equilibration, and they increase with pressure due to mineral reactions changing mineral assemblages from plagioclase-bearing and garnet-free to garnetbearing and plagioclase-free. TheV p -density correlation is high, and shows two clear trends: one for iron-poor ultramafic rocks and another for all the other rocks considered. Mineral reactions, which occur at high pressures, changeV p and density of anhydrous magmatic rocks following the well-known Birch (or a similar) law.Felsic, intermediate and mafic rocks can be well distinguished in theV p -V p /V s - diagram, although their values ofV p can be close to one another. TheV p -V p /V s -density diagrams together with calculated phase diagrams can serve as efficient instruments for petrologic interpretation of seismic velocities.  相似文献   

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