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1.
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 Mg2SiO4Fe2SiO4. 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   

2.
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   

3.
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 Na0.5M0.53+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   

4.
To calculate accurately the pressure interval and mineral proportions (i.e. yields) across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non-ideality of Fe-Mg mixing in these (Mg,Fe)2SiO4 solid solutions. In order to constrain the activity-composition relations that describe non-ideal mixing, Fe-Mg partitioning experiments have been conducted between magnesiowüstite and (Mg,Fe)2SiO4 olivine, wadsleyite and ringwoodite as a function of pressure at 1400°C. Using known activity-composition relations for magnesiowüstite the corresponding relations for the three polymorphs were determined from the partitioning data. In all experiments the presence of metallic iron ensured redox conditions compatible with the Earth’s transition zone. The non-ideality of the (Mg,Fe)2SiO4 solid solutions was found to decrease in the order WwadsleyiteFeMg>WringwooditeFeMg>WolivineFeMg. These partitioning data were used, along with published phase equilibria measurements for the Mg2SiO4 and Fe2SiO4 end-member transformations, to produce an internally consistent thermodynamic model for the Mg2SiO4-Fe2SiO4 system at 1400°C. Using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations. By combining these results with Fe-Mg partitioning data for garnet, the widths of transition zone phase transformations in a peridotite composition were calculated. The olivine to wadsleyite transformation at 1400°C in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation. This transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow. The entire range of estimated widths for the 410 km discontinuity (4-35 km) could, therefore, be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and H2O contents. The wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400°C. This transformation yield was also found to be near linear.  相似文献   

5.
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 (cij) for (Mg, Fe3 +)(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+-Si4 + 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   

6.
We present ab initio calculations of the zero-temperature iron high- to low-spin crossover in (Mg1 ? xFex)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   

7.
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 (Mg0.8Fe0.2)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   

8.
We report observations of seasonal and local time variation of the averaged electron and iron concentrations, as well as simultaneous measurements of the two species, above the Arecibo Observatory (18.35°N, 66.75°N), Puerto Rico. The average Fe profile between 21:00 and 24:00 LT has a single peak at about 85 km with the exception of the summer when an additional peak exists at about 95 km. The higher Fe peak in the summer is correlated with higher electron concentrations in this season. The three nights of simultaneous measurements of electron and iron concentrations show that narrow layers of Fe and electrons are well correlated. Comparison of the climatological and simultaneous Fe and electron data suggests that recombination of Fe+ plays an important role in determining the Fe profile in the upper part of the Fe layer. Above 93 km, the Fe concentration appears to increase after sunset if the electron concentration exceeds about 4000 electrons cm−3. The average rate of Fe production is about 0.1 atom cm−3 s−1 for all seasons at 100 km in the early evening hours. A chemical model reveals that the concentration of Fe+ must be 50–80% of the total ionization over Arecibo for typical equinox conditions to explain the observed rate of Fe production. These high relative Fe+ concentrations are consistent with in situ observations that Fe+ is usually the dominant ion in sporadic E layers in the nighttime lower E region. This suggests that the source of Fe+ is provided by sporadic E layers descending over Arecibo after sunset. The Fe density between 80 and 85 km decreases during the night, for all seasons. This is attributed to the formation of stable molecular Fe species, such as FeOH, due to the increase in O3 and decrease in atomic O and H during the night at these altitudes.  相似文献   

9.
Experimental study of the phase boundary for the disproportionation of the inverse spinel Mg2SnO4 into its isochemical mixed oxides indicates a slope dP/dT = 40 ± 10bars/°K. This positive slope is consistent with the large entropies of inverse (relative to normal) spinels predicted from high-temperature entropy-molar volume systematics. Thermodynamic data do not support the existence of a distinctly negative slope for the proposed disproportionation of Mg2SiO4 normal spinel. Evidence from X-ray and phase equilibria studies suggests the possibility that Si4+, Mg2+, and Fe2+ share the octahedral sites in silicate spinels under mantle conditions. The consequences of this partial inverse character are a positive slope for the postulated spinel-mixed oxide phase boundary near 650 km depth, removal of a widely accepted constraint on mantle-wide convection, and anomalous elasticity-density behaviour within the transition zone.  相似文献   

10.
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   

11.
Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe2+/(Fe2++Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe3+/(Fe3++Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe3+/(Fe3++Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H2O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H2O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H2O>1 wt.%, and the log fO2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H2O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.  相似文献   

12.
Shock observations on melting of iron by Brown and McQueen with the inner core boundary (ICB) density contrast estimated by Masters are used with the assumption that the light ingredient of the outer core is oxygen to calculate the boundary temperature TICB = (5000 ± 900) K. Adiabatic extrapolation to the core-mantle boundary (CMB) gives TICB = (3800 ± 800) K. The temperature increment across the D″ layer is not well constrained, but is estimated to be TD″ = (800 ± 400) K and a slightly superadiabatic extrapolation to 670 km gives T670 + = (2300 ± 950) K. This is only about 300 K higher than the extrapolation to the same level from the upper mantle, T670? = (1970 ± 150) K. The difference is far too small to make a viable mid-mantle boundary layer. Remaining unceertainties are too large to discount such a boundary layer with certainty, but agreement of our new temperature profile with temperatures deduced from equation of state studies on the lower mantle and core encourages the view that we are converging to a well-determined temperature profile for the Earth.  相似文献   

13.
The concentration of rock-forming elements, the static magnetic susceptibility κ, spectra of electron paramagnetic resonance, and their relative intensities I are studied in samples from a borehole drilled in Cenozoic sedimentary deposits of southern Western Siberia. All measured values experience appreciable irregular variations with depth. A linear dependence exists between κ and I within the range of their medium and large values; κ and I have maximum values in the same sample, and κmax = 1920 × 10?6SI, κmin = 210 × 10?6 SI, and κav = 630 × 10?6 SI. The magnetic properties of the samples are controlled by Fe2+ ions present in clastic material and by microphases (clusters) with Fe3+ ions of the goethite and lepidocrocite type present in the cement. The theoretically possible magnetic susceptibility of the Fe2+ ion system (provided that all iron exists in this form) is quite comparable with κmin but, even with very high concentrations of Fe2+, does not reach half of κav: (154 < κ(Fe2+) < 254) × 10?6 SI. Anomalously high values of κ are due to a large number of clusters with Fe3+ ions if structural units FeOOH do not dissociate and the interaction of the clusters with hydroxides of aluminum and precipitation medium impedes the process of their coagulation. Otherwise, the cluster sizes gradually increase, an antiferromagnetic structure develops in clusters, and the magnetic susceptibility decreases.  相似文献   

14.
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   

15.
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   

16.
A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al2O3, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.  相似文献   

17.
Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prp25–35), sodium augite (Jd10–25) with a small amount of ilmenite. There are two stages of retrometamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and lherzolite. Due to intensive serpentinitization, only 5%–40% of the relic olivine (Fo92–93) are preserved. The orthopyroxenes are Mg-rich (En87–93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe?Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988)K D=4.06–5.28;T=793–919°C,P=1.5 GPa are estimated for the garnet pyroxenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emplaced into the lower crust in the solid state and then uplifted to the shallow depth. Obviously, this kind of garnet pyroxenite must be petrogenetically related to its host rock. The REE distribution pattern and the Ni?Co?Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle rocks.  相似文献   

18.
Current views favour the presence of sulphur in the core, giving a composition of Fe + FeS. It is argued that the core composition is close to the eutectic and that this eutectic composition is Fe2S. The consequences for the thermal regime in the core are examined in terms of the most likely properties of the Fe2S eutectic. This leads to much lower temperatures than would be expected for an iron or FeSi core.Consideration of the thermal regime in the mantle and the probable thermal properties of lower-mantle assemblages leads to a similar low temperature for the core-mantle boundary. These temperatures require a temperature gradient near the adiabatic in the mantle, implying a convective thermal history.  相似文献   

19.
A new method of reconstruction of the temperature profile in the lunar mantle from the velocities of seismic P- and S-waves for different models of chemical composition is developed. The procedure of the solution of an inverse problem is realized with the help of the minimization of the Gibbs free energy and the equations of state of a mantle substance, taking into account phase transformations, anharmonicity, and the effects of inelasticity. The geophysical and geochemical constraints on composition and temperature distribution in Moon’s mantle are established. The upper mantle can be composed of olivine pyroxenite, depleted by low-volatile oxides (∼2 wt % of CaO and Al2O3). On the contrary, the lower mantle must be enriched by low-volatile oxides (∼4–6 wt % of CaO and Al2O3). Its composition can be represented by a mineral association of the olivine + clinopyroxene + garnet or olivine + orthopyroxene + clinopyroxene + garnet type, which is close in composition to pyrolite. The temperature distribution at depths 50–1000 km are approximated by the equation: T(°C) = 351 + 1718[1–exp (−0.00082H)]. The constraints inferred make it possible to conclude that the published values of the velocities of P- and S-waves for the lunar mantle, obtained by processing the data of seismic experiments of the Apollo lunar mission are inconsistent with each other at depths below 300 km. Otherwise, the variations in the velocities of P- and S-waves disturb the symmetry between the petrological model (composition), the temperature profile, and the seismic profile.  相似文献   

20.
The elastic moduli of a synthetic single crystal of pyrope (Mg3Al2Si3O12) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)3 (Al, Fe, Cr)2 Si3O12. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe2+/Mg2+ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg2+ and Fe2+ in these different minerals.  相似文献   

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