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1.
Coupled substitutions involving hydrogen plus trivalent elements (Al, Eu, Fe, Ga, Gd, Lu, Mn, Nd, Pu, Sc, Y and Yb) in forsterite (Mg2SiO4) are studied using atomistic simulation methods. Incorporation of hydrogen is energetically favourable when included in the forsterite lattice as hydroxyl groups (OH−) at O3 sites while the trivalent cations replace either magnesium or silicon. Our calculations show a strong dependence on the ionic radius of the impurity species and some variation with pressure. There are also significant structural distortions around the impurity defects. At low pressure (0 GPa), the smaller trivalent cations, (e.g. Al, Fe, Mn and Ga) substitute into forsterite by replacing Si as: . The larger trivalent cations (e.g. Eu, Gd, Lu, Nd, Pu, Y and Yb) however, replace Mg at the M2 site coupled with an Mg1 vacancy as described by . At 12 GPa, the large cations are more stable at Mg1 relative to Mg2, but both are predicted to be less stable than configurations associated with Si vacancies. The trivalent ionic radius has a significant effect on the H incorporation mechanism, however, the high formation energy of Si vacancies suggests that the presence of H in forsterite could inhibit incorporation of these elements, particularly at high pressure.  相似文献   

2.
A theoretical study of some thermoelastic and structural properties of forsterite, at non-ambient conditions, is presented in this work. A site-dependent potential (NP) has been developed, and successfully used to carry out theoretical investigations on the lattice parameters, specific heat and tensorial elastic properties of forsterite, at P and T conditions representative of the actual thermobaric environment of the upper mantle of the Earth. Calculations have been performed in the framework of lattice dynamics and of quasi-harmonic approximation. The results, from high temperature or high pressure simulations, have been compared with experimental data. Calculations at high pressure and high temperature have provided unprecedented data, which are not easy to achieve experimentally. Received: October 10 1997 / Revised, accepted: January 26 1998  相似文献   

3.
Magnesium-rich olivine (Mg0.9Fe0.1)2SiO4 is considered to be a major constituent of the Earth's upper mantle. Because of its major geophysical importance, the temperature and pressure dependence of its crystal structure, elastic and dielectric constants, long-wavelength phonon modes and specific heat have been measured using a variety of experimental techniques. Theoretical study of lattice dynamics provides a means of analyzing and understanding a host of such experimental data in a unified manner. A detailed study of the lattice dynamics of forsterite, Mg2SiO4, has been made using a crystal potential function consisting of Coulombic and short-range terms. Quasiharmonic lattice dynamical calculations based on a rigid molecular-ion model have provided theoretical estimates of elastic constants, long-wavelength modes, phonon dispersion relation for external modes along the three high symmetry directions in the Brillouin zone, total and partial density of states and inelastic neutron scattering cross-sections. The neutron cross-sections were used as guides for the coherent inelastic neutron scattering experiment on a large single crystal using a triple axis spectrometer in the constant Q mode. The observed and predicted phonon dispersion relation show excellent agreement. The inelastically scattered neutron spectra from a powder sample have been analyzed on the basis of a phonon density of states calculated from a rigid-ion model, which includes both external and internal modes. The experimental data from a powder sample show good agreement with the calculated spectra, which include a multiphonon contribution in the incoherent approximation. The computed phonon densities of states are used to calculate the specific heat as a function of temperature using both the rigid molecular-ion and rigid ion models. These results are in very good agreement with the calorimetric measurement of the specific heat. The interatomic potential developed here can be used with some confidence to study physical properties of forsterite as a function of pressure and temperature.  相似文献   

4.
End-member synthetic fayalite and forsterite and a natural solid-solution crystal of composition (Mg1.80,Fe0.20)SiO4 were investigated using Raman spectroscopy. Polarized single-crystal spectra were measured as a function of temperature. In addition, polycrystalline forsterite and fayalite, isotopically enriched in 26Mg and 57Fe, respectively, were synthesized and their powder spectra measured. The high-wavenumber modes in olivine consist of internal SiO4 vibrations that show little variation upon isotopic substitution. This confirms conclusions from previous spectroscopic studies that showed that the internal SiO4 vibrations have minimal coupling with the lower-wavenumber lattice modes. The lowest wavenumber modes in both forsterite and fayalite shift in energy following isotopic substitution, but with energies less than that which would be associated with pure Mg and Fe translations. The low-wavenumber Raman modes in olivine are best described as lattice modes consisting to a large degree of mixed vibrations of M(2) cation translations and external vibrations of the SiO4 tetrahedra. The single-crystal spectra of forsterite and Fo90Fa10 were recorded at a number of temperatures from room temperature to about 1200 °C. From these data the microscopic Grüneisen parameters for three different Ag modes for both compositions were calculated, and also the structural state of the solid solution crystal was investigated. Small discontinuities observed in the wavenumber behavior of a low-energy mixed Mg/T(SiO4) mode between 700 and 1000 °C may be related to minor variations in the Fe–Mg intracrystalline partitioning state in the Fo90Fa10 crystal, but further spectroscopic work is needed to clarify and quantify this issue. The mode wavenumber and intensity behavior of internal SiO4 vibrations as a function of temperature are discussed in terms of crystal field and dynamic splitting and also 1 and 3 coupling. Crystal-field splitting increases only very slightly with temperature, whereas dynamical-field splitting is temperature dependent. The degree of 1–3 coupling decreases with increasing temperature.  相似文献   

5.
Enstatites (Mg2Si2O6) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO6 octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg2+ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, Mg2SiO4 and fayalite, Fe2-SiO4) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)0.6Mg1.4Si2O6 stable at room temperature using the 488 nm or 514.5 nm lines of an Ar+ ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.  相似文献   

6.
The pressure dependence of the Raman spectrum of forsterite was measured over its entire frequency range to over 200 kbar. The shifts of the Raman modes were used to calculate the pressure dependence of the heat capacity, C v, and entropy, S, by using statistical thermodynamics of the lattice vibrations. Using the pressure dependence of C v and other previously measured thermodynamic parameters, the thermal expansion coefficient, α, at room temperature was calculated from α = K S (?T/?P) S C V/TVK T, which yields a constant value of (? ln α/? ln V)T= 6.1(5) for forsterite to 10% compression. This value is in agreement with (? ln α/? ln V)T for a large variety of materials. At 91 kbar, the compression mechanism of the forsterite lattice abruptly changes causing a strong decrease of the pressure derivative of 6 Raman modes accompanied by large reductions in the intensities of all of the modes. This observation is in agreement with single crystal x-ray diffraction studies to 150 kbar and is interpreted as a second order phase transition.  相似文献   

7.
The dissolution of labradorite, microcline, enstatite, augite and forsterite in acidified deionized water was investigated at near standard temperature and pressure and constant pH of 4.00 to determine the kinetics of the release of silica, and cations. Saturation indices and mass balance calculations suggest that after 700 hours, the release of silica from forsterite and augite was controlled by the precipitation of a solid silica phase, whereas silica mass transfer from the feldspars and enstatite was essentially as silicic acid. Iron release from the pyroxenes and olivine was probably controlled by the precipitation of iron oxyhydroxide phases. Linear-rate constants calculated after 700 hours for release of magnesium ranged from 10?15.2 to 10?14.4 M · cm?2 s?1 for augite and forsterite respectively. Linear-rate constants for the release of cations from feldspars ranged from 10?15.8 to 10?15.3 M · cm?2 s?1.  相似文献   

8.
Polarized single-crystal absorption and reflection spectra of fundamental modes in both the mid- and far-infrared are presented for microscopic crystals of forsterite and fayalite. All modes predicted by symmetry were observed for forsterite, but two B3u modes were not observed for fayalite. Consideration of previously determined frequency shifts for isotopically and chemically substituted olivines, along with symmetry analysis, produced a complete set of band assignments satisfying all constraints for forsterite. A plausible assingment was derived for fayalite by analogy. The frequency shifts from forsterite to fayalite are consistently small for bands assigned to SiO4 stretching and bending, moderate for rotations, and large for translations of M-site ions, suggesting that in olivine, SiO4 groups vibrate separately from the lattice. Allocating the bending and external modes among multiple continua in Kieffer's (1979c) model considerably improves prediction of quasiharmonic heat capacityC v and entropy for forsterite (～1% discrepancy from 200–1000 K). The experimental entropy of fayalite is closely accounted for (1.8 to 0.1%) by summing lattice, electronic (from Burns' (1985) optical band assignment), and constant magnetic contributions above 200 K.S magnetic determined from the difference of the experimental and model lattice entropies shows inflection points at the two magnetic transition temperatures (23 and 66 K) and indicates that complete spin disorder is not achieved below 680 K.  相似文献   

9.
We report in this paper results of lattice dynamical calculations of the thermodynamic properties, namely, the equation of state and the melting point of forsterite. The root mean square displacements of atoms and thermal parameters are also evaluated. It is observed from our results, that, a modified Lindemann criterion for melting may be proposed for an ionic ‘molecular’ solid like forsterite. Agreement among various theoretical and experimental results is satisfactory indicating that lattice dynamical studies based on microscopic quasiharmonic formulations help in understanding the macroscopic properties of forsterite, over a wide range of temperature and pressure.  相似文献   

10.
We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.  相似文献   

11.
Laser-melted magnesium silicate droplets, supercooled 400–750°C below their equilibrium liquidus temperatures before crystallization, were examined to provide a comparison with meteoritic and lunar chondrules and to examine physicochemical parameters that may indicate the conditions of their formation. Internal textures of the spherules strikingly resemble textures observed in some chondrules. Definite trends in crystal morphology, crystal width and texture were established with respect to nucleation temperature and bulk composition. Such trends provide a framework for determining the nucleation temperature of chondrules. The only phase to nucleate from the supercooled forsterite-enstatite normative melts was forsterite, which was present in more-than-normative amounts. Highly siliceous glass (~65wt. % SiO2) was identified interstitially to the forsterite crystals in seven of the spherules and is thought to be present in all. The presence of enstatite and the large proportion of crystals in some meteoritic chondrules implies that they were maintained at temperatures considerably in excess of 600°C at some point in their history.  相似文献   

12.
Drop calorimetry measurements made between 900 and 1800 K are reported for six MO-SiO2 liquids (M = Li2, K2, SrandBa) and two titanium alkalisilicate melts. These results, together with data from the literature, are used to derive a model for calculating the heat capacity of Al-free silicate melts as a function of temperature and chemical composition. Twenty-one major or minor oxides have been considered and, except for K2O-bearing melts, the available data do not indicate deviations of the heat capacities from an additive function of composition. Simple energy calculations show that large variations of the temperature of the liquids result in structural changes of a magnitude similar to those of crystal-liquid transitions. It is suggested that network-modifier cations play an important role in changing the configuration of the liquid in response to temperature variations. The specificity of the behavior of the cations is shown by the lack of a simple relationship between the heat capacities of the liquids and characteristics of the alkali and alkaline-earth cations such as ionic potential or field strength.  相似文献   

13.
The adiabatic single-crystal elastic moduli of superhydrous B, Mg10Si3O14(OH)4, have been measured at ambient conditions using Brillouin spectroscopy. This material is the first hydrous phase found to be stable at the extreme conditions of 20 GPa and 1400 °C. The single-crystal moduli, in GPa, are: C 11=280.0±1.5, C 22=307.4±1.6, C 33=293.4±1.4, C 44=90.0±1.1, C 55=99.2±0.8, C 66=89.6±0.6, C 12=66.1±2.2, C 13=105.6±2.6, C 23=81.8±2.6. With aggregate elastic properties of K VRH =154.0±4.2 and μ VRH =97.0±0.7 GPa, superhydrous B is approximately 16% suffer than forsterite and 20% softer than magnesium silicate spinel; it is also considerably more elastically isotropic than forsterite. The single-crystal moduli are compared to those of forsterite, magnesium silicate spinel and periclase, materials that are both structurally and compositionally similar to superhydrous B. The longitudinal moduli of superhydrous B and forsterite follow similar trends and appear to be dominated by the incompressibility and rotation of silicon tetrahedra. The shear and off-diagonal moduli more closely resemble those of periclase and spinel and may reflect the properties inherent to layers of magnesium octahedra.  相似文献   

14.
《地学前缘(英文版)》2018,9(6):1777-1794
Sedimentary rocks cover-73% of the Earth's surface and metamorphic rocks account for approximately91% of the crust by volume. Understanding the average behavior and variability of heat production for these rock types are vitally important for developing accurate models of lithospheric temperature. We analyze the heat production of ～204,000 whole rock geochemical data to quantify how heat production of these rocks varies with respect to chemistry and their evolution during metamorphism. The heat production of metaigneous and metasedimentary rocks are similar to their respective protoliths. Igneous and metaigneous samples increase in heat production with increasing SiO_2 and K_2 O, but decrease with increasing FeO, MgO and CaO. Sedimentary and metasedimentary rocks increase in heat production with increasing Al_2 O_3, FeO, TiO_2, and K_2 O but decrease with increasing CaO. For both igneous and sedimentary rocks, the heat production variations are largely correlated with processes that affect K_2 O concentration and covary with other major oxides as a consequence. Among sedimentary rocks,aluminous shales are the highest heat producing(2.9 μW~(-3)) whereas more common iron shales are lower heat producing(1.7 μW m~(-3)). Pure quartzites and carbonates are the lowest heat producing sedimentary rocks. Globally, there is little definitive evidence for a decrease in heat production with increasing metamorphic grade. However, there remains the need for high resolution studies of heat production variations within individual protoliths that vary in metamorphic grade. These results improve estimates of heat production and natural variability of rocks that will allow for more accurate temperature models of the lithosphere.  相似文献   

15.
Experiments on water solubility in forsterite in the systems Mg2SiO4–K2Mg(CO3)2–H2O and Mg2SiO4–H2O–C were conducted at 7.5–14.0 GPa and 1200–1600 °C. The resulting crystals contain 448 to 1480 ppm water, which is 40–70% less than in the forsterite–water system under the same conditions. This can be attributed to lower water activity in the carbonate-bearing melt. The water content of forsterite was found to vary systematically with temperature and pressure. For instance, at 14 GPa in the system forsterite–carbonate–H2O the H2O content of forsterite drops from 1140 ppm at 1200 °C to 450 ppm at 1600 °C, and at 8 GPa it remains constant or increases from 550 to 870 ppm at 1300–1600 °C. Preliminary data for D-H-bearing forsterite are reported. Considerable differences were found between IR spectra of D-H- and H-bearing forsterite. The results suggest that CO2 can significantly affect the width of the olivine-wadsleyite transition, i.e., the 410-km seismic discontinuity, which is a function of the water content of olivine and wadsleyite.  相似文献   

16.
The nine adiabatic elastic stiffness constants of synthetic single-crystal fayalite, Fe2SiO4, were measured as functions of pressure (range, 0 to 1.0 GPa) and temperature (range, 0 to 40° C) using the pulse superposition ultrasonic method. Summary calculated results for a dense fayalite polycrystalline aggregate, based on the HS average of our single-crystal data, are as follows: Vp = 6.67 km/s; Vs = 3.39km/s; K= 127.9 GPa; μ = 50.3 GPa; (?K/?P)T = 5.2; (?μ/?P)T=1.5;(?K/?T)P= ?0.030 GPa/K;and,(?/?T)P =-0.013 GPa/K (the pressure and temperature data are referred to 25° C and 1 atm, respectively). Accuracy of the single-crystal results was maintained by numerous cross and redundancy checks. Compared to the single-crystal elastic properties of forsterite, Mg2SiO4, the fayalite stiffness constants, as well as their pressure derivatives, are lower for each of the on-diagonal (C ij for which i=j) values, and generally higher for the off-diagonal (C ij for which i≠j) data. As a result, the bulk moduli (K) and dK/dP for forsterite and fayalite are very similar, but the rigidity modulus (μ) and dμ/dP for polycrystalline fayalite are much lower than their forsterite counterparts. The bulk compression properties derived from this study are very consistent with the static-compression x-ray results of Yagi et al. (1975). The temperature dependence of the bulk modulus of fayalite is somewhat greater (in a negative sense) than that of forsterite. The rigidity dependencies are almost equivalent. Over the temperature range relevant to this study, the elastic property results are generally consistent with the data of Sumino (1978), which were obtained using the RPR technique. However, some of the compressional modes are clearly discrepant. The elastic constants of fayalite appear to be less consistent with a theoretical HCP model (Leibfried 1955) than forsterite, reflecting the more covalent character of the Fe-O bonding in the former.  相似文献   

17.
The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe)2SiO4) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo70. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.  相似文献   

18.
We perform first principles molecular dynamics simulations of Mg2SiO4 liquid and crystalline forsterite. On compression by a factor of two, we find that the Grüneisen parameter of the liquid increases linearly from 0.6 to 1.2. Comparison of liquid and forsterite equations of state reveals a temperature-dependent density crossover at pressures of ∼12-17 GPa. Along the melting curve, which we calculate by integration of the Clapeyron equation, the density crossover occurs within the forsterite stability field at P = 13 GPa and T = 2550 K. The melting curve obtained from the root mean-square atomic displacement in forsterite using the Lindemann law fails to match experimental or calculated melting curves. We attribute this failure to the liquid structure that differs significantly from that of forsterite, and which changes markedly upon compression, with increases in the degree of polymerization and coordination. The mean Si coordination increases from 4 in the uncompressed system to 6 upon twofold compression. The self-diffusion coefficients increase with temperature and decrease monotonically with pressure, and are well described by the Arrhenian relation. We compare our equation of state to the available highpressure shock wave data for forsterite and wadsleyite. Our theoretical liquid Hugoniot is consistent with partial melting along the forsterite Hugoniot at pressures 150-170 GPa, and complete melting at 170 GPa. The wadsleyite Hugoniot is likely sub-liquidus at the highest experimental pressure to date (200 GPa).  相似文献   

19.
The natural occurrence of critical assemblages among the phases clinohumite, calcite, dolomite, tremolite, forsterite, diopside, chlorite, and spinel in metamorphosed impure limestones, together with experimental and thermodynamic data, permits the calculation of phase equilibria governing the stability of clinohumite in terms of the variables P, T, and composition of a CO2-H2O-HF fluid. Equilibrium constant expressions are given for 23 equilibria that describe the stable phase relations between the above phases. Pure OH-clinohumite is considered to be metastable at relatively low pressures. The occurrence of clinohumite in natural marbles is the result of nonideal fluorine substitution which increases the stability of clinohumite. The stability field for clinohumite +calcite, governed primarily by the equilibrium 4forsterite+dolomite+H2O = clinohumite+calcite +CO2, expands to more CO2-rich fluid compositions with increasing fluorine contents and decreasing total pressure. The F/(F+OH) ratio of clinohumite coexisting with calcite, dolomite, and forsterite is a sensitive indicator of the composition of the mixedvolatile fluid phase. The thermodynamic model is in good agreement with observed phase relations and can be used to gain useful information concerning the P-T-X fluid conditions responsible for the formation of clinohumite.  相似文献   

20.
Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO2-CO2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO2-CO2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO2 bubbles in peridotite nodules do not represent free CO2 in mantle peridotite along normal geotherms. (4) CO2 is as effective as H2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO2 at depths shallower than 80 km produces basic magmas, becoming more SiO2-undersaturated with depth. (6) The solubility of CO2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO2-bearing SiO2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.  相似文献   

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