共查询到20条相似文献,搜索用时 15 毫秒
1.
The results of an experimental study of the pyrope (Mg 3Al 2Si 3O 12)-jadeite (NaAlSi 2O 6) system at P = 7.0 and 8.5 GPa and T = 1300?C1800°C are summarized in this paper. The main phases that were obtained in the experiments are garnet, pyroxene, kyanite (in some cases corundum), and quenched melt. Garnets are characterized by a stable Na 2O admixture (up to 0.6 wt % at 7.0 GPa and up to 0.8 wt % at 8.5 GPa) and the high silicon content (Si = 3.016?C3.166). The maximal sodium concentrations in garnet were found at the solidus of the system, which results from an increase of the coefficient of sodium partitioning between garnet and melt during a temperature decrease. 相似文献
2.
The pseudo-binary system Mg 3Al 2Si 3O 12–Na 2MgSi 5O 12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet
is a liquidus phase of the system up to 60 mol% Na 2MgSi 5O 12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases.
Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na 2O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus ( T = 1,840°C), the equilibrium concentration of Na 2O in garnet is 0.7–0.8 wt% (∼6 mol% Na 2MgSi 5O 12). With the temperature decrease, Na concentration in Grt increases, and the maximal Na 2MgSi 5O 12 content of ∼12 mol% (1.52 wt% Na 2O) is gained at the solidus of the system ( T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing
<0.4 wt% Na 2O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations
(>1 wt% Na 2O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring
Na-bearing garnets is restricted by the eclogite/garnetite bulk composition. 相似文献
5.
The composition of a reduced C–O–H fluid was studied by the method of chromatography–mass spectrometry under the conditions of 6.3 GPa, 1300–1400°C, and fO 2 typical of the base of the subcratonic lithosphere. Fluids containing water (4.4–96.3 rel. %), methane (37.6–0.06 rel. %), and variable concentrations of ethane, propane, and butane were obtained in experiments. With increasing fO 2, the proportion of the CH 4/C 2H 6 peak areas on chromatograms first increases and then decreases, whereas the CH 4/C 3H 8 and CH 4/C 4H 10 ratios continually decrease. The new data show that ethane and heavier HCs may be more stable to oxidation, than previously thought. Therefore, when reduced fluids pass the “redox-front,” carbon is not completely released from the fluid and may be involved in diamond formation. 相似文献
6.
Diamond crystallization has been studied in the SiO 2–H 2O–С, Mg 2SiO 4–H 2O–С and H 2O–С subsystems at 7.5 GPa and 1,600°C. We found that dissolution of initial graphite is followed by spontaneous nucleation
of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [α = M Dm/(M Dm + M Gr)100, where M Dm is mass of obtained diamond and M Gr mass of residual graphite] reached 100% in H 2O-rich fluids but was only 35–50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of α has been established
at the weight ratio H 2O/(H 2O + SiO 2) ≤ 0.16 or H 2O/(H 2O + Mg 2SiO 4) ≤ 0.15. Our results indicate that α is a function of the concentration of water, which controls both the kinetics of diamond
nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization
in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates
solute. In H 2O-poor silicate melts diamond formation is questionable. 相似文献
7.
The crystal structure of a knorringite-type compound, Mg 3(Cr 1.58Mg 0.21Si 0.21)Si 3O 12, synthesized in a multi-anvil press at P = 16 GPa and T = 1,600 °C, was refined from single-crystal X-ray diffraction data up to R = 2.36 % for 314 independent reflections. Garnet was found to be cubic and have space group Ia $\overline{3}$ d, with the unit cell parameters a = 11.5718 (1) Å, V = 1,549.54 (2) Å 3. The knorringite crystal studied contains 21 mol% of majorite end-member. The structural characterization of knorringitic garnet is important because the study of its thermodynamic constants provides new constraints on thermobarometry of peridotitic garnet assemblages of the lowermost upper mantle. The Raman spectra of synthetic knorringite have been obtained for the first time. 相似文献
8.
Phase equilibria in the ternary system H 2O-CO 2-NaCl were studied at 800 °C and 9 kbar in internally heated gas pressure vessels using a modified synthetic fluid inclusion
technique. The low rate of quartz overgrowth along the `b' and `a' axes of quartz crystals was used to avoid fluid inclusion
formation during heating, prior to attainment of equilibrium run conditions. The density of CO 2 in the synthetic fluid inclusions was calibrated using inclusions in the binary H 2O-CO 2 system synthesised by the same method and measured on the same heating-freezing stage. In the two-phase field, two types
of fluid inclusions with different densities of CO 2 were observed. Using mass balance calculations, these inclusions are used to constrain the miscibility gap and the orientation
of two-phase tie-lines in the H 2O-CO 2-NaCl system at 800 °C and 9 kbar. The equation of state of Duan et al. (1995) approximately describes the P- T section of the ternary system up to about 40 wt% of NaCl. At higher NaCl concentrations the measured solubility of CO 2 in the brine is much smaller than predicted by the EOS. A “salting out” effect must be added to the equation of state to
include coulomb interaction in the model of Anderko and Pitzer (1993) and Pitzer and Jiang (1996). The new experimental data
together with published data up to 5 kbar (Shmulovich et al. 1995) encompass practically all subsolidus crustal P- T conditions. A feature of the new experimental results is the large compositional range in the H 2O-CO 2-NaCl system occupied by the stability fields of halite + CO 2-rich fluid ± H 2O-NaCl brine. The prediction of halite stability in equilibrium with CO 2-rich fluid in deep-crustal rocks is supported by recent petrological and fluid inclusion studies of granulites.
Received: 29 June 1998 / Accepted: 17 March 1999 相似文献
9.
To characterize the influence of alkaline metal chlorides on the phase ratios under melting of upper mantle eclogites, the eclogite–CaCO3–NaCl–KCl system with Н2О + СО2-fluid was studied in the experiments under 4 GPa and 1200–1300°C. A low difference in temperatures (<100°C) was registered between the eclogite solidus and liquidus (>1200 and <1300°C, respectively), which is characteristic for the near-eutectic compositions. The phase proportions were peculiar for the absence of any silicate melt over the entire temperature range considered. The carbonate melt coexisted with clinopyroxene and garnet within 1200–1250°C, whereas a carbonate melt exclusively occurred under above-liquidus conditions at 1300°C. The melt quenching resulted in the formation of a multiphase fine-grained mixture of Ca, Na, and K carbonates and chlorides containing microinclusions of clinopyroxene and garnet. The occurrence of a high-calcium carbonate melt in Cl-containing eclogite systems might play a significant role in the mantle metasomatism of subduction zones characterized by the water–alkaline–chloride type of fluids. 相似文献
10.
The paper presents results of experiments aimed at diamond synthesis in the Fe–C–S system at 5.3–5.5 GPa and temperatures of 1300–1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (Fe 3C and Fe 7C 3 carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = Fe 7C 3) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed. 相似文献
11.
Experiments on water solubility in forsterite in the systems Mg 2SiO 4–K 2Mg(CO 3) 2–H 2O and Mg 2SiO 4–H 2O–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–H 2O the H 2O 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 CO 2 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
13.
Compressibility of perovskite-structured Ca 3Al 2Si 3O 12 grossular (GrPv) was investigated at high pressure and high temperature by means of angle-dispersive powder X-ray diffraction using a laser-heated diamond anvil cell. We observed the Pbnm orthorhombic distortion for the pure phase above 50 GPa, whereas below this pressure, Al-bearing CaSiO 3 perovskite coexists with an excess of corundum. GrPv has a bulk modulus ( K 0 = 229 ± 5 GPa; \(K_{0}^{{\prime }}\) fixed to 4) almost similar to that reported for pure CaSiO 3 perovskite. Its unit-cell volume extrapolated to ambient conditions ( V 0 = 187.1 ± 0.4 Å 3) is found to be ~2.5 % larger than for the Al-free phase. We observe an increasing unit-cell anisotropy with increasing pressure, which could have implications for the shear properties of Ca-bearing perovskite in cold slabs subducted into the Earth’s mantle. 相似文献
14.
Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the join VIIICa 3VI(CaGe) IVGe 3O 12– VIIICa 3VIFe 2IVGe 3O 12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO 3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe 2O 5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 ( XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mössbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe 3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe 2+ at the octahedral sites of the garnet structure. The relative abundance of Fe 2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa 3VI(CaGe) IVGe 3O 3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg 3VI(MgSi) IVSi 3O 3 majorite. 相似文献
16.
Doklady Earth Sciences - The phase relations upon eclogitization of basalt and melting of H2O-bearing eclogite were studied experimentally for basalt–H2O in the range of P = 3.7–4.0 GPa... 相似文献
17.
Partitioning of more than 35 elements between coexisting phases in the apatite (Apt)–carbonate (Carb)–H2O system was studied experimentally at P = 0.5 GPa and T = 1200°C for estimation of the efficiency of fluid transport during the formation of carbonatite in platform alkaline intrusions. The interphase partition coefficients of elements (D) range from n × 10–2 to 100 and higher, which provides evidence for their effective fractionation in the system. The following elements were distinguished: (1) Apt-compatible (REE, Y, Th, Cu, and W), which are concentrated in apatite; (2) hydrophile (Na, K, Mg, Ba, S, Mn, Pb, U, W, and Re), which are preferably distributed into fluid or the carbonate melt. The high hydrophilicity of alkali metals controls the alkaline character of postmagmatic fluids and related metasomatic rocks, whereas the high D(Fl/Apt) and D(Fl/LCarb) for S, Zr, W, Re, and U show their high potential in relation to U–W–Re mineralization. 相似文献
18.
Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe 0-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe 3C with an N-rich fluid in a graphite-saturated system produce the ε-Fe 3N phase (space group P6 3/ mmc or P6 322) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity ( fH 2), fluids vary from NH 3- to H 2O-rich compositions (NH 3/N 2?>?1 in all cases) with relatively high contents of alkanes. The fluid derived from N-poor samples contains less H 2O and more carbon which mainly reside in oxygenated hydrocarbons, i.e., alcohols and esters at MMO-buffered fH 2 and carboxylic acids at unbuffered fH 2 conditions. In unbuffered conditions, N 2 is the principal nitrogen host (NH 3/N 2?≤?0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-Fe, melt, or Fe 3N) are stable at the upper mantle pressures and temperatures. According to our estimates, the metal/fluid partition coefficient of nitrogen is higher than that of carbon. Thus, nitrogen has a greater affinity for iron than carbon. The general inference is that reduced fluids can successfully transport volatiles from the metal-saturated mantle to metal-free shallow mantle domains. However, nitrogen has a higher affinity for iron and selectively accumulates in the metal phase, while highly mobile carbon resides in the fluid phase. This may be a controlling mechanism of the deep carbon and nitrogen cycles. 相似文献
19.
The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = ?1.7 to ?1.2, where ΔIW is the deviation of the oxygen fugacity ( fO 2) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < ?3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO 2, whereas the solubility of Si decreased with increasing fO 2. The carbon-bearing phases were graphite, Fe 3C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < ?6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core. 相似文献
20.
Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability
and mutual solubility of the K 2ZrSi 3O 9 (wadeite) and K 2TiSi 3O 9 cyclosilicates under upper mantle conditions. The K 2ZrSi 3O 9–K 2TiSi 3O 9 join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating
that K 2ZrSi 3O 9 is the more refractory end member. At 2 GPa, in the more complex K 2ZrSi 3O 9–K 2TiSi 3O 9–K 2Mg 6Al 2Si 6O 20(OH) 4 system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions
[Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO 2 partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa,
the partitioning behaviour of TiO 2 changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K 2(Zr,Ti)Si 3O 9 phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree
of melting to forsterite and liquid, following the major substitution VITi+ VI□=2 VIMg.
Received: 26 January 1999 / Accepted: 10 January 2000 相似文献
|