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1.
The reaction between enstatite (En95.3Fs4.7) and CaCO3 has been studied at pressures between 23 and 77 kbars and at temperatures between 800° and 1400°C. At 1000°C enstatite and CaCO3 react to form dolomite and diopside solid solutions at pressures below approximately 45 kbars and magnesite and diopside solid solutions at higher pressures. The curve for the reaction dolomitess + enstatitess ? magnesitess + diopsidess lies between 40 to 45 kbars at 1000°C and between 45 and 50 kbars at 1200°C. It is very close to the graphite-diamond transition curve. These experimental results indicate that calcite (or aragonite) is unstable in the presence of enstatite, and that dolomite and magnesite are the stable carbonates at high pressures. The forsterite + aragonite assemblage is, however, stable to at least 80 kbars at 800°C. It is suggested that in the upper mantle where enstatite is present, dolomite is stable to depths of about 150 km and magnesite is stable at greater depths in the continental regions, assuming that the partial pressure of CO2 is equal or close to the total pressure. It is also suggested that carbonate inclusions in pyroxene can be used as an indicator of the depth of their equilibration; dolomite inclusions in enstatite would be formed at depths shallower than 150 km and magnesite inclusions in diopside at greater depths. Eclogite and peridotite inclusions in kimberlite may be classified on this basis.  相似文献   

2.
An experimental determination of the reaction MgCO3 + MgSiO3 = Mg2SiO4 + CO2 between 20 and 40 kbars and in the range 1000–1500°C yields an average pressure effect on the equilibrium of 44 bars/°C. This result shows that the assemblage forsterite and carbon dioxide is not stable under most pressure and temperature conditions expected in the upper mantle. Hypotheses requiring the presence of free CO2 in the low-velocity zone, CO2 as a drive mechanism for kimberlite emplacement, or action of a free CO2 phase in ultramafic rocks may need considerable revision.  相似文献   

3.
In the join CaCO3-CaSiO3 at 30 kbars, calcite melts at 1615°C, wollastonite II at 1600°C, and a binary eutectic occurs at 1365°C with liquid composition 43 wt.% CaCO3, 57 wt.% CaSiO3. The eutectic liquid quenches to a glass with few quench crystals. In the join MgCO3-MgSiO3 at 30 kbars, magnesite melts at 1590°C, enstatite at 1837°C, and the fields for the primary crystallization of magnesite and enstatite are separated by a thermal barrier near 1900°C for the melting of forsterite in the presence of CO2. Only about 10 wt.% MgSiO3 dissolves in the carbonate liquid. These data, are considered together with incomplete results for joins CaMgSi2O6-CaMg(CO3)2, CaMgSi2O6-MgCO3, CaMgSi2O6-CaCO3, and other published data in the system CaO-MgO-SiO2-CO2. A thermal barrier separates the silicate and carbonate liquids in MgO-SiO2-CO2 but, in the quaternary system, silicate liquids with dissolved CO2 can follow fractionation paths around the forsterite field to the fields for the primary crystallization of carbonates. This suggests that fractional crystallization of CO2-bearing ultrabasic magma at 100 km depth can produce residual carbonatite magma.  相似文献   

4.
Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg4Si4O12Mg3Al2Si3O12, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe4Si4O12Fe3Al2Si3O12. At 1,000°C, the solid solubility of enstatite (MgSiO3) in pyrope (Mg3Al2Si3O12) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg3(Al0.8Mg0.6Si0.6)Si3O12. In the MgSiO3-rich field, the three-phase assemblage of β- or γ-Mg2SiO4, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe4Si4O12Fe3Al2Si3O12 shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO3) in almandine (Fe3Al2Si3O12) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.  相似文献   

5.
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   

6.
Radiogenic Ar diffusion from phlogopite mica has been measured at 900° and 1080°C at 15 kbars pressure, using a Griggs-type hot creep tester operated at hydrostatic pressure, and followed by standard mass spectrometric analysis. The diffusion coefficients fall within the uncertainty of the diffusion results reported by Giletti (1974) at 1 kbar at 900° and on extrapolation of that Arrhenius plot to 1080°C. That linear plot is given by D0 = 0.75 and Q = 57.9kcal/g-atom Ar. In the observed temperature range, the effect of pressure up to 15 kbars is not significant. A second Ar diffusion, which employed Ca(OH)2-CaO as a water buffer, was run at 1 atm H2O pressure and 550°C for 781 days. The resulting diffusion coefficient agrees, within the uncertainty, with the extrapolation of the earlier curve. This result, and both earlier ones at 600°, are similar in that they fall below the curve. Consequently, despite the agreement within the uncertainty, it is possible that diffusion of Ar from phlogopite below 650°C is somewhat slower than given by the above equation parameters.  相似文献   

7.
The stability field of knorringite (Mg3Cr2Si3O12) is studied experimentally. Knorringite is stable at pressures above 10.5 GPa at 1200°C and 11.8 GPa at 1400°C. Below these pressures, knorringite decomposes to enstatite + eskolaite. A phase diagram of the pyrope-knorringite system is described based on the available experimental data. The solubility of the knorringite molecule in pyrope is essentially dependent only on pressure, and the Cr/Cr+Al value of garnet is considered to be an indicator of the minimum pressure of equilibration. Consideration of the genesis of Cr-rich pyrope and other peridotitic inclusions in diamonds indicates that the fractionation process should have taken place, at least at depths to ca. 240 km, to give rise to the Cr-rich complement of Cr-poor upper mantle materials such as undepleted lherzolite. The knorringite-rich peridotitic suite in diamond will be identified with this complement, which may be the material constituting the deep upper mantle.  相似文献   

8.
Two synthetic end-members and two natural solid solutions of the system Mg3Al2Si3O12-Fe3Al2Si3O12 have been found to display successive phase transformations at increasingly high pressures when they were compressed in a diamond-anvil cell and heated with a YAG laser to temperatures of approximately 1400–1800°C. X-ray diffraction studies of the quenched samples show that the iron-rich garnets apparently first transform to a garnet-related high-pressure phase, then disproportionate into a mixture of magnesiowüstite plus an unknown phase(s). The latter phase(s) may further transform to a still denser unknown phase(s). The ultimate high-pressure phase may be a perovskite-like structure as was previously found for the magnesium-rich garnets. One of the unknown phases may be the high-pressure phase of Al2O3 · nSiO2 compounds. Magnesium-rich garnets display similar phase transformations as do the iron-rich garnets with the exception of the garnet-related high-pressure phase. These results disagree with a previous interpretation for the high-pressure phase of iron-silicate garnets recovered in shock-wave experiments reported by Ahrens and Graham (1972).  相似文献   

9.
Synthesis of pyrope-knorringite solid solution series   总被引：1，自引：0，他引：1  
The garnet solid solution series between pyrope Mg3Al2Si3O12 and knorringite Mg3Cr2Si3O12 has been synthesized from oxide mixtures at pressures of 60–80 kbars and 1400–1500°C. Lattice parameters and refractive indices of solid solutions vary linearly with (molecular) composition within the limits of measurement. The lattice parameter of pure knorringite is 11.600Åand its refractive index is 1.83. The genetic significance of mineral inclusions in natural diamonds is discussed, particularly in the light of the very high knorringite contents often found in garnet inclusions. It is suggested that the most common mineral assemblage occurring as inclusions in diamonds (olivine + knorringite-rich garnet + enstatite) might be explained in terms of subduction into the mantle of olivine + chrome-spinel + enstatite cumulates originally formed by crystallization of mafic magmas within the oceanic crust. The cumulate assemblage experienced alteration by circulating hydrothermal solutions, resulting in the introduction of some carbonate and serpentine minerals. During subduction, this assemblage was partially melted at depth below 150 km, accompanied by reduction of carbonate, to form a reconstituted assemblage consisting of olivine + knorringite-rich garnet + enstatite ± diamond.  相似文献   

10.
Phase behaviour in the system diopside-jadeite (CaMgSi2O6NaAlSi2O6) have been investigated in the pressure region 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The omphacite solid solution extends from 30 to at least 200 kbar for the entire system. Omphacites, ranging in composition from pure diopside to more than 40 mole % jadeite, transform to diopside (II) at pressures greater than 230 kbar. Diopside (II), which probably possesses a perovskite-type structure, cannot be preserved when experiments are quenched to ambient conditions. Jadeite-rich omphacites were found to decompose into an assemblage of NaAlSiO4(CaFe2O4-type structure) + stishovite + diopside (II) (?) at pressures greater than about 260 kbar. These results suggest that an eclogitic model mantle would not display the 400-km seismic discontinuity. Moreover, sodium in the transition zone and lower mantle would most likely be accommodated in phases of omphacite and diopside (II).  相似文献   

11.
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12.
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   

13.
Tadao  Nishiyama  Aiko  Tominaga  Hiroshi  Isobe 《Island Arc》2007,16(1):16-27
Abstract We carried out hydrothermal experiments in the system dolomite‐quartz‐H2O to track the temporal change in reaction rates of simultaneous reactions during the development of reaction zones. Two types of configurations for the starting materials were prepared: dolomite single crystals + quartz powder + water and quartz single crystals + dolomite powder + water, both sealed separately in gold capsules. Runs at 0.1GPa and 600°C with cold seal pressure vessels gave the following results. (i) In short duration (45–71 h) runs metastable layer sequences involving wollastonite and talc occur in the reaction zone, whereas they disappear in longer duration (168–336 h) runs. (ii) The layer sequence of the reaction zones in short duration runs differs from place to place on the dolomite crystal even in the same run. (iii) The diversity of layer sequences in the short duration runs merges into a unique layer sequence in the longer duration runs. (iv) The reaction zone develops locally on the dolomite crystal, but no reaction zone was observed on the quartz crystal in any of the runs. The lines of evidence (i)–(iii) show that the system evolves from an initial transient‐ to a steady‐state and that the kinetic effect is important in the development of reaction zones. A steady diffusion model for the unique layer sequence Qtz/Di/Fo + Cal/Dol + Cal/Dol shows that the Dol + Cal layer cannot be formed by diffusion‐controlled process and that the stability of the layer sequence Qtz/Di/Fo + Cal/Dol depends not only on L‐ratios (a = /LCaOCaO and b = /LMgOMgO) but also on the relative rate P = (−2ξ1 − ξ2)/(–ξ1 − 2ξ2) of competing reactions: Dol + 2Qtz = Di + 2CO2 (ξ1) and 2Dol + Qtz = Fo + 2Cal + 2CO2 (ξ2). For smaller P the stability field will shift to higher values of a and b. The steady diffusion model also shows that the apparent‐non‐reactivity on the quartz surface can be attributed to void formation in a large volume fraction in the diopside layer.  相似文献   

14.
At 30 kbar, calcite melts congruently at 1615°C, and grossularite melts incongruently to liquid + gehlenite (tentative identification) at 1535°C. The assemblage calcite + grossularite melts at 1450°C to produce liquid + vapor, with piercing point at about 49 wt.% CaCO3. Vapor phase is present in all hypersolidus phase fields except for those with less than about 7% CaCO3 or 8% Ca3Al2Si3O12. These results, together with known liquidus data for CaO—SiO2—CO2 and inferred results for CaO—Al2O3—CO2 and Al2O3—SiO2—CO2, permit construction of the position of the CO2- saturated liquidus surface in the quaternary system, and estimation of the positions of liquidus field boundaries separating some of the primary crystallization fields on this surface. The field of calcite is separated from those for grossularite and quartz by a field boundary with about 50% dissolved CaCO3. Crystallization paths of silicate liquids in the range Ca2SiO4—Ca3Al2Si3O12—SiO2, with some dissolved CO2, will terminate at a quaternary eutectic on this field boundary, with the precipitation of calcite together with grossularite and quartz, at a temperature below 1450°C. Addition of Al2O3 to CaO—SiO2—CO2 in amounts sufficient to stabilize garnet thus causes little change in the general liquidus pattern as far as carbonates and silicates are concerned. With addition of MgO, we anticipate that silicate liquids with dissolved CO2 will also follow liquidus paths to fields for the precipitation of carbonates; we conclude that similar paths link kimberlite and some carnbonatite magmas.  相似文献   

15.
A brief report is made of current laboratory investigations on phase relations among olivine, pyroxene, anorthite, magnetite, tridymite, liquid and gas in the system Mg2SiO4-CaAl2Si2O8-FeO-Fe2O2-SiO2 over a wide range of oxygen partial pressures. Courses of fractional crystallization under various conditions of oxygen partial pressure are depicted using an anorthite saturation diagram. Starting with a basalt-like composition in the system, fractional crystallization at a moderate oxygen partial pressure (10 atm.) results in an andesite-like residual liquid of composition 55 SiO2, 14 iron oxide, 6 MgO, 9 CaO, 16 Al2O3 at a temperature of 1155°C. With fractional crystallization in a closed system, the end liquid approaches the composition of 45 SiO2, 38 iron oxide, 6 CaO and 11 Al2O3, at a temperature of 1050°C and oxygen partial pressure of about 10?12 atm. The andesitic final liquid in this system would be expected to further differentiate toward dacitic and rhyolitic compositions if alkalies and water were present in the system. On the basis of these studies, the derivation of liquids of andesitic, dacitic or rhyolitic composition from primary basalts by fractional crystallization seems entirely possible if the oxygen partial pressure is maintained at a moderate or high level.  相似文献   

16.
The melting curve of forsterite has been studied by static experiment up to a pressure of 15 GPa. Forsterite melts congruently at least up to 12.7 GPa. The congruent melting temperature is expressed by the Kraut-Kennedy equation in the following form: Tm(K)=2163 (1+3.0(V0 ? V)/V0), where the volume change with pressure was calculated by the Birch-Managhan equation of state with the isothermal bulk modulus K0 = 125.4 GPa and its pressure derivative K′ = 5.33. The triple point of forsterite-β-Mg2SiO4-liquid will be located at about 2600°C and 20 GPa, assuming that congruent melting persists up to the limit of the stability field of forsterite. The extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite-enstatite system should shift toward the forsterite component with increasing pressure, and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure, although no evidence on incongruent melting has been obtained in the present experiment.  相似文献   

17.
Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different Xco2 conditions in the model system CaO-MgO-SiO2-CO2-H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at Xco2= 0.01 and 0.5 are described. In the calculated P-T grid at Xco2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to Xco2 < 0.02 and their high-P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.  相似文献   

18.
Viscosity of anhydrous albite melt, determined by the falling-sphere method in the solid-media, piston-cylinder apparatus, decreases with increasing pressure from 1.13 × 105 P at 1 atm to 1.8 × 104 P at 20 kbar at 1400°C. The rate of decrease in viscosity is larger between 12 and 15 kbar than in other pressure ranges examined. The density of the quenched albite melt increases with increasing pressure of quenching from 2.38 g/cm3 at 1 atm to 2.53 g/cm3 at 20 kbar. The rate of increase in density is largest at pressures between 15 and 20 kbar. The melting curve of albite shows an inflexion at about 16 kbar. These observations strongly suggest that structural changes of albite melt would take place effectively at pressures near 15 kbar. Melt of jadeite (NaAlSi2O6) composition shows very similar changes in viscosity and density and a melting curve inflexion at pressures near 10 kbar. Difference in pressure for the suggested effective structural changes of albite and jadeite melts is 5–6 kbar, which is nearly the same as that between the subsolidus reaction curves nepheline + albite= 2jadeite and albite=jadeite + quartz. The structural changes of the melts are, however, continuous and begin to take place at pressures lower than those of the crystalline phases.  相似文献   

19.
40Ar/39Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (～3°C Ma?1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ～1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40Ma ago, causing temperatures to rise to～350°C in some places. During this event, accumulated40Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the40Ar/39Ar age spectra gives a local estimate of this partial pressure of ～10?4atm. The region finally cooled below 100°C about 280 Ma ago.40Ar/39Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess40Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess40Ar with respect to radiogenic40Ar, appears to be responsible for the saddle-shaped age spectra.  相似文献   

20.
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   

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