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1.
Under hydrous conditions the stability field of the assemblage Mg-cordierite+K feldspar+quartz is limited on its low-temperature side by the breakdown of cordierite+K feldspar into muscovite, phlogopite and quartz, whereas the high-temperature limit is given by eutectic melting. The compatibility field of the assemblage ranges from 530° C to 745° C at 1 kbar , from 635 to 725° C at 3 kbars , from 695 to 725° C at 5 kbars and terminates at 5.5 kbars . Most components not considered in the model system will tend to restrict this field even more. However, the condition < P total will increase the range of stable coexistence drastically, making the assemblage common at elevated temperatures from contact metamorphic rocks up to intermediate pressure granulites of appropriate bulk composition.  相似文献   

2.
Stability relations of Fe-Mg cordierite with K feldspar have been determined for conditions of muscovite-quartz instability, applicable to highgrade metamorphism of pelitic rocks. Fe cordierite, K feldspar, and water break down to Fe biotite, sillimanite, and quartz at pressures above a line through 640 ° C, 2kbar and 710 ° C, 2.7 kbar. A P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K D value of 0.53 if Al content of biotite and cordierite water of hydration are taken into account.At higher temperatures Fe cordierite breaks down alone to almandine, sillimanite, quartz and water at pressures above a line through 650 ° C, 3.41 kbar and 760 ° C, 2.9 kbar. For the Fe-Mg reaction, P-X data up to 4 kbar may be extrapolated with use of natural K D values increasing toward one with increasing temperatures.Lines of constant cordierite composition for the two reactions intersect in an Fe-Mg univariant reaction of sillimanite-biotite-quartz to cordieritealmandine-K feldspar-water which is metastable relative to melt at = P tot Reduced water pressure and impurities in the garnet and K feldspar greatly reduce the temperature of this reaction so that it becomes a reasonable reaction for upper amphibolite and granulite facies conditions.The results demonstrate that (1) cordierite may be used as a geobarometer if temperature and approximate can be estimated, (2) almandine low in Mn and Ca does not participate in cordierite reactions where muscovite is present, and (3) the reaction which forms cordierite, almandine, and K feldspar is a possible melt-forming reaction which, under reduced , occurs about 50 ° C above the muscovite melting reaction.  相似文献   

3.
Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A12O3-SiO2-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al2O3-SiO2-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for =P total (770° C, 5.9 kb) and =O (740° C, 4.8 kb). A low , implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and Ptotal=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs.  相似文献   

4.
The partition of iron and magnesium between cordierite and garnet depends on as well as temperature. The apparently conflicting experimental data on the values of K D may be reconciled by considering the pertaining during the different experiments.  相似文献   

5.
Cordierite and garnet are common in migmatites adjoining Caldeonian (sensu lato) synorogenic intrusions in the Highlands of Scotland. Migmatitic hornfelses of the Huntly-Portsoy area, of particular interest in being closely associated with the regional andalusite/kyanite boundary, contain both of the pressure-sensitive sub-assemblages (cordierite-garnet-sillimanite) and (cordierite-garnet-ortho-pyroxene). In other areas regional metamorphism was of higher-pressure (kyanite-sillimanite) type, the metamorphic patterns having been subsequently modified at lower pressures. The subassemblage cordierite-garnet-sillimanite is found in pelites occurring near contacts of the Strontian Granodiorite and the nearby, but probably earlier, Glen Scaddle basic complex, where andalusite also occurs in the contact zone. The sub-assemblage cordierite-garnet-orthopyroxene is studied in migmatitic hornfelses of the Lochnagar aureole. Zoning of cordierite and garnet is found in all specimens, and results largely from retrograde diffusive reactions.The calibrations of the garnet-cordierite-sillimanite geobarometer by Thompson (1976) and by Holdaway and Lee (1977) are used to estimate pressures of migmatization. These calibrations give results 2–3 kbar lower than that of Hensen and Green (1973). In the Huntly-Portsoy area, where the pressure must have been close to that of the Al2SiO5 triple point, the Thompson result is 5.5 ±0.1 kbar. The calibration by Holdaway and Lee (1977), in which the H2O content of cordierite is treated, gives a maximum pressure estimate (for P H 2 O=P) of 6.1 ±0.2 kbar. Rough calculations in which is also estimated, using a simplified biotite-sillimanite-quartz dehydration reaction, give P=4.9±0.1 kbar with 0.6P.The Glen Scaddle area gives P = 5.6–5.9 kbar, P by this method, compared with P=5.2–5.3 kbar by the Thompson calibration. The Strontian area gives lower values (<5kbar) despite being structurally below the Glen Scaddle area; this would indicate late origin of the cordierite-bearing migmatites and is consistent with their interpretation in terms of the thermal effects of the Strontian Granodiorite. The Lochnagar aureole is also relatively low-pressure.Estimates of maximum temperatures are 700–740 °C for the Strontian and Huntly-Portsoy areas, and 780–820 °C for the higher grade localities in the Glen Scaddle and Lochnagar areas, where orthopyroxene coexists with potash felspar and cordierite (but not sillimanite). It is likely that cordierite in all the rocks studied was produced as a result of melting reactions that are also responsible for migmatization.  相似文献   

6.
In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O2=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S2+1.90 H2O+10.62 CO2. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.  相似文献   

7.
Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P tot to the assemblage enstatite+forsterite+spinel+H2O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H2O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.  相似文献   

8.
The Al-in-hornblende barometer, which correlates Altot content of magmatic hornblende linearly with crystallization pressure of intrusion (Hammarstrom and Zen 1986), has been calibrated experimentally under water-saturated conditions at pressures of 2.5–13 kbar and temperatures of 700–655°C. Equilibration of the assemblage hornlende-biotite-plagioclase-orthoclasequartz-sphene-Fe-Ti-oxide-melt-vapor from a natural tonalite 15–20° above its wet solidus results in hornblende compositions which can be fit by the equation: P(±0.6 kbar) = –3.01 + 4.76 Al hbl tot r 2=0.99, where Altot is the total Al content of hornblende in atoms per formula unit (apfu). Altot increase with pressure can be ascribed mainly to a tschermak-exchange ( ) accompanied by minor plagioclase-substitution ( ). This experimental calibration agrees well with empirical field calibrations, wherein pressures are estimated by contact-aureole barometry, confirming that contact-aureole pressures and pressures calculated by the Al-in-hornblende barometer are essentially identical. This calibration is also consistent with the previous experimental calibration by Johnson and Rutherford (1989b) which was accomplished at higher temperatures, stabilizing the required buffer assemblage by use of mixed H2O-CO2 fluids. The latter calibration yields higher Altot content in hornblendes at corresponding pressures, this can be ascribed to increased edenite-exchange ( ) at elevated temperatures. The comparison of both experimental calibrations shows the important influence of the fluid composition, which affects the solidus temperature, on equilibration of hornblende in the buffering phase assemblage.  相似文献   

9.
Near-liquidus melting experiments were performed on a high-K latite at fO2's ranging from iron-wustite-graphite (IWG) to nickel-nickel oxide (NNO) in the presence of a C-O-H fluid phase. Clinopyroxene is a liquidus phase under all conditions. At IWG , the liquidus at 10 kb is about 1,150° C but is depressed to 1,025° C at NNO and . Phlogopite and apatite are near-liquidus phases, with apatite crystallizing first at pressures below 10 kb. Phlogopite is a liquidus phase only at NNO and high . Under all conditions the high-K latites show a large crystallization interval with phlogopite becoming the dominant crystalline phase with decreasing temperature. Increasing fO2 affects phlogopite crystallization but the liquidus temperature is essentially a function of . The chemical compositions of the near-liquidus phases support formation of the high-K latites under oxidizing conditions (NNO or higher) and high . It is concluded from the temperature of the H2O-saturated liquidus at 10 kb, the groundmass: crystal ratio and presence of chilled latite margins around some xenoliths that the Camp Creek high-K latite magma passed thru the lower crust at temperatures of 1,000° C or more.  相似文献   

10.
The compositions of coexisting pyroxmangites, rhodonites, rhodochrosites and manganese calcites in regional metamorphosed manganese cale-silicate marbles from Val Scerscen and Alagna were analysed by microprobe and permit definition of critical tie lines at metamorphic grades appropriate to temperatures between 400 and 450 °C.Variations in composition of coexisting mineral pairs in one and the same locality are attributed to variations in and not to metamorphic temperatures. From the analysed assemblages isothermal plots (with SiO2 as excess component) were constructed for the system CaO-MnO-SiO2-CO2.  相似文献   

11.
Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in and . The chemical potential gradients, locally as high as 72 cal/m in and 9 cal/m in , controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where fell below 0.80, at near 10–23 bars; the andalusite-biotite isograd records where fell below 0.25, at near 10–22 bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO2 which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining.  相似文献   

12.
The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO2-H2O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has 0.8$$ " align="middle" border="0"> , and there are indications that during their progressive metamorphism is approximately equal to P total.  相似文献   

13.
Iron chlorites with compositions intermediate between the two end-members daphnite (Fe5Al2Si3O10(OH)8) and pseudothuringite (Fe4Al4Si2O10(OH)8) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing traversing the magnetite field for P total = =2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C.  相似文献   

14.
Thermal aureoles surrounding intrusions of the Nain complex, Labrador, contain many unique or unusual mineral assemblages in aluminous gneisses and granulites, ironstones, and ultramafic rocks. Some of the limiting assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite ±biotite±magnetite): Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Ga-Hy-Ol-Qz, Cd-Hy-Os-Qz, Ga-Cd-Sp-Si, Cd-Sp-Co-Si, Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Hy-Ol-Qz-Aug, and Ol-Hy-Sp(±Chl±Ca-amph±Aug). On the basis of some of these assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite from 3.7–6.6 kbar and temperatures from 645–915 ° C. The paucity of hydrous phases, the preponderance of Ksp-Plag-Qz or even Cd-Ksp-Qz without evidence of a melt at these temperatures, the stable occurrence of osumilite, and the common presence of graphite suggest that was extremely low. The presence of graphite-pyrrhotite, the compositions of ilmenites, the compositions of coexisting Fe-Ti oxides, and other mineralogical data indicate the was relatively low and, despite the wide range in bulk compositions and rock types, may have followed an approximately buffered trend. It is possible that the magmas of the Nain complex may have acted as an external buffer for the aureole rocks. The unique nature of these mineral assemblages appears to be a function of the extremely low .  相似文献   

15.
Gneisses in the Guri area of the Venezuelan Guayana Shield contain mineral assemblages with cordierite, garnet, sillimanite, hypersthene, biotite and Fe-Ti oxide intergrowths.Analysis of mineral assemblages and compositional relationships in the light of experimental data indicate metamorphic conditions of 725–800° C, 5–6 kb P T , <P T for the highest grade rocks and 650–700° C, 5–7 kb P T , approximating P T for the lowest grade rocks. Oxygen fugacities in different lithologies ranged between those of the MH and QFM buffers.The distribution coefficient K D (Mg-Fe) (gar-bio), decreases by 0.006 per atom percent increase in (Mn/Mn+Mg+Fe)gar, falls in the range of K D typical of the sillimanite-K feldspar zone and granulite facies, and is systematically lower in lower grade rocks-all in accord with observation in other localities. K D (Mg-Fe) (cord-bio) ranges from 3.0 in the highest grade rocks to 10.0 in the lowest grade rocks, appears independent of FeO/MgO of cordierite or biotite, and varies systematically with grade. In contrast with conclusions based on observation in other localities, data from the Guri area suggest -KD(cord-bio) may be a sensitive index of grade.A number of mineralogic and geologic observations are difficultly reconciled with existing experimental data.  相似文献   

16.
The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H2O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given , equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H2O (1)The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in , this factor being otherwise buffered by the reaction (1) assemblage.The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2).The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, = 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature.The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C.It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in . It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.  相似文献   

17.
The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO2, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6 for the biotite granites and 13.2 ± 0.8 for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO2 and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with =1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at 6 kbar, <1 and temperatures 800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, <1, the relatively high TiO2 and Ba concentrations and the relatively low values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low- biotite granites generated at a deeper, hotter region and the high- tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high- melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.  相似文献   

18.
Three Al-Cr exchange isotherms at 1,250°, 1,050°, and 796° between Mg(Al, Cr)2O4 spinel and (Al, Cr)2O3 corundum crystalline solutions have been studied experimentally at 25 kbar pressure. Starting from gels of suitable bulk compositions, close approach to equilibrium has been demonstrated in each case by time studies. Using the equation of state for (Al, Cr)2O3 crystalline solution (Chatterjee et al. 1982a) and assuming that the Mg(Al, Cr)2O4 can be treated in terms of the asymmetric Margules relation, the exchange isotherms were solved for Δ G *, and . The best constrained data set from the 1,250° C isotherm clearly shows that the latter two quantities do not overlap within three standard deviations, justifying the choice of asymmetric Margules relation for describing the excess mixing properties of Mg(Al, Cr)2O4 spinels. Based on these experiments, the following polybaric-polythermal equation of state can be formulated: , P expressed in bars, T in K, G m ex and W G,i Sp in joules/mol. Temperature-dependence of G m ex is best constrained in the range 796–1,250° C; extrapolation beyond that range would have to be done with caution. Such extrapolation to lower temperature shows tentatively that at 1 bar pressure the critical temperature, T c, of the spinel solvus is 427° C, with dTc/dP≈1.3 K/kbar. The critical composition, X c, is 0.42 , and changes barely with pressure. Substantial error in calculated phase diagrams will result if the significant positive deviation from ideality is ignored for Al-Cr mixing in such spinels.  相似文献   

19.
APL computer programs for the thermodynamic calculation of devolatilization and solid-solid equilibria operate using stored values for the molar volume and entropy of solids, the free energies of H2O and CO2, and the free energies of formation for 110 geologically-important phases. P-T-X CO 2 calculations of devolatilization equilibria can be made at pressures from 0.2 through 10 kb, and temperatures from 200 through 1,000° C. P-T-X calculations of solid-solid equilibria may be accomplished at pressures to 30 kb and temperatures to 1,000° C. Calculations can be extrapolations from experimental points, or direct calculations from thermochemical data alone. Options are available in these programs to consider effects of: real vs. ideal gas mixing, thermal expansion and compressibility, solid solution, fluid pressure differing from solid pressure, and uncertainties in high-temperature entropies.A collection of thermodynamic data programs accompanies the programs for calculating P-T-X CO 2 equilibria. Over a wide range of physical conditions, the data functions report free energies, entropies, fugacities of H2O and CO2, high temperature entropies of solids, and activities of components in H2O-CO2 mixtures.List of Symbols Activity of H2O and CO2 - Gf Free energy of formation of a phase from elements - Gr Free energy change of reaction - G r o Standard state free energy change of a reaction - Free energies of pure H2O and CO2 - H r o Standard state enthalpy change for a reaction - K Equilibrium constant - R Gas constant - S r o Standard state entropy change of reaction - S s o Standard state entropy change of solids in a reaction - Vs o Standard state volume change of a reaction - Vs o Standard state volume change of solids in a reaction - Mole fraction of H2O and CO2 - Activity coefficient of H2O and CO2  相似文献   

20.
Eclogites are found as lenses or layers in the Precambrian gneiss terrain of the Bitlis Massif in eastern Turkey. Kyanite-eclogites from the region of Gablor Hill in the Bitlis Massif exhibit relatively minor alteration, and consist of garnet, omphacite, kyanite, zoisite, calcic amphibole, phengite, rutile and quartz. In terms of geological setting and mineral compositions, Gablor eclogites are very similar to eclogites from high-grade gneiss terrains. Metamorphic conditions during the eclogite crystallisation are determined as 625±35° C and 16±3 kbars. The coexistence of white mixa, omphacite and kyanite constrains between 0.4 and 1. Primary calcic amphiboles from the Gablor eclogites exhibit conflicting textures, indicating stable coexistence with, as well as growth from omphacite and garnet. This is explained by a buffering reaction between amphibole, garnet, omphacite, zoisite and kyanite during the eclogite crystallisation, whereby is controlled and buffered by the mineral assemblage.  相似文献   

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