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1.
The enthalpies of solution of several synthetic garnets on the join Mg3Al2Si3O12-Ca3Al2Si3O12 (pyrope-grossular) and of several synthetic clinopyroxenes on the join CaMgSi2O6-CaAl2SiO6 (diopside-Ca-Tschermak's molecule) were measured in a melt of composition 2PbO · B2O3 at 970 K. The determinations were made with sufficient precision so that thermochemical characterizations of the solid solutions could be achieved.The pyrope-grossular solutions show positive enthalpies of mixing. The non-ideality in the range 0–30 mole % grossular is relatively the largest and is in good agreement with the predictions of Ganguly and Kennedy (1974) based largely on cation partitioning of natural high grade metamorphic garnets with biotite, and with the deductions of Hensenet al. (1975) based on measurement of the compositions of synthetic pyrope-rich garnets equilibrated with anorthite, Al2SiO5 and quartz. However, the garnets show smaller excess enthalpies at higher grossular contents. This would lead to an asymmetric solvus with a critical temperature lower than predicted by the symmetrical regular solution model of Ganguly and Kennedy (1974). The composition-dependent non-ideality can be understood by simple ionic size considerations in solid substitution and is analogous to the situations for the calcite-dolomite and enstatite-diopside solvi.The heats of solution of pyropes crystallized in the range 1000–1500°C were all the same, within the precision of measurement, and thus we have found no evidence for temperature-dependent cation disordering as a possible explanation of the high entropy of pyrope, as suggested by Charluet al. (1975). Positional disorder of dodecahedral Mg is a more probable reason.The diopside-CaTs join is also non-ideal, with the larger positive enthalpy deviations near the diopside end. The calorimetric data in the magnesian range are consistemt with the model for completely disordered tetrahedral Si and Al which results from the free energy derivations of wood (1975) based on syntheses of diopside-rich aluminous pyroxenes in the presence of anorthite and quartz. At higher Al concentrations the calorimetric data seem more consistent with the ‘local charge-balance’ model of Wood (1975).No evidence for temperature-dependent disorder was found for either the diopside or CaTs end-members.  相似文献   

2.
The heats of solution of synthetic anhydrous Mg-cordierite and of its high-pressure breakdown assemblage sapphirine + quartz (+ enstatite?) have been measured in a lead borate melt at 694°C. The ΔH of this reaction at this temperature and one atmosphere is 6.1±1 kilocalorie per mole of cordierite. A P-T stability diagram of cordierite relative to other synthetic phases in the system MgO-Al2O3-SiO2 was constructed which satisfies the heat of reaction data and all other reliable observations pertaining to the stability of anhydrous cordierite. The stability field of cordierite is limited by boundaries of very small dP/dT slopes. The maximum pressure of cordierite stability is about 8 kilobars. Above an invariant point near 950°C the sapphirine-bearing assemblage is the stable breakdown product of cordierite. Below 950°C the stable breakdown assemblage is enstatite + sillimanite + quartz. New heat of solution data for orthorhombic enstatite are presented which allow a calculation of the lower-temperature breakdown boundary. This calculation is in good agreement with the boundary deduced above. The calculated breakdown pressure of cordierite at 700°C is 5.6±1.5 kilobars. This is much lower than estimates of earlier workers and shows that cordierite stability is greatly restricted under very dry conditions. Heat of solution data of natural low-iron cordierite and sapphirine samples are presented. These indicate that synthetic cordierite is energetically close to natural cordierite and is therefore an adequate stability model to apply to natural occurrences but that the synthetic sapphirine prepared by the breakdown of cordierite is quite different from natural sapphirine. An estimate of the breakdown relations of cordierite relative to natural sapphirine is presented, which looks quite like the diagram of the synthetic system except that the invariant point is shifted to considerably lower temperatures. A consequence of the present work is that if conditions of metamorphism were very dry, pressures of only six to eight kilobars would have been necessary to produce the dense anhydrous assemblages equivalent to natural cordierite which are found in some ancient granulites. The subcrustal pressures considered necessary by some workers should not be regarded as established by presently available evidence.  相似文献   

3.
Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe2O3 in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al2O3–Fe2O3–SiO2–H2O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe3+ content and a high XMg= (Mg/ (Mg+Fe2+)) under these P–T conditions.  相似文献   

4.
Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al2O3 (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al2O3), aluminous orthopyroxene (up to 8.93 wt% Al2O3), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al2O3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe2+-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).  相似文献   

5.
Forty-six reversed determinations of the Al2O3content of enstatite in equilibrium with garnet were made in the P/T range 15–40 kbar/900–1,600° C in the MgO-Al2O3-SiO2 system. Starting materials were mixtures of synthetic pyrope+Al-free enstatite and pyrope+enstatite (5–12% Al2O3). Al2O3 contents in reversal run pairs closely approached common values from both the high- and low-Al sides. Most experiments were done in a piston-cylinder device using a NaCl medium; some runs at very high temperatures were made in pyrex/NaCl or pyrex/talc assemblies. The measured enstatite compositions, expressed as mole fractions of Mg2(MgAl)(AlSi3)O12(X Opy En ) were fitted by a Monte-Carlo method to the equilibrium condition: $$\begin{gathered} \Delta H_{970}^0 - 970\Delta S_{970}^0 \hfill \\ + \mathop \smallint \limits_1^P \Delta V_{970}^0 dP - \mathop \smallint \limits_{970}^T \Delta S_T^0 dT + RT\ln X_{Opy}^{En} = 0 \hfill \\ \end{gathered}$$ where the best fit parameters of ΔH, ΔS and ΔV (1 bar, 970 K) for the reaction pyrope=opy are 2,040 cal/mol, 2.12 eu and 9.55 cc/mol. In addition to the determination of Al2O3 contents of enstatite, the univariant reaction pyrope+forsterite=enstatite+spinel was reversibly located in the range 1,100–1,400°C. A “best-fit” line passes through 22, 22.5 and 25 kbar at 1,040, 1,255 and 1,415°C, respectively. Our results for the univariant reaction are in agreement with previous studies of MacGregor (1974) and Haselton (1979). However, comparison of the experimentally determined curve with thermochemical calculations suggests that there may be a small error in the tabulated ΔH f(970,1) 0 value for enstatite. A value of?8.32 rather than?8.81 kcal/mole (Charlu et al. 1975) is consistent with the present data. Application of garnet-enstatite-spinel-forsterite equilibria to natural materials is fraught with difficulties. The effects of nonternary components are poorly understood, and the low solubilities of Al2O3 in enstatite under most geologically reasonable conditions make barometric or thermometric calculations highly sensitive. More detailed studies, including reversed determinations in low-friction assemblies, are sorely needed before the effects of important diluents such as Fe, Ca and Cr can be fully understood.  相似文献   

6.
Crystallization of garnet in high-chromium restite formed under the conditions of partial melting in the spinel facies and subsequently subducted into the garnet depth facies was studied experimentally in the MgO–Al2O3–Cr2O3–SiO2 system. The crystallization of garnet and the dependence of its composition on the temperature and bulk composition of the system with low Al concentration were studied as well. Experiments in the knorringite–majorite–pyrope system with 5, 10, and 20 mol % Prp were carried out at 7 GPa. The phase associations for the starting composition of pure knorringite Mg3Cr2Si3O12 included chromiumbearing enstatite MgSiO3 (up to 3.2 wt % Cr2O3) and eskolaite Cr2O3. Addition of Al resulted in crystallization of high-chromium majoritic garnet. The portion of garnet in the samples always exceeded the concentration of pyrope in the starting composition owing to the formation of the complex majorite–knorringite–pyrope series of solid solutions. With increasing content of pyrope (from 5 to 20 mol %) and increasing temperature, the modal concentration of garnet increased significantly (from 6–12 to 22–37%). The garnet was characterized by high concentrations of the pyrope (23–80 mol %) and knorringite (22–70 mol %) components. The excess of Si (>3 f.u.) with decreasing Cr concentration provided evidence for the contribution of the majorite–knorringite trend to the variation in garnet composition. On the basis of the natural data, most of the garnets composing xenoliths of ultrabasic rocks in kimberlites and occurring as inclusions in diamonds are low-chromium; i.e., their protolith was not subjected to partial melting, at least in the spinel depth facies.  相似文献   

7.
Sapphirine, coexisting with quartz, is an indicator mineral for ultrahigh‐temperature metamorphism in aluminous rock compositions. Here a new activity‐composition model for sapphirine is combined with the internally consistent thermodynamic dataset used by THERMOCALC, for calculations primarily in K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O (KFMASH). A discrepancy between published experimentally derived FMAS grids and our calculations is understood with reference to H2O. Published FMAS grids effectively represent constant aH2O sections, thereby limiting their detailed use for the interpretation of mineral reaction textures in compositions with differing H2O. For the calculated KFMASH univariant reaction grid, sapphirine + quartz assemblages occur at P–T in excess of 6–7 kbar and 1005 °C. Sapphirine compositions and composition ranges are consistent with natural examples. However, as many univariant equilibria are typically not ‘seen’ by a specific bulk composition, the univariant reaction grid may reveal little about the detailed topology of multi‐variant equilibria, and therefore is of limited use for interpreting the P–T evolution of mineral assemblages and reaction sequences. Calculated pseudosections, which quantify bulk composition and multi‐variant equilibria, predict experimentally determined KFMASH mineral assemblages with consistent topology, and also indicate that sapphirine stabilizes at increasingly higher pressure and temperature as XMg increases. Although coexisting sapphirine and quartz can occur in relatively iron‐rich rocks if the bulk chemistry is sufficiently aluminous, the P–T window of stability shrinks with decreasing XMg. An array of mineral assemblages and mineral reaction sequences from natural sapphirine + quartz and other rocks from Enderby Land, Antarctica, are reproducible with calculated pseudosections. That consistent phase diagram calculations involving sapphirine can be performed allows for a more thorough assessment of the metamorphic evolution of high‐temperature granulite facies terranes than was previously possible. The establishment of a a‐x model for sapphirine provides the basis for expansion to larger, more geologically realistic chemical systems (e.g. involving Fe3+).  相似文献   

8.
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9.
The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al2O3–SiO2 (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe2O3, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al2O3–SiO2–O (FMASO) and FeO–MgO–Al2O3–SiO2– TiO2–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe2O3 and TiO2 on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.  相似文献   

10.
A unique sapphirine + orthopyroxene + quartz granulite from Mt. Riiser-Larsen in the Tula Mountains of Enderby Land, East Antarctica, preserves two generations of coarse and texturally equilibrated orthopyroxene and sapphirine coexisting with quartz. Initial subhedral orthopyroxene porphyroblasts retain core compositions enriched in Al2O3 (12.2 ± 0.5 wt%) compared with their rims and finer orthopyroxene (9.6 ± 0.5 wt% Al2O3) that forms granoblastic textures with sapphirine. Sapphirine and quartz also form symplectites on and along cleavage planes within orthopyroxene. These compositional and textural features are consistent with the reaction [2MgAl2SiO6=Mg2Al4 SiO10 + SiO2] leading to the formation of sapphirine + quartz at the expense of aluminous orthopyroxene. Calculations in the MAS and FMAS systems and theoretical considerations involving the phases enstatite, sapphirine, sillimanite, quartz and cordierite indicate that the reaction above progresses from left to right with decreasing temperature in the orthopyroxene + sapphirine + quartz field, at pressures of ca. 8–10 kbar. The temperature difference required to account for the ca. 2.5–3 wt% decrease in Al2O3 in orthopyroxene is at least 60–80 °C, and implies peak temperatures for the initial assemblage of at least 1120 °C if the second granoblastic assemblage equilibrated at 1040 °C, the P–T conditions required by the sapphirine + quartz association and other P–T-sensitive assemblage indicators in the Napier Complex. It is not possible to distinguish whether the two assemblages are simply related by cooling and re-equilibration or reflect a polyphase evolution involving the superposition of a second UHT event on an earlier, even higher temperature, UHT metamorphism. Preliminary thermodynamic modelling of the reaction above incorporating the observed range in orthopyroxene Al2O3 zoning indicates that present estimates for the entropy of high-temperature sapphirine are potentially too high by 15–18% compared with sapphirine entropy estimates that are consistent with MAS system experiments. The Mt. Riiser-Larsen sapphirine–quartz rocks preserve the first definitive record of regional metamorphic temperatures in excess of 1120 °C in the Napier Complex, or indeed any UHT granulite terrain worldwide. Similarly high peak temperatures may be retrieved from detailed studies of sapphirine–quartz granulites from other regions, further expanding the thermal realm of crustal metamorphism, but progress will critically depend on the experimental acquisition of new entropy data for sapphirine. Received: 3 September 1998 / Accepted: 8 November 1999  相似文献   

11.
The following equilibrium among tremolite forsterite, diopside, and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials: Ca2Mg5Si8O22(OH)2+Mg2SiO4 =2 CaMgSi2O6+5MgSiO3+H2O A significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite. For example, in nearly pure H2O with the H2 content of the fluid buffered by nickel-bunsenite at one kilobar (108 pascals), the breakdown of the assemblage with synthetic amphibole occurs at 708±20° C. The breakdown of the assemblage with natural amphibole, Ca2.16Mg4.94Fe0.03Si7.92 Al0.01O22(OH)2F0.03 occurs at 841±47° C. The shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments. The reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the NB, OH (XG, COH) buffer. Analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160.92 joules/degree is used. The resulting enthalpy of reaction is 113.96±1.82 kilojoules with the natural amphibole and 104.83±0.12 kilojoules with synthetic tremolite. Deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability.  相似文献   

12.
Fly ash is a product arising from coal combustion in thermal power plants. It represents a major source of environmental pollution. It is well known by its chemical composition rich of SiO2 and Al2O3. With the aim of preserving the environment against this contamination, fly ash was used along with the starting materials for producing glass cordierite (2MgO, 2Al2O3, 5SiO2). Four formulations were developed by mixing the silica gel, magnesium chloride (MgCl2.6H2O) and fly ash in the percentages enclosing the stoichiometry of cordierite (2MgO, 2Al2O3, 5SiO2). Different experimental techniques (DTA/TGA, X-ray diffraction, FTIR and SEM) were used to characterise the prepared formulations. The results shown that for all formulations, a cordierite phase was obtained at 1200 °C along with several secondary phases such as mullite, cristobalite, silicon oxide, enstatite and spinel. At 1300 °C, pure indialite (α-cordierite) was obtained along with a small amount of spinel. The four formulations sintered at 1200 °C exhibit a homogenous morphology and high porosity. The acicular-shaped indialite grains were observed in both formulations with excess of alumina and excess of magnesia.  相似文献   

13.
Enthalpies of solution of synthetic enstatite (Mg2Si2O6), forsterite (Mg2SiO4), akermanite (Ca2MgSi2O7), monticellite (CaMgSiO4), and merwinite (Ca3MgSi2O8) and their component oxides were determined in eutectic (Li, Na)BO2 at 1073 K. Resulting enthalpies of formation at 1073 are enstatite: ?8.10 ± 0.42 kcal; forsterite: ?14.23 ± 0.45 kcal; akermanite: ?42.60 ± 0.39 kcal; monticellite: ?25.05 ± 0.41 kcal; and merwinite: ?51.10 ± 0.49 kcal. The value for the synthetic monticellite of composition Mo.965Fo.035 was corrected slightly for non-stoichiometry based on experimental monticellite-forsterite phase equilibrium relations.The enthalpies of formation of enstatite and forsterite are somewhat less negative than yielded by several other solution calorimetric studies but are in good agreement with the recent Pb2B2O5 solution calorimetry of Kiselevaet al. (1979), and are in good agreement with values to be derived from reliable phase equilibrium data in the system MgO-Al2O3-SiO2. The enthalpies of formation of akermanite, monticellite and merwinite are all much less negative than values tabulated by robieet al. (1978) and helgesonet al. (1978) but are shown to be compatible with reliable phase equilibrium data for the system CaO-MgO-SiO2, whereas the tabulated values are not. Several methods of analysis yield an entropy of monticellite at 1000 K of 69.9 ± 0.2 cal/K.  相似文献   

14.
The univariant high-pressure reaction of aluminous enstatite and spinel to pyrope and forsterite in the MgO-Al2O3-SiO2 system has been determined in the temperature range 900 °–1100 °C by hydrothermal reversals in the piston-cylinder apparatus using the low-friction NaCl pressure medium. A mixture of synthetic minerals, including an enstatite with 6 wt% Al2O3, with product and reactant assemblages in nearly equal amounts, was the starting material. The equilibrium pressure of 19.3±0.3 kbar at 1000 ° C and average dP/dT slope of 8.0 bars/ ° C confirm the strong curvature of the equilibrium below 1200 ° C deduced by Obata (1976) from a theoretical study of experimental Al2O3 isopleths of enstatite in the garnet field. His prediction of an absolute minimum pressure near 18 kbar of the garnet peridotite assemblage in the ternary system is undoubtedly correct.Three reversed determinations of the equilibrium Al2O3 content of enstatite in the presence of spinel +forsterite were made at points adjacent to the univariant curve. The points are 5.5 wt% Al2O3 at 950 ° C and 20 kbar, 6.2 wt% at 1000 ° C and 20 kbar and 7.2 wt% at 1080 ° C and 20 kbar. These values are somewhat higher than given by the MacGregor (1974) isopleth set and quite close to those predicted by Fujii (1976) from experimental synthesis data at higher temperatures, using the Wood and Banno (1973) model of ideal solution of the Mg2Si2O6 and MgAl2SiO6 components in enstatite to reduce the data.All of the available spinel-field isopleth data can be systematized with the use of the ideal solution model. A value of H 0 of 9000 cal fits the reduced data well, and is in agreement with the calorimetrically determined value of 8500±1900 calories. An accurate calculation of the dP/dT slope of the univariant equilibrium at 1000 ° C based on calorimetry gives 7±2bars/ °C, also in good agreement with experiment. Thus, all of the available experimental and calorimetric data are consistent with the ideal-solution aluminous enstatite model.The dP/dT slopes of the spinel-field isopleths are too large to permit their use as an accurate geobarometric scale. They do have considerable potential as a thermometric indicator for certain natural peridotites, however. The southwestern Oregon overthrust peridotite masses of Cretaceous age have enstatite of 5.6 wt% Al2O3 with spinel of nearly 80 mole% MgAl2O4. The present reduced isopleth data directly give 930 ° C for the equilibration, assuming 12 kbar pressure. A first order correction based on ideal solution departures from the ternary system, as suggested by Stroh (1976) gives 1000 ° C. Thus, the high temperatures deduced by Medaris (1972) are confirmed. The pressure cannot be deduced independently from the pyroxene Al2O3 contents.  相似文献   

15.
Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The XMg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al2O3-SiO2-(H2O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history.  相似文献   

16.
Use of simple mixing models of orthopyroxene and garnet solid solutions enables extrapolation of experimentally determined equilibria in the MgSiO3-Al2O3 system to uninvestigated parts of pressure-temperature-composition space. Apparent discrepancies in the experimental data for simple and multicomponent systems may be explained by considering the effect of CaO and FeO on reducing pyrope activity in the garnet solid solutions. Equilibration pressures of natural garnet-orthopyroxene assemblages may be calculated, provided temperatures are known, from a combination of the experimental data on the MgSiO3-Al2O3 system and analyses of coexisting natural phases.Despite the presence of a compositional gap in the system, the solubility of enstatite in diopside coexisting with orthopyroxene can also be approximately treated by an ideal solution model. An empirical approach has been developed to take account of Fe2+ on the orthopyroxene-clinopyroxene miscibility gap in natural systems in order to calculate equilibration temperatures of 2-pyroxene assemblages. The model presented reproduces almost all of the available experimental data for multicomponent systems to within 60° C.  相似文献   

17.
Silica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al2O3‐SiO2 (FMAS) and KFMASH (+K2O + H2O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.  相似文献   

18.
Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al2O2−SiO2) under low fO2 and for various H2O/CO2 conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H2O, CO2 and H2O−CO2 (±CH4) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO2-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO2. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H2O and CO2 are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg2Al4SiO10 due to AlAl=MgSi substitutions [100(Al2O3/Al2O3+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.  相似文献   

19.
The solubility of alumina in enstatite was determined in the range of 1100–1500° C and 10–25 kbar. The alumina content in enstatite coexisting with sapphirine and quartz increases with increasing temperature and pressure, while that in enstatite coexisting with sapphirine and sillimanite or with pyrope decreases with increasing pressure and decreasing temperature. Two univariant lines, pyrope = enstatitess + sillimanite + sapphiriness and enstatitess + sillimanite =sapphiriness + quartz were confirmed. The invariant point involving these phases is metastable. The alumina content of orthopyroxene can not be used either as a pressure indicator or as a temperature indicator without taking the mineral assemblage into account.  相似文献   

20.
Osumilite, approximate composition K(Mg,Fe)2 Al5Si10O30, has been reported recently from two granulite localities. The mineral has been synthesised in a model pelitic composition at 1000 and 1100 ° C and 3.6–6.3 kb under conditions of low water and oxygen fugacity. Osumilite coexists, apparently stably, with hypersthene, cordierite and quartz (?) thus duplicating the mineral assemblage of one of the natural occurrences. Osumilite is in a divariant reaction relationship with cordierite and hypersthene i.e. osumilite ? cordierite + hypersthene + orthoclase+quartz. This reaction runs to the right with increasing pressure. Experimental data and field observations suggest that the joins osumilite-garnet and osumilite-sillimanite are not stable. It is suggested osumilite is involved in an invariant point in the system K2O-MgO-FeO-A12O3-SiO2 with the phases cordierite, hypersthene, sapphirine, spinel, orthoclase and quartz. The invariant point should occur at 1000 ± 100 °C and 7± 2kb.  相似文献   

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