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1.
Enthalpies of solution have been measured on a series of muscovite—paragonitemicas in 20.1% HF at 50C under isoperibolic conditions. Themolar enthalpy of formation of paragonite at 298 K, for whichno calorimetrically measured value is currently available, hasbeen determined to be –5937.5 (3) kJ. An inversion ofall calorimetric, volumetric and phase equilibrium data hasbeen performed, giving a range of mixing models compatible withmost experimental data. The following expressions of the mixingproperties of 2M1 micas for enthalpy (Hex) and volume (Vex)at pressures up to 10 kbar, forcing excess entropy (Sex) tobe zero and using a subregular mixing model are favoured: Hex(kJ) = [10.6+4.45(1–2Xms)]Xms(1–Xms) Vex(J/bar) = 0.452Xms(1–Xms). However, mixing models of higher order with asymmetric negativeSex are also possible. KEY WORDS: muscovite; paragonite; solvus; calorimetry; solid solution
*Corresponding author. 相似文献
2.
Reversed Na-K exchange data between mica and a 2 molal aqueous(Na,K)Cl fluid (Flux & Chatterjee, 1986) have been employedto model the thermodynamic mixing behaviour of muscovite-paragonitecrystalline solutions on the basis of the Redlich-Kister equation.For these binary micas, Gexm may be expressed as
where A=11222+1.389 T+0.2359 P, B=–1134+6.806 T–0.0840 P, and C=–7305+9.043 T, with T in K, P in b, Gexm, A, B, and C in joules/mol. Gmex is well constrained between 450 and 620?C, and may be extrapolatedbeyond that range with caution. The calculated solvi are skewedtoward the paragonite end member. In the range up to 15 kb,the critical temperature, Tc and the critical composition, Xcmay be expressed as a function of P by the relations:
and
with P indicated in bars. Calculated phase relations of muscovite-paragonite crystallinesolutions have been depicted in terms of the system KAlSi3O8-NaAlSi3O8-Al2O3-SiO2-H2O.These data may be applied to appropriate assemblages involvingmica, alkali feldspar, an Al2 polymorph, and quartz to estimateP, T and aH2O conditions of their equilibration. In principle,the muscovite limb of the solvus may be used to obtain geothermometricdata for coexisting muscovite-paragonite pairs, provided theequilibrium pressure is independently known. However, such applicationmust be restricted for the present to micas on the ideal muscovite-paragonitejoin. Mica-alkali feldspar-Al2SiO5-quartz or mica-plagioclase-Al2SiO5-quartzassemblages may be used to deduce aH2O in the coexisting fluid,if P, and T of equilibrium are independently known. Examplesof such geological applications are given. 相似文献
3.
The basement of Sardinia represents a nearly complete sectionof a segment of the Variscan belt that experienced a polyphasetectono-metamorphic evolution and Barrovian metamorphism. Thisbasement is well suited to investigate the relationship betweentectono-metamorphic evolution and argon isotope records in whitemica. Micaschists from the garnet zone (maximum T of up to 520–560°C)contain two texturally and chemically resolvable generationsof white mica: (1) deformed celadonite-rich flakes, defininga relict S1 foliation preserved within the main S2 foliationor enclosed in rotated albite porphyroblasts; (2) celadonite-poorwhite micas aligned along the main S2 foliation. The S1 foliationdeveloped earlier and at a deeper crustal level with respectto that at which the thermal peak was reached. From the staurolitezone (T of up to 590–625°C) to the sillimanite + K-feldsparzone, white mica is nearly uniform in composition (muscovite)and is predominantly aligned along the S2 foliation or is oflater crystallization. In situ 40Ar–39Ar laser analysesof white mica yielded ages of 相似文献
4.
Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]–1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
5.
Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation
where Gij* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. Xio = Xi/(Xi+Xj))and Xi is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (Wij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K–1 mol –1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T–2.The Wij data are: Opx: W12 = W21 = 25 W13 = (13?1–0-015T),W31 = (3?37–0?005T), W23 = 20, W32 = 16, W24 = 5, W42= 7, W34 = 15, W43 = 15; Cpx: W12 = (25?484+0?0812P), W21 =(31?216–0?0061P),W31 = W13 = 0W14 = (93?3–0?045T), W41 = (–20?0+0?028T),W23 = 24, W32 = 15, W24 = 12, W42 = 12, W34 = (16?941+0?00592P),W43 = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed. 相似文献
6.
The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (fo2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr2O3 at a log fo2= –3 to1?4 wt.% at a log fo2=–12?8. The Cr2+/Cr3+ of melt increaseswith decreasing fo2 and is estimated by assuming a constantpartitioning of Cr3+ between chromite and melt at constant temperature.The estimated values of Cr2+/Cr3+ in the melt are at fo2 valuesof 4–5 orders of magnitude lower than the equivalent Fe2+/Fe3+values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of fo2 below log fo2=–6.Five experiments at 10 kb indicate that Cr2O3 dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at fo2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al2O3 content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite. 相似文献
7.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2 = 43.7–45.7 wt.per cent, A12O3 = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe2+)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H2O,CO2, and H2. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al2O3.Conversely, lowering CaO/Al2O3 reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H2O in the vapor (XH2Ov). The data alsoshow that some CO2 may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of XH2Ov = 0.5–0.25 (XCO2v= 0.5–0.75). Such activities of H2O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite. 相似文献
8.
Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications 总被引：1，自引：0，他引：1
We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe2+. A mixture of PbO with about 55 mol per cent PbF2 provedvery effective as a flux. The experimental results do not suggest any significant dependenceof KD on Fe/Mg ratio at T 1000?C. The lnKD vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to KD and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr3+ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe2+ in garnetand about 1 wt. per cent Al2O3 in orthopyroxene. 相似文献
9.
A new thermobarometer, based on the equilibrium:
has been calibrated with experiments carried out in the piston-cylinderapparatus. Reversed equilibria were obtained using well-calibrated2.54 cm NaCl furnace assemblies and Ag80Pd20capsules with fO2bufferedat or near iron-wustite. The equilibrium is located between5.2–5.4, 6.6–6.8, and 8.6–8.8 kb at 880, 940,and 1020?C, respectively, and at 5.2 and 8.8 kb between 865–880and 1020–1030?C, respectively. X-ray refinement data indicate that the hercynite (a = 8.15546?) has approximately 18 per cent inverse character. M?ssbauerspectra reveal that 4 mol per cent of the Fe is ferric (2 percent magnetite component). Broad Mossbauer lines and a Fe2+energy level splitting of 3.7 kJ mol–1 calculated fromthe Mossbauer spectra are consistent with the X-ray determineddegree of inversion, although no separate octahedral Fe2+ spectraldoublet is resolved. Calibration of this equation allows calculation of the equilibrium:
Thermobarometers based on the above equilibria are widely applicablein granulite fades rocks and yield pressure/temperature datathat are consistent with other well-calibrated barometers andthermometers. 相似文献
10.
The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 0–5 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900–l550?C subsolidus range. The ‘dry’monzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H2O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.5–3 wt.% initial H2O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for Opx–Cpx–Plg primocrysts in the dioritescovers c. 1100–950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO2near NNO. A solidus temperature of c. 900?C is inferred forthis ‘dry’ system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050–690?C and{small tilde}900–680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid. 相似文献
11.
Experiments defining the distribution of H2O [Dw = wt % H2O(melt)/wt% H2O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H2O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H2O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H2O vapour-saturatedconditions Dw ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedDw decreases to minimum values of 2·6–5·0at moderate to low cordierite H2O contents (0·6–1·1wt %). At very low aH2O, cordierite contains less than 0·2–0·3wt % H2O and Dw increases sharply. The Dw results are consistentwith melt H2O solubility models in which aH2O is proportionalto Xw2 (where Xw is the mole fraction of H2O in eight-oxygenunit melt) at Xw 相似文献
12.
In situ eclogitic schist lenses occur in the coherent low-gradeepidote-zone Ward Creek metabasite unit of the Central Franciscanbelt. They contain almandine garnet, clinopyroxene, and rutile.They have slightly higher Mn content (0–5–1–0wt.%) than the coexisting Type III metabasites (0–12–0–25wt%) which contain epidote + glaucophane + actinolite + chlorite+ omphacite + quartz + sphene ? aragonite? lawsonite ? pumpellyite+ albite. The in situ eclogitic schists (130–140 Ma) canbe distinguished from older tectonic eclogites (150–160Ma) in Ward Creek as follows: (1) they are medium grained, whereasType IV tectonic eclogites are coarse grained; (2) they haveunaltered spessartine-rich idioblastic (0–4–10 mm)garnets, whereas Type IV tectonic eclogites have larger xenoblasticto hypidiomorphic spessartine-poor garnets which were corrodedand chloritized along the rim during retrograde metamorphism;(3) clinopyroxenes are chloromelanite in in situ eclogitic schistsbut omphacite in Type IV tectonic eclogites; (4) barroisiticamphiboles occur both as inclusions in garnets and as matrixminerals in Type IV tectonic eclogites but not in in situ eclogiticschists; (5) albite is present in in situ eclogitic schistsbut not in Type IV tectonic eclogites; and (6) the estimatedP-T condition of in situ eclogitic schists is 290 ?C < T<350 ?C, P = 8–9 kb, whereas that of Ward Creek Type IVtectonic eclogites is 500?C< r<540?C, P< 10–11–5kb. Medium-grained eclogites occur as individual blocks in WardCreek; they are different from Type IV tectonic eclogites butare very similar to in situ eclogitic schists. They have unalteredidioblastic garnet with high almandine and spessartine content(Alm47Sp23Gr20Py10), and they have chloromel-anitic clinopyroxeneand quartz but no barroisite. Paragonite is also stable in theseeclogites. The blocks formed at 380 ?C< r<400?C, and 9–5kb<P< 14 kb. They are presumably in situ eclogites formedat the highest-temperature part of the Ward Creek metabasiteunit and may be younger than Type IV tectonic eclogites. Such low-temperature occurrences of eclogitic assemblages aredue to the compositional effect on reactions between blueschistand eclogite that are insensitive to pressure and shift towardslower temperatures as bulk-rock MnO content and XFe/(Fe+Mg)increase. The Mn/(Mn + Fe) ratio of bulk rock is an importantfactor in controlling the P-T positions of these reactions attemperatures below 450 ?C, whereas the Fe/(Fe + Mg) ratio ofbulk-rock becomes important at temperatures higher than 450?C. 相似文献
13.
Carbonate scapolite is a potentially powerful mineral for calculatingCO2 activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-XCO2 equilibriainvolving meionite in the CaO-Al2O3-SiO2-CO2-H2O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The fG?m, 298 for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-XCO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (PTotal=PFluid), being stableto XCO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-XCO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state.
Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100 相似文献
14.
Contact-mctamorphic assemblages in ophicarbonate from the Bergellaureole correspond either to model isobaric invariant T-XCO2points [Atg-Cal-Di-Tr-Fo (6 samples) and Atg-Cal-Tr-Fo-Dol (2)]or to isobaric univariant T-XCO2, curves [Tr-Cal-Di-Atg (18),Tr-Dol-Atg-Cal (1), Atg-Cal-Fo-Di (1), and Atg-Cal-Tr-Fo (1)].Calcite-dolomite thermometry and mineral-fluid equilibria inthe invariant assemblages record T=440–540C at P=3•5kbar. Equilibrium metamorphic fluids were very H2O rich withX CO2,=0•001–0•027. In the invariant assemblagesTr + Fo were produced by prograde decarbonation-dehydrationreactions. In contrast, measured modes and reaction texturesin samples with univariant assemblages indicate thai Tr wasproduced by carbonation reactions. The apparent paradox of simultaneousdecarbonation reactions in the model isobaric invariant assemblagesand carbonation reactions in univariant assemblages is resolvedby local mineral-fluid equilibrium and fluid flow through ophicarbohatesin the direction of decreasing temperature as the aureole heated.Time-integrated flux (q) was computed from measured reactionprogress in 28 samples for models of both horizontal and verticaldown-temperature flow. Results are similar, with q decreasingrapidly from (0•2–5•1) 105 cm3 fluid/cm2 rock1•3–1•7 km from the intrusion to 0–0•6105cm3/cm2 at 1•8–4•0 km. The decrease in q ismore consistent with vertical than horizontal flow. Variationsin time-integrated flux of more than an order of magnitude arerecorded by samples from the same outcrop. The absence of carbonatein adjacent metaperidotite indicates that flow was confinedto the ophicarbonate. Channelized, spatially heterogeneous,vertical flow can be explained by the brecciation and strongvertical foliation of the ophicarbonate relative to surroundingmassive metaperidotite. Generation of metamorphicfluids by decarbonation-dehydrationreactions within the ophicarbonates explains larger averageflux 1–2 km from the intrusion compared with more distalpoints. KEY WORDS: Bergell; contact metamorphism; fluid flow; ophicarbonate
*Telephone: (410) 516-8121. Fax: (410) 516-7933 相似文献
15.
Thermodynamic calculations based on addition of mass balanceequations to the Gibbs Method (Spear, 1986) are used to modelthe cordierite-producing reaction in pelitic gneiss from theMcCullough Range, southern Nevada. Calculations which treatthe model paragenesis as a system open to transfer of H2O areconsistent with textural relations. Results indicate that cordieritegrew by the continuous net-transfer reaction: 0?76 BIO+1?72 SILL+3? 55 QTZ+0?27 PLG+0?005 GRT +0?06Al2R2+–1Si–1[BIO]1?02 KSP+0?76 H2O +0?30 FeMg–1[CRD]+0?15FeMg–1[BIO]+0?0005 FeMg–1[GRT] +0?005 CaNaAl–1Si–1[PLG] with decreasing P, decreasing T, and increasing aH2O The steepretrograde dP/dT path for these low-pressure granulites contrastswith isobaric cooling paths typical of higher pressure granulites,and suggests uplift and erosion were active during Proterozoicgranulite-grade metamorphism in this area. 相似文献
16.
Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia 总被引：1，自引：0，他引：1
The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A12O3-Fe2O3-SiO2-H2O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the XFe3+ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe3+ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-XFe3+ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H2O, where the XFe3+ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-XFe3+ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant. 相似文献
17.
The unmetamorphosed equivalents of the regionally metamorphosedclays and marls that make up the Alpine Liassic black shaleformation consist of illite, irregular mixed-layer illite/montmorillonite,chlorite, kaolinite, quartz, calcite, and dolomite, with accessoryfeldspars and organic material. At higher grade, in the anchizonalslates, pyrophyllite is present and is thought to have formedat the expense of kaolinite; paragonite and a mixed-layer paragonite/muscovitepresumably formed from the mixed-layer illite/montmorillonite.Anchimetamorphic illite is poorer in Fe and Mg than at the diageneticstage, having lost these elements during the formation of chlorite.Detrital feldspar has disappeared. In epimetamorphic phyllites, chloritoid and margarite appearby the reactions pyrophyllite + chlorite = chloritoid + quartz+ H2O and pyrophyllite + calcite ± paragonite = margarite+ quartz + H2O + CO2, respectively. At the epi-mesozone transition,paragonite and chloritoid seem to become incompatible in thepresence of carbonates and yield the following breakdown products:plagioclase, margarite, clinozoisite (and minor zoisite), andbiotite. The maximum distribution of margarite is at the epizone-mesozoneboundary; at higher metamorphic grade margarite is consumedby a continuous reaction producing plagioclase. Most of the observed assemblages in the anchi-and epizone canbe treated in the two subsystems MgO (or FeO)-Na2O–CaO–Al2O3–(KAl3O5–SiO2–H2O–CO2).Chemographic analyses show that the variance of assemblagesdecreases with increasing metamorphic grade. Physical conditions are estimated from calibrated mineral reactionsand other petrographic data. The composition of the fluid phasewas low in XCO2 throughout the metamorphic profile, whereasXCH4 was very high, particularly in the anchizone where aH2Owas probably as low as 0.2. P-T conditions along the metamorphicprofile are 1–2 kb/200–300 °C in the anchizone(Glarus Alps), and 5 kb/500–550 °C at the epi-mesozonetransition (Lukmanier area). Calculated geothermal gradientsdecrease from 50 °C/km in the anchimetamorphic Glarus Alpsto 30 °C/km at the epi-mesozone transition of the Lukmanierarea. 相似文献
18.
Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany 总被引：3，自引：0，他引：3
Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-fo2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An17·3Ab82·3Or0·4-An20·0Ab78·1Or1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H2O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO2 between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na2O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi3O8 component accompanied by a simultaneous increasein CaAl2Si2O8 (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H2O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO2) and sillimanite(49 mol. per cent SiO2) are Fe2O3 and TiO2 rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al2O3) occurs with corundumwhich has high Fe2O3 and TiO2 (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO2 contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An1·9Ab26·0Or72·1-An1·3Ab15·9Or82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K2O from 8·6 to less than 1 wt. per cent and oxidetotals (less H2O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An2·0Ab34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO(total iroq) and up to 10 percent normative corundum. Near liquidus biotite with higher Al2O3and TiO2 than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO2) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl2O4—66·6–45·0;MgAl2O4—53·0–18·7; Fe3O4—6·9–28·1;MnAl2O4—1·2–1·5; Fe2TiO4—0·6–6·2.Rims are generally enriched in the Fe3O4 component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe3+, Al and decreasing Ti4+, Fe2+ inilmenite, and increasing Fe2+, Ti4+ and decreasing Fe3+ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al2O3, TiO2, FeO, Fe2O3, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-fo2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. fo2 values of metamorphismand melting were between the Ni-NiO and Fe2O3-Fe3O4 buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb Pload = Pfluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers. 相似文献
19.
On the pseudobinary join CaO:3MgO:Al2O3:2SiO2:xH2O–CaO:1.25MgO:2.75 Al2O3: 0.25SiO2:xH2O clintonite mixed crystals Ca(Mg1+ xAl2 – x) (Al4 – xSixO10)(OH)2 with x rangingfrom 0.6 to 1.4 occur in the temperature range 600–830?C, 2 kb fluid pressure. On the MgSirich side clintonites coexistwith chlorite, forsterite, diopside, and calcite (due to smallamounts of CO2 in the gas phase) and, at lower temperatures,also with idocrase, hydrogrossularite, and aluminous serpentine.Decomposition of clintonite over a divariant temperature rangeoccurs above 830 ?C, 2 kb; clintonite-free subsolidus assemblagescomprising three or four solid phases are formed in the temperatureranges 890 ?–1120 ?C. The subsolidus assemblages can berepresented in a polyhedron defined by the corners forsterite,diopside, melilite, spinel, anorthite, corundum, and calciumdialuminate. Above 1120 ?C partial melting occurs. The upper thermal stability limits of three selected compositionshave been reversed in the P-T range 0.5–20 kb and 730–1050 ?C, respectively. Below some 4 kb breakdown is dueto the divariant reactions: (1)Ca(Mg2.25Al0.75)(Al2.75)(Si1.25O10)(OH)2 spinel+diopsidess+forsterite+clintonitess+vapor, (2)Ca(Mg2Al)(Al3SiO10)(OH)2 spinelx002B;melilitess+anorthite+clintonitess+vapor, (3)Ca(Mg1.75Al1.25)(Al3.25)(Si0.75O10)(OH)2 spinel+melilitess+corundum+clintonitess+vapor, At the terminations of the divariant temperature ranges (1)melilitess, (2) diopsidess, and (3) anorthite enter those assemblagesand clintonitess disappears completely. The reactions can berepresented by the following equations (1)log,H2O = 10.2879–8113/T+0.0856(P–1)/T, (2)log = 9.5852–7325/T+0.0794(P–1)/T, (3)log = 7.8358–5250/T+0.077(P–1)/T, with P expressed in bars and Tin ?K. Above 4 kb the upper thermalstability limit of clintonite is defined by incongruent melting,with grossularite participating at pressures above 9 kb. Thesecurves exhibit a very steep, probably even negative slope inthe P-T diagram. There is a close correspondence between natural clintonite-bearingassemblages and thosefound experimentally. The rarity of clintonitein nature is not due to special conditions of pressure and temperaturebut rather due to special bulk compositions of the rocks. 相似文献
20.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?7–45?7 wt. percent, Al2O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe2+)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite. 相似文献