Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O¹⁸/O¹⁰ ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at P_s 11 kb, T 550? C, and aH₂o low enough (Pe(H₂o) sufficiently lessthan P_s) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao₂ are equilibrated locally(within each sample), but all the samples have equilibratedto a common O¹⁸/O¹⁶ ratio. A possible explanation is that afluid medium had sufficient oxygen in H₂O to control the O¹⁸/O¹⁶ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao₂ of the fluid.     

Pumpellyite-Actinolite Facies Schists of the Taveyanne Formation near Lo?che, Valais, Switzerland

Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO₄(OH) for CaTiSiO₅. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A1₂O₃, TiO₂,and Na₂O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na₂O and TiO₂. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs₂ and indicating low fo₂. Ratios Mg: Fe^* (Fe^* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving K_D =(Mg/Fe^*) _actlnolIte/(Mg/Fe^*)_chlorle = 1.72. Mg/Fe^* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A1₂ in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A1₂O₃ contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe^* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe²O₃, FeO, and MgO as separate componentsand it may also be necessary to regard CO² as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow X_co2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco₂ at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe³⁺)+chlorite+quartz+H₂=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.     

Petrology of an Andalusite-Type Regional Metamorphic Terrane, Panamint Mountains, California

The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH₂O – X_Fe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H₂O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and X_H2O (assuming P_H2O +P_CO3 = P_total) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H₂O-rich. A flux of H₂O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.     

Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan

The major mineral assemblages of the metabasites of the Omoiji-Nagasawaarea in central Shikoku are hematite+epidote+chlorite+actinolite,riebeckitic actinolite+epidote+chlorite, epidote+chlorite+actinolite,and pumpellyite+epidote+chlorite+actinolite. The constituentminerals are often heterogeneous and assemblages in the fieldof a thin section sometimes do not obey the phase rule, butif grains apparently in non-equilibrium with others are excludedand domains of chemical equilibrium are appropriately chosenthe assemblages approximately obey the phase rule. The stability of hematite, pumpellyite, and epidote associatedwith chlorite and actinolite can be dealt with in terms of aternary system with appropriate excess phases. By fixing theFe²⁺/(Fe²⁺ +Mg) ratio of chlorite, it is dealt with in termsof stability relations in the system Ca₂Al₃Si₃O₁₂(OH)–Ca₂AlFe₂Si₃O₁₂(OH)with excess chlorite, actinolite, quartz, and controlled P_H2O.The maximum and minimum Fe³⁺ contents of epidote in this modelsystem are determined by hematite+epidote+chlorite+actinoliteand pumpellyite+epidote+chlorite+actinolite assemblages. Themaximum Fe³⁺ of the three phase assemblage epidote+chlorite+actinoliteis insensitive to temperature, but the minimum Fe³⁺ contentof epidote is sensitive to temperature and can be used to definethe metamorphic grade by a continuous quantity related to temperature.The phase relations expected for the model system are in goodagreement with the parageneses of the Sanbagawa terrain in centralShikoku and offer an explanation to the rule of Miyashiro &Seki (1958a) that the compositional range of epidote enlargeswith increasing temperature. The model also makes it possibleto estimate semi-quantitatively the temperature range in whichthe assemblage pumpellyite+epidote+chlorite+actinolite is stable.The possible maximum range is about 120 ?C, but the assemblageis stable in metabasite only for about 90 ?C. The higher temperaturelimit of the pumpellyite-actinolite facies defined by the disappearanceof pumpellyite in metabasite corresponds to the temperatureat which epidote with Fe³⁺/(Fe³⁺ +Al) = 0.10 0.15 coexistswith pumpellyite, actinolite, and chlorite. The compositions of epidotes in the metabasites of the Omoiji-Nagasawaarea cluster around Fe³⁺/(Fe³⁺ +Al) = 0.33. The grade of thisarea is close to the lower temperature stability limit of thepumpellyite+epidote+chlorite+actinolite assemblage.     

The effect of whole-rock MnO content on the stability of garnet in pelitic schists during metamorphism

Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H₂O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with X_Mn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (X_Mn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.     

The Solubility of Alumina in Orthopyroxene Coexisting with Garnet in FeO-MgO--Al2O3--SiO2 and CaO--FeO--MgO--Al2O3--SiO2

The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al₂O₃ in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al₂O₃—SiO₂(FMAS). These results have been extended into the CaO—FeO—MgO—Al₂O₃—SiO₂(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al₂O₃ contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters G^o_y= –5436 + 2.45T cal mol^–1,and W^M1_FeA1= –920 cal mol^–1. The volume change ofreaction, V^o, the entropy change, S^o₉₇₀ and the enthalpy changeH^o_1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term W^ga_CaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al₂O₃ contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.     

A Biotite Isograd in South-Central Maine, U.S.A.: Mineral Reactions, Fluid Transfer, and Heat Transfer

The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO₂-H₂O mixtures with X_CO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H₂O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H₂O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.     

Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

Mineral Compositions and Equilibria in the Metamorphosed Iron Formation of the Gagnon Region, Quebec, Canada

Forty-seven specimens of the Wabush Iron Formation were collectedfrom ten outcrop areas. Twenty-five specimens contain the assemblage(1), quartz+clinopyroxene+calcite with or without orthopyroxene,grunerite, magnetite, ankerite, and siderite. Five specimenscontain assemblage (2), quartz+clinopyroxene+actinolite+calcite+magnetite+hematite,and two contain assemblage (3), quartz+orthopyroxene+actinolite+magnetite+hematite.In three specimens of assemblage (1), graphite occurs in theabsence of magnetite; pyrrhotite and pyrite occur separatelyor together in specimens with assemblage (1). Thirty-nine clinopyroxenes, 38 orthopyroxenes, 18 grunerites,7 actinolites, 16 calcites, 1 ankerite, and 1 siderite wereanalyzed for iron, manganese, and calcium by X-ray emissionspectrography. Magnesium contents were estimated by assumingstoichiometric proportions. Minerals occurring with hematite show low Fe/(Fe+Mg) ratios,and those in the other assemblages show higher values with awide range of variation. In orthopyroxene, Fe/(Fe+ Mg) rangesfrom 0·17 (with hematite) to 0·77. Regularity in the distributions of Fe, Mn, and Ca between pairsof coexisting minerals shows that equilibrium was attained inmost of the rocks studied. This regularity is also accomplishedin the distribution of Mn between calcite and coexisting silicatesas well as between the silicates themselves. Small differencesin the distributions of Ca and Fe depend on both outcrop areaand mineral assemblage. Phase rule considerations suggest that the specimens with dolomite-ankeriteor magnesitesiderite do not represent equilibrium assemblages.Variations in orthopyroxene compositions in assemblages withpyrite or pyrrhotite, or both, and magnetite indicate non-equilibrationof sulfides with silicates. The presence of the oxygen buffer,magnetite+hematite, attests to the immobility of oxygen duringmetamorphism. Within each outcrop area, over which the temperature and pressureare assumed to have been uniform, variations in the compositionsof the silicates in the sub-assemblages quartz+ orthopyroxene+gruneriteand quartz+orthopyroxene+clinopyroxene+calcite indicate gradientsof µ_H2O µ_CO2 and respectively. As characterizedby the composition of orthopyroxene, both gradients are relativelylow along strike, and high across strike. The direction of gradientsacross strike is almost without reversals, which is consistentwith intergranular diffusion of H₂O and CO₂. Phase rule restrictionsfor a majority of assemblages are not in accord with the simultaneousimposition of µ_H2O and µ_CO2 gradients on the rocks,nor the formation of an H₂O-CO₂ fluid phase during metamorphism.     

Distribution of the Period Four Transition Elements among Olivine, Calcic Clinopyroxene and Mafic Silicate Liquid: Experimental Results

The distribution of Ti⁴⁺, V³⁺, Cr³⁺, Mn²⁺, Fe(total), Co²⁺ andNi²⁺ among synthetic olivine, calcic clinopyroxene and maficsilicate liquid has been studied between 1125 and 1250 ?C underanhydrous conditions at 1 bar total pressure. The distributionof iron and magnesium among the three phases was concluded tobe independent of temperature and may be described by the twoequations Titanium and vanadiumdid not enter olivine in significant amounts. The mean valueof the ratio (wt. per cent TiO₂ in Cpx)/(wt. per cent TiO₂ inL) was 0.29?0.04 for assemblages in which the liquid had botholivine and hypersthene in the norm but the ratio was greaterif the liquid was nepheline normative. Vanadium was concentratedin the pyroxene in some experiments and in the liquid in others,but it was not possible to conclude whether the change in distributionbehavior was due to varying temperature or changing liquid composition.Equilibrium partitioning of chromium was not achieved but theresults indicate that Cr₂O₃ was most strongly enriched in clinopyroxeneand showed a slight preference for olivine over the liquid.The divalent transition elements were each enriched in olivinerelative to clinopyroxene and the degree of enrichment increasedin the order predicted by crystal field theory. The mean (wt.per cent oxide in 01)/(wt. per cent oxide in Cpx) ratios were2.0 for MnO, 2.4 for FeO, 3.9 for CoO and 5.6 for NiO. Manganesewas enriched in olivine relative to the liquid and in the liquidrelative to the clinopyroxene. Cobalt and nickel were more concentratedin the crystalline phases than in the liquid but the degreeof enrichment was markedly less in the experiments in whichthe liquids were more mafic.     

Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History

Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and X_Spa, (from 0•2 to 0•05), X_Gra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with X_An increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. X_An of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H₂O.Plagioclase is consumed to provide Ca for growing garnet, andX_An, Fe/(Fe + Mg) of garnet, X_Gra, and X_Spa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and X_An increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. ^* Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

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 (X_H2O^v). 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 X_H2O^v = 0.5–0.25 (X_CO2^v= 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.     

Prehnite--Epidote Phase Relations in the Nordre Aputit?q and Kruuse Fjord Layered Gabbros, East Greenland

Layered gabbros at Nordre Aputit?q and Kruuse Fjord were emplacedduring extensional tectonism that led to the formation of theNorth Atlantic basin in the Early Tertiary. Sub-solidus reactionsbetween the gabbros and hydrothermal fluids formed superimposedalteration assemblages in fractures, cavities, and the adjacentgabbros. The earliest secondary minerals are Ca-Al amphibole+ clinopyroxene + biotite ? plagioclase that form thin veinsor porous pegmatitic masses. These minerals are crosscut, overgrownor partially replaced by one or more generations of prehniteand epidote bearing assemblages associated with filling of thefractures and cavities, and with extensive wall rock albitization. Wide variations in the partitioning of Fe³⁺ and Al between coexistingprehnite and epidote solid solutions occur in these alteredgabbros. The partitioning data define distinct clusters in termsof associated mineralogy and paragenetic relations. This, togetherwith prehnite and epidote compositions from active geothermalsystems, are used to evaluate the thermodynamic properties ofthe intercrystalline exchange reaction:When compared to thecompositions of prehnite and epidote in the Nordre Aputit?qand Kruuse Fjord intrusions, it is concluded that the latestand lowest temperature generations of prehnite and epidote displaydisequilibrium partitioning of Fe³⁺ and Al, manifested by theoccurrence of prehnite that is relatively enriched in Fe³⁺ Thermodynamic analysis of phase relations in the system Na₂O-CaO-Al₂O₃-Fe₂O₃-FeO-SiO₂-H₂O-HClis used to determine local equilibrium constraints on Fe³⁺-Alsubstitution in prehnite and epidote. It appears that parageneticand compositional relations of prehnite and epidote are sensitiveindicators of local fluctuations in fluid composition and temperature.The complex magmatic and structural history of the gabbros atNordre Aputit?q relative to Kruuse Fjord is considered to beresponsible for the differences in mineral paragenesis and compositionsof prehnite and epidote within these intrusions.     

Mineral zoning as a record of P,T history of Precambrian eclogites and schists in western Tasmania

Ferromagnesian minerals, particularly garnet but also phengite, omphacite and talc, from eclogites and surrounding schists from the Lyell Highway-Collingwood river area, western Tasmania are compositionally zoned.In rocks which have suffered little secondary alteration the Mg-value ($\text{100 Mg}\text{Mg+Fe}{}^{\mathrm{++}}$) of granets increases from core to rim, while the Mg-value of the most important coexisting ferromagnesian phases (clinopyroxene, phengite and talc in different assemblages) decreases from core to rim. CaO decreases from core to rim in garnet. MnO may show little or no variation in garnet, or decrease from core to rim.When compared with experimental data, the zoning of these minerals can be uniquely explained by growth during changing P,T conditions. The eclogites and the surrounding schists have the same prograde P,T history.When determining the K_D-values of garnet and its coexisting ferromagnesian phases it is important to consider secondary rim alterations as well as the prograde zoning of the mineral.     

Upper Mantle Amphiboles and Micas and TiO2, K2O, and P2O5 Abundances and 100 Mg/(Mg+Fe2+ Ratios of Common Basalts and Andesites: Implications for Modal Mantle Metasomatism and Undepleted Mantle Compositions

Modal mantle metasomatism, involving the re-enrichment of depletedmantle by the introduction or production of new hydrous phases,apatite and other minerals, has been proposed as a criticalprecursor to alkaline volcanism. The merits of the modal metasomatismmodel are evaluated by examining whole-rock 100 Mg/(Mg+Fe²⁺)ratios and the abundances of TiO₂, K₂O and P₂O₅ in mafic volcanicsspanning the mafic alkaline-subalkaline compositional spectrum.Upper mantle amphiboles and micas are also discussed becausethey would be major donors of Ti, Fe, and K to melts duringanatexis of either modally metasomatized depleted mantle orundepleted mantle. Compared with tholeiitic and calc-alkaline basalts and andesites,basanites and alkali basalts and alkali andesites are neitherdistinctive nor unique by virtue of persistant or well-definedhigher abundances of TiO₂, K₂O, and P₂O₅ or lower 100 Mg/(Mg+Fe₂₊)ratios, features which might reflect precursor modal metasomatismof the alkaline sources. Some basanites and alkali basalts dohave higher abundances of TiO₂, K₂O, and P₂O₅ than some tholeiitesbut these abundances may be the result of lower degrees of meltingof similar undepleted mantle sources for both magma types. The most widespread mantle phases of inferred metasomatic originare interstitial amphiboles and micas in Group I spinel peridotitexenoliths. These have high 100 Mg/(Mg+Fe) ratios ({small tilde}90) and high Cr₂O₃ and low TiO₂ abundances, and the K₂O/Na₂Oratios of the amphiboles (chromian pargasites) are low, generallyless than 0?3. Interstitial amphiboles and micas developed asa result of near-isochemical hydration reactions which largelyinvolved Cr-spinel and Cr-diopside. Their formation was probablyinduced in many instances by fluids derived from crystallizingmafic magmas. Metasomatized Group I xenoliths with interstitialhydrous phases remain depleted in TiO₂, K₂O, and P₂O₅, and theyretain the high 100 Mg/(Mg+Fe) ratios characterizing depletedGroup I xenoliths. Together with the low K₂O/Na₂O ratios, thesefeatures preclude such peridotites as suitable sources of mostalkaline (and subalkaline) volcanics. It is suggested that modalmetasomatism plays an insignificant role in the genesis of mostmantle-derived mafic volcanics. Compared with the interstitial phases, kaersutitic amphibolesand titaniferous micas from vein, Group II inclusion and megacrystupper mantle parageneses have lower 100 Mg/(Mg+Fe) ratios andCr₂O₃ contents, and much higher TiO₂ abundances. K₂O/Na₂O ratiosof the Ti-amphiboles are also much more wide-ranging (0?3 togreater than 1?0). These Fe, Ti-rich amphiboles and micas areneither widespread nor pervasive phases in metasomatized mantle.They are directly related to alkaline magmatism in the uppermantle where they may be associated with incompatible elementenrichment of peridotite wallrocks in the immediate vicinityof frozen conduits of alkaline mafic magmas. The varying K₂O/Na₂O ratios of mafic volcanics (MORB constitutea major exception) indicate that the principal K-bearing phasesin undepleted mantle are kaersutitic amphibole and titaniferousmica, in varying proportions. The former is probably the majorsource of Ti and K for low K/Na volcanics (K₂O/Na₂O < 0?5)and also many medium K/Na types (0?5 < K₂O/Na₂O < 1?0),whereas mica is more likely to be the major K-bearing phasein the source regions of high K/Na extrusives (K₂O/Na₂ >1?0). Experimental data indicate that kaersutitic amphibole,mica and apatite probably coexist in undepleted spinel- andgarnet lherzolites at pressures up to 25 kb, with mica persistingto pressures as high as 50 kb. It is proposed that undepleted asthenospheric mantle is heterogeneouswith respect to its amphibole, mica, and apatite contents (andhence TiO₂, K₂O, and P₂O₅ abundances and K₂O/Na₂O ratios), andalso with respect to 100 Mg/(Mg+Fe²⁺ ) ratios which may be significantlyless than the ratios generally assigned to undepleted mantle,namely 88–90.     

Fe--Al Oxides: Phase Relationships below 1,000?C

The subsolidus phase relationships of magnetite, hercynite,hematite, corundum, wostite, and iron are described. The phaseswere synthesized from chemical mixtures. Reactions and solidsolution between them were induced under controlled conditionsof composition, temperature, total vapor pressure, and partialpressure of oxygen. Reaction rates are slow, so that the experimentslasted from 1 to 40 days, and quenching is completely successful. A solvus was determined which limits solid solution along themagnetitc-hercynite join at temperatures below 860^o?15^oC. Compositionsof the spinel solid solutions were determined by measuring theshift of the (440) reflection, using a powder X-ray diffractometer.The calibration curve, 20 vs. composition, was made from measurementsof spinel solid solutions synthesized in the one-phase region.The cell edge a_o changes from 8–391?0.002 A (magnetic,Fe⁺²Fe₂⁺²O₄OJ to 8.150?0.004 (hercynite, Fe⁺²Al₂O₄)by a_o?8.391–0.00190x- 0.5X²10^-5 (X is mol per cent FeAl₂O₄ in solid solution). In the system Fe-Al₂-O₃-O there are five univariant assemblages: 1. Hematite-corundum+magnetite +V (vapor) 2. Corundum+magnetite+hercynite+V 3. Magnetite+hercynite+w?stite+V 4. Hercynite+wilstite+iron+V 5. Hercynite+iron+corundum+V The lines were located by determining the composition of themagnetite, hercynite, hematite, and corundum solid solutionsfor each assemblage. The diagrams provide a basis for the discussion of the paragenesisof the oxide minerals. The progressive metamorphism of lateritedeposits can be represented by (1) laterites and bauxites: hematiteH+hydratedaluminum oxides; (2) diasporites: hematite+diaspore+corundum,with magnetite as a rare accessory; (3) emery: corundum+magnetite,with hematite as an accessory. The path of these mineral changeson the diagrams shows the decrease in oxygen content of thesolids with decrease in the partial pressure of oxygen and relatesthe aluminum content of the magnetite to temperature. The occurrences of hercynite are discussed. It is a rare mineralbecause it requires unusual conditions to grow, i.e. relativelylow oxygen pressure and an extremely Fe-Al-rich environment.     

Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications

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 K_D on Fe/Mg ratio at T 1000?C. The lnK_D 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 K_D 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.     

Metamorphism of Calcareous Rocks in Three Roof Pendants in the Sierra Nevada, California

Two roof pendants in the Hope Valley area, Alpine County, containabundant calc-silicate assemblages which can be related to univariantor invariant equilibria in the CaO-Al₃O₃-SiO₂-H₂O-CO₂ system.Such assemblages are considered to represent components of reactionsthat buffered the chemistry of the pore fluid. Through dataobtained from microprobe analysis it is concluded that solidsolution in plagioclase, garnet, and clinozoisite are importantvariables such that on a TXco₂ projection each sample had aunique path during metamorphism. Differences in the plagioclasecomposition of nearby samples with assemblages related by thereaction: grossularite_(s.s)+quartz = anorthite_(s.s.)+wollastonite, suggest unique equilibration temperatures for assemblages inlocal domains. In the Twin Lakes pendant in Fresno County, thereaction: clinohumite+calcite+CO₂= 4forsterite+dolomite+H₂O, is importantin magnesian marbles. Contrasting parageneses, which are relatedby this equilibrium, are considered to reflect variations influid composition. Constrasting assemblages in calc-silicaterocks, which are linked by the reactions: calcite+quartz= wollastonite+CO₂, tremolite+calcite= dolomite+diopside+CO₂+H₂O, exist down to the scale of a thin section. Variation in Ti contentof idocrase may be an important factor in assemblages linkedby reactions involving this phase. This study suggests that during contact metamorphism of calcareousrocks in the Sierra Nevada, H₂O and CO₂ behaved as ‘initialvalue components’ (Zen, 1963) whose activities were controlledby reactions withion local systems.     

Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents

Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB₂O₃, whereas the ion microprobe Li₂O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li₂O=0–0?19 wt.%; BeO=0–0?032 wt.%;B₂O₃=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li₂O, O066 wt.% BeO, and 4?72 wt.% B₂O₃. Thesum B+Al+Fe³⁺+Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B₂O₃) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B₂O₃ contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and K^krn-BtD=(F/OH)^Krn/(F/(OH)^Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B₂O₃ content is a measureof B₂O₃ activity in associated metamorphic fluid, whereas sillimaniteB₂O₃ content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe³⁺ content.In Fe³⁺-poor kornerupines, b increases with Mg and with (Mg+ Fe²⁺) but the effect of Mg on b via the substitution ^VIMg+^IVSi=^VIAl+^IVAloverwhelms the effect of Fe²⁺=Mg substitution.     

Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.