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1.
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 KD =(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 Xco2 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. 相似文献
2.
The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks 总被引：33，自引：10，他引：33
Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO2 vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe2+/(Mg + Fe2+) (the Cr–Al trend); increasingFe3+, Fe2+/(Mg + Fe2+) and T iO2 at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe2+/(Mg + Fe2+); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel 相似文献
3.
Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan 总被引：2，自引：0，他引：2
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 PH2O.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. 相似文献
4.
Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction. 相似文献
5.
Mineral assemblages and textures are described from clinopyroxene-bearingmeta-syenites and related rocks from a small area in the PenninicBasement Complex of the south-east Tauern Window. Evidence from mineral textures, mineral compositions and geobarometryindicate that the clinopyroxene, a sodic salite, crystallizedas part of an equilibrium albite-epidote-amphibolite faciesparagenesis in the 35–40 Ma meso-Alpine metamorphic event.Phase relations in co-facial quartz + albite + K-feldspar +sphene-bearing meta-syenites and meta-granites are examinedusing a projection from these minerals onto the plane (A12O3+ Fe2O3)-CaO-(MgO + FeO + MnO). The projection demonstratesthat salitic clinopyroxene can only be a stable phase in suchrocks if the bulk-rock Al/Na + K ratios are low. This is confirmedby comparing the whole-rock analyses of clinopyroxene-bearingmeta-syenites with those of clinopyroxene-free meta-syenitesand meta-granites. Mineral assemblages in a variety of lithologies from the south-eastTauern Window are used to construct a generalized AKM diagramfor magnesian albite + epidote + quartz-bearing rocks of thealbite-epidote-amphibolite facies. Thermochemical calculations indicate that the meta-syeniteswere metamorphosed at temperatures close to 500 C and at a pressureof 6+2 –4 kb. Fluids in equilibrium with meta-syeniteand meta-granite mineral assemblages had XH2O values of 0–95,assuming XH2O + XCO2O= 1.0. 相似文献
6.
Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na2O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(XFe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO2 values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al2O3content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al2O3 of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al2O3 greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures. 相似文献
7.
Four pairs of associated calcic and sodic amphiboles from blueschistfacies metamorphic rocks were analyzed with the electron microprobeand studied by single-crystal X-ray diffraction techniques.Except for ranges in the ratios Mg/(Mg+Fe) and Fe3+/(Fe3++Al+Ti),the sodic amphiboles are similar in chemical composition. Theamount of calcium in the M(4)-site ranges only from 0·18to 0·21 ion per formula unit. The calcic amphiboles,in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio(0·29–0·48). Three of the calcic amphibolescontain less than 1·5 calcium ions per formula unit,indicating a significant solid solution of sodic amphibole componentsin the calcic amphibole phase. The a and b unit-cell parametersof the calcic amphiboles decrease with increased content ofthe sodic component. 相似文献
8.
Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California 总被引：1，自引：0，他引：1
The Cazadero blueschist allochthon lies within the Central MelangeBelt of the Franciscan assemblage in the northern Coast Rangeof California. Mineral compositions and assemblages of morethan 200 blueschists from Ward Creek were investigated. Theresults delineate lawsonite-, pumpellyite-, and epidote-zones.The lawsonite and pumpellyite zones are equivalent to the TypeII metabasites of Coleman & Lee (1963) and are characterizedby well-preserved igneous textures, relict augite, and pillowstructures, whereas epidote zone rocks are equivalent to theType III strongly deformed and schistose metabasites. Chlorite,phengite, aragonite, sphene, and minor quartz and albite areubiquitous. The lawsonite zone metabasites contain lawsonite ( < 3 wt.per cent Fe2O3), riebeckite-crossite, chlorite, and Ca-Na-pyroxene;some rocks have two distinct clinopyroxenes separated by a compositionalgap. The clinopyroxene of the lowest grade metabasites containsvery low Xjd. In pumpellyite zone metabasites, the most commonassemblages contain Pm + Cpx + Gl + Chl and some samples withhigher Al2O3 and/or Fe2O3 have Pm + Lw + Cpx + Chl, Actinolitejoins the above assemblage in the upper pumpellyite zone wherethe actinolite-glaucophane compositional gap is well defined.The epidote zone metabasites are characterized by the assemblagesEp + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep+ Pm + two amphiboles + Chl depending on the Fe2O3 content ofthe rock. In the upper epidote zone, winchite appears, Fe-freelawsonite is stable, pumpellyite disappears and omphacite containsvery low Ac component. Therefore, the common assemblages areEp + winchite + Lw, and Lw + Omp + winchite. With further increasein metamorphic grade, epidote becomes Al-rich and lawsoniteis no longer stable. Hence Ep + winchite + omphacite ? garnetis characteristic. Mineral assemblages and paragenetic sequences delineate threediscontinuous reactions: (1) pumpellyite-in; (2) actinolite-in;and (3) epidote-in reactions. Using the temperatures estimatedby Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes,the P–T positions of these reactions and the metamorphicgradient are located. All three metabasite zones occur withinthe aragonite stability field and are bounded by the maximumpressure curve of Ab = Jd + Qz and the maximum stabilities ofpumpellyite and lawsonite. The lawsonite zone appears to bestable at T below 200?C with a pressure range of 4–6?5kb; the pumpellyite zone between 200 and 290?C and the epidotezone above 290?C with pressure variation between 6?5 and 9 kb.The metamorphic field gradient appears to have a convex naturetowards higher pressure. A speculative model of underplatingseamounts is used to explain such feature. 相似文献
9.
Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (1?1?5 km) introducing Upper Jurassic sediments, Marlborough,New Zealand. The ultrabasic-gabbroic rocks contain lenses ofkaersutite pegmatite and sodic syenite pegmatite and are intrudedby ring dykes of titanaugite-ilmenite gabbro and lamprophyre.The margin of the intrusion is defined by a ring dyke of alkaligabbro. The plutonic rocks are cut by a swarm of hornblende-biotite-richlamprophyre dykes. Thermal metamorphism has converted the sedimentsto a hornfels ranging in grade from the albite-epidote hornfelsfacies to the upper limit of the hornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo82-74),endiopside (Ca45Mg48Fe7-Ca36Mg55Fe9), titanaugite (Ca40Mg50Fe10-Ca44Mg39Fe17),plagioclase (An73-18), and ilmenitetitaniferous magnetite, withvarious amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between end-iopside and titanaugitewith the coupled substitution Ry+z+Si(Ti+4+Fe+3)+Al+3 and asympathetic increase in CaAl2SiO6 (0?2-10?2 percent) and CaTiAl2O6(2?1-8?1 per cent) with fractionation. Endiopside shows a small,progressive Mg enrichment along a trend subparallel to the CaMgSi2O6-Mg2Si2O6boundary, and titanaugite is enriched in Ca and Fe+2+Fe+3 withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1?0–5?7 per cent TiO2)kaersutite (6?4 per cent TiO2)Fe-richhastingsite (18?0–19?1 per cent FeO as total Fe). Biotiteis high in TiO2 (6?6–7?8 per cent). Ilmenite and titaniferousmagnetite (3?5–10?6 per cent TiO2) are typically homogeneousgrains; their composition can be expressed in terms of R+2RO3:R+2O:R2+3O4. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe2O3, TiO2, MnO, and V; almost linear increase in Al2O3and late stage increase in alkalis and P2O3, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high PH2Oand fairly high, constant PO2; PH2 and PO2 increased duringthe formation of the gabbroic rocks until fracturing of thechamber roof occurred. The abundance of euhedral amphibole inthe latter injection phases suggests that amphibole accumulatedfrom a hydrous SiO2 undersaturated magma when an increase inPO2, stabilized its crystallization. Plutonic complexes similar to Blue Mountain are found withinand beneath the volcanic piles of many oceanic islands, e.g.Canaries, Reunion, and Tahiti, and those intruding thick sedimentarysequences, as at Blue Mountain, e.g. the pipe-like intrusionsof the Monteregian Hills, Quebec. 相似文献
10.
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
11.
Distribution of Ferric Iron in some Upper-Mantle Assemblages 总被引：16，自引：5，他引：11
The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using 57Fe Mssbauer spectroscopyto determine the Fe3+/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe3+ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe3+. Whole-rock Fe3+ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe3+ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe3+ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe3+ between mantle phases resultsin complicated patterns of the activities of the Fe3+ -bearingcomponents, and thus in calculated equilibrium fO2, which showlittle correlation with whole-rock Fe3+ or degree of depletion.Whether Fe3+ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose fO2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe3+ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry
*Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada 相似文献
12.
Activity diagrams in the system KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HClhave been calculated in terms of aK+/aH+ and aN+/aH+ from existingexperimental data. They show the effect of temperature, pressure,and aH2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, aH2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al2SiO5+H2O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO2+2 K+=3 K-feldspar+2 H+ muscovite+6 SiO2+3 Na+=3 Albite+K++2 H+ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H+=3 Al2SiO5+3 SiO2+3 H2O+2 K+ is favored at high temperature and pressure and low aH2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria. 相似文献
13.
Phase Relations on the Actinolite-Pargasite Join 总被引：1，自引：0，他引：1
Phase relations along the join Ca2Mg4Fe2+Si8O22 (OH)2 (Actinolite)-NaCa2Mg3?2Fe0?82+AlSi6Al2O22(OH)2 (Pargasite) have been studied at PH2O = 1 kb andthe oxygen fugacities defined by the iron-wustite(IW) buffer. Actinolite and bornblende are separated by a solvus and thefield of actinolite+hornblende+vapor is present in the regionbetween Ac85Pa15 and Ac55 Pa45 at 680 ?C. Complete miscibilityis achieved at 720 ?C. At temperatures higher than the solvusthere is a continuous solid solution series between the twoend members. The stability field of amphibole solid solutiongradually increases with increasing pargasite content in actinolite.The phase assemblages at temperatures higher than those of asolid solution series between the two end members change withincreasing pargasite content in the bulk composition as follows;Act+Cpx+Qz+V, ActHbl+Cpx+Opx+Qz+V, Hbl+Cpx+Opx+Pl+V and Hbl+Cpx+Pl+Ol+V. In comparison with the Fe-free system, the extent of the miscibilitygap between actinolite and hornblende is reduced by an increasein the Fe2+ content. The present study should provide an adequatebasis for the interpretation of actinolite-hornblende pairsin metamorphic rocks. 相似文献
14.
Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas 总被引：1，自引：0，他引：1
Sillimanite from a variety of high-grade metamorphic rocks containsfrom 0.13 to 1.82 weight per cent Fe2O3 and less than 0.1 weightper cent TiO2. The iron is trivalent and substitutes for Alonly. Ilmenite associated with the sillimanite contains no morethan 0.4 weight per cent Al2O3, SiO2, CaO, and MnO; and MgOdoes not exceed 1.6 weight per cent. It ranges in compositionfrom Ilm99Hem1 to Ilm85Hem15. A least squares fit of precision unit cell data on 10 analyzedsillimanites gives the following cell dimensions for iron-freesillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694Á and V = 331.15 Á3. The projected increase incell volume with substitution of 10 mole per cent Fe2SiO3 is1.66 per cent. A regular increase in the Fe2O3 content of sillimanite withincreasing Fe2O3 content of associated ilmenite in 15 of 21samples analyzed suggests that sillimanite and ilmenite crystallizedin equilibrium in the 15 samples. The compositions of the tensillimanite-ilmenite pairs analyzed by the author fit the followingempirical curve (sol;(XFe2O3)Il = 1.110 x 10–3. This regularincrease in Fe2O3 contents fits a model of Fe3+ substitutionfor Al on two independent sites in sillimanite and a coupledsubstitution of for Fe2+ Ti on two sites in ilmenite. Sillimaniteand ilmenite are behaving as ideal solutions over the compositionalrange 0 < XFe2SIO3 < 0.013 in sillimanite and 0 < XFe2O3< 0.15 in ilmenite. Equations have been derived for expressing the variation inFe2O3 content of sillimanite associated with quartz and ilmeniteor hematite as a function of pressure, temperature, and Fe2O3content of the oxide minerals. For example, the Fe2O3 contentof a sillimanite with 1.5 mole per cent Fe2SiO3 coexisting withTi-free hematite is calculated to decrease 11 per cent witha 5 kb increase in pressure. The rate of increase with temperatureof the Fe2O3 content of sillimanite is greater in hematite-bearingassemblages than in ilmenite-bearing assemblages. 相似文献
15.
Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO2, aq + K+ + H2Oor alternatively beryl —K—feldspar + H2O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimanite—K—feldspar, muscovite—K—feldspar—sillimanite,and annite—magnetite—spinel—sillimanite—garnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at P—T conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H+ = chrysoberyl + quartz+ Na+ + H2O or alternatively beryl + albite + H2O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H+ =chrysoberyl + quartz + sillimanite + Na+ + K+ + H2O or alternativelyberyl + albite + muscovite + H2O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H2O = chrysoberyl + phenakite + melt at P—Tconditions between the K—feldspar—quartz—H2Osolidus and the K—feldspar—albite—quartz-H2Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high P—T conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO2-poor country rocks, or of the assimilationof Al2O3-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites. 相似文献
16.
The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe2O3 (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe2O3‐saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe2+‐Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages. 相似文献
17.
Fractionation of Nb and Ta from Zr and Hf at Mantle Depths: the Role of Titanian Pargasite and Kaersutite 总被引：8，自引：4，他引：8
TIEPOLO M.; BOTTAZZI P.; FOLEY S. F.; OBERTI R.; VANNUCCI R.; ZANETTI A. 《Journal of Petrology》2001,42(1):221-232
Selective enrichment or depletion in either Zr and Hf (HFSE4+)or Nb and Ta (HFSE5+) is a feature commonly observed in manymantle-derived melts and amphiboles occurring as either disseminatedminerals in mantle xenoliths and peridotite massifs or in veinassemblages cutting these rocks. The fractionation of Nb fromZr seen in natural mantle amphiboles suggests that their incorporationis governed by different crystal-chemical mechanisms. An extensiveset of new partitioning experiments between pargasite–kaersutiteand melt under upper-mantle conditions shows that HFSE incorporationand fractionation depends on amphibole major-element compositionand the presence or absence of dehydrogenation. Multiple regressionanalysis shows that Amph/LDNb/Zr is strongly dependent on themg-number of the amphibole as a result of a combination of amphiboleand melt structure effects, so that the following generalizationsapply: (1) high-mg-number amphiboles crystallized from unmodifiedmantle melts more easily incorporate Zr relative to Nb leadingto an increase of the Nb/Zr ratio in the residual melt; (2)low-mg-number amphiboles, such as those found in veins cuttingperidotites, may strongly deplete the residual melt in Nb andcause very low Nb/Zr in residual melts. Implications and applicationsto mantle environments are discussed. KEY WORDS: trace elements; high field strength elements; partition coefficients; amphibole; upper mantle 相似文献
18.
Tourmaline has been synthesized hydrothermally at 200 MPa between 300 and 700 °C from oxide mixtures with Mg-Al ratios for the end members dravite NaMg3Al6(Si6O18)(BO3)3(OH)3(OH) and Mg-foitite &ding6F;(Mg2Al)Al6 (Si6O18)(BO3)3(OH)3(OH). Six different Na concentrations were investigated to determine the distribution of Na between tourmaline and fluid in the SiO2-saturated system Na2O-MgO-Al2O3-SiO2-B2O3-H2O-HCl. Synthetic tourmaline ranges from X-site vacant (&ding6F;) tourmaline (Mg-foitite) to nearly ideal dravite with Na=0.95 apfu. There are small, but significant, amounts of proton deficiency and negligible tetrahedral Al. Chemical variation is primarily caused by the substitutions Al&ding6F;Mg-1Na-1 and minor AlMg-1H-1. Varying amounts of Na and &ding6F; determine the Mg/Al ratios. Besides tourmaline and quartz, additional Mg-Al phases are chlorite and, at 700 °C, cordierite. Albite is also present at high Na concentrations in the bulk composition. The c dimension of the tourmaline crystals increases with Na in tourmaline. The amount of Na in the X-site depends strongly on the bulk concentration of Na in the system as well as on the temperature. These factors in turn control the phase assemblage and the composition of the fluid phase. For the assemblage tourmaline + quartz + chlorite/cordierite + fluid, a linear relationship exists between Na concentration in the fluid (quenched after the run) and tourmaline with temperature: T °C [ᆭ °C]=(Nafluid/Natur)앾.878-14.692 (r2=0.96). For the assemblage tourmaline + albite + quartz + fluid, it is: T °C [ᆣ °C]=(Nafluid/Natur)욝.813-6.231 (r2=0.95), where Nafluid is the concentration of Na+ in the final fluid (mol/l) and Natur is the number of Na cations in the X-site of tourmaline. The equations are valid in the temperature range of 500-715 °C. Our experiments demonstrate that the occupancy of the X-site in combination with the changing concentrations of Al and Mg can be used to monitor changes in the fluid composition in equilibrium with a growing tourmaline crystal. Currently, this relation can be applied qualitatively to natural tourmaline to explain zoning in Na- and Al/(Al+Mg). 相似文献
19.
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
20.
E. H. Brown 《Contributions to Mineralogy and Petrology》1977,64(2):123-136
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
- lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
- lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
- crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
- crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
- calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
- pumpellyite + chlorite + quartz = epidote + actinolite + H2O