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1.
Phase Equilibria of Amphibolites from the Post Pond Volcanics, Mt. Cube Quadrangle, Vermont 总被引：1，自引：0，他引：1
Amphibolites of the Post Pond Volcanics, south-west corner ofthe Mt. Cube Quadrangle, Vermont, are characterized by a greatdiversity of bulk rock types that give rise to a wide varietyof low-variance mineral assemblges. Original rock types arebelieved to have been intrusive and extrusive volcanics, hydrothermallyaltered volcanics and volcanogenic sediments with or withoutadmixtures of sedimentary detritus. Metamorphism was of staurolite-kyanitegrade. Geothermometry yields a temperature of 535 ± 20°C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic,indicating a close approach to chemical equilibrium was attained.Relative enrichment of Fe/Mg is garnet > staurolite >gedrite > anthophyllite cummingtonite hornblende > biotite> chlorite > wonesite > cordierite dolomite > talc;relative enrichment in Mn/Mg is garnet > dolomite > gedrite> staurolite cummingtonite > hornblende > anthophyllite> cordierite > biotite > wonesite > chlorite >talc. between coexisting amphiboles varies as a function ofbulk Fe/Mg, which is inconsistent with an ideal molecular solutionmodel for amphiboles. Mineral assemblages are conveniently divided into carbonate+ hornblende-bearing, hornblende-bearing (carbonate-absent)and hornblende-absent. The carbonate-bearing assemblages allcontain hornblende + dolomite+ calcite + plagioclase (andesineand/or anorthite) + quartz with the additional phases garnetand epidote (in Fe-rich rocks) and chlorite ± cummingtonite(in magnesian rocks). Carbonate-bearing assemblages are restrictedto the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocksof lower CaO content than the carbonate-bearing assemblages.All of these assemblages contain hornblende + andesine ±quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenitein Fe-rich rocks). In rocks of low Al content, cummingtoniteand two orthoamphiboles (gedrite and anthophyllite) are common.In addition, garnet is found in Fe-rich rocks and chlorite isfound in Mg-rich rocks. Several samples were found that containhornblende + cummingtonite + gedrite + anthophyllite ±garnet +chlorite + andesine + quartz + Fe-Ti oxide ±biotite. Aluminous assemblages contain hornblende + staurolite+ garnet ± anorthite/bytownite (coexisting with andesine)± gedrite ± biotite ± chlorite ±andesine ± quartz ± ilmenite. Hornblende-absentassemblages are restricted to Mg-rich, Ca-poor bulk compositions.These rocks contain chlorite ± cordierite ± staurolite± talc ± gedrite ± anthophyllite ±cummingtonite ± garnet ± biotite ± rutile± quartz ± andesine. The actual assemblage observeddepends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a firstapproximation, by the model system SiO2–Al2O3–MgO–FeO–CaO–Na2O–H2O–CO2;graphical representation is thus achieved by projection fromquartz, andesine, H2O and CO2 into the tetrahedron Fe–Ca–Mg–Al.The volumes defined by compositions of coexisting phases filla large portion of this tetrahedron. In general, the distributionof these phase volumes is quite regular, although in detailthere are a large number of phase volumes that overlap otherphase volumes, especially with respect to Fe/Mg ratios. Algebraicand graphical analysis of numerous different assemblages indicatethat every one of the phase volumes should shift to more magnesiancompositions with decreasing µH2O. It is therefore suggestedthat the overlapping phase volumes are the result of differentassemblages having crystallized in equilibrium with differentvalues of µH2O or µCO2 and that the different valuesmay have been inherited from the original H2O and CO2 contentof the volcanic prototype. If true, this implies that eithera fluid phase was not present during metamorphism, or that fluidflow between rocks was very restricted. 相似文献
2.
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
3.
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
4.
The Wabush Iron Formation, of late Precambrian (Proterozoic)age is part of the Labrador Trough in southwestern Labrador,Canada. It is the regionally metamorphosed equivalent of lowgrade metamorphic (chlorite zone) iron-rich sediments of thecentral part of the Labrador Trough. The metamorphic grade iskyanite-staurolite zone, as concluded from conformably underlyingpelitic schist assemblages. Sedimentary textural features suchas very pronounced banding and a very rare occurrence of relicgranules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite,cummingtonite-grunerite, and ferrodolomite-ankerite. Less commonare actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite,ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite,garnet (almandine, spessartine, calderite), siderite, rhodochrosite,calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyseshave been made of thirty-four phases belonging to the abovelist. Six additional electron probe analyses have been madeof phases from the underlying pelitic schists. All conventionallyanalyzed phases are characterized by complete optical, unitcell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonateiron formation include grunerite-ferrosalite, grunerite-eulite-siderite,grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine,rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite,magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite,and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages.Of the volatile components, O2, CO2, and H2O, O2, is concludedto have behaved as an inert (buffered) component. Variationsin the activity of CO2 are concluded to have existed betweensilicate-oxide and carbonate-oxide members of the iron formation.It is not clear, however, whether CO2 has acted as a perfectlymobile component with strong aco2 gradients throughout the area,or as an inert component in some parts of the area. H2O is consideredto have been perfectly mobile. An increase in Mg/(Mg+Fe) ratioin ferromagnesian silicates is correlated with an increase inthe oxidation state of the assemblage. A similar increase in(Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicateswith increasing activity of O2. A number of ferromagnesian silicatescontain large amounts of Na+ and Fe3+ as a result of the verylow Al2O3 content of the iron formation. The P and T conditionsof metamorphism are deduced from experimental studies applicableto the underlying pelitic schists. 相似文献
5.
The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO2 and periclase–wüstite solid solutions, (MgO · FeO)ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO2 react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO2–CaSiO3 system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO3 (bridgmanite) + L = SiO2 + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO3 (bridgmanite) + (MgO · FeO)ss (ferropericlase), and basic minerals, (FeO · MgO)ss (magnesiowüstite) + SiO2 (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions. 相似文献
6.
Lin-gun Liu 《Contributions to Mineralogy and Petrology》2002,144(1):16-21
Diamonds containing ferropericlase (Mg,Fe)O and other silicate (enstatite [(Mg,Fe)SiO3], in particular) assemblages are generally believed to be derived from the Earth's lower mantle. On the basis of the observed ratio between ferropericlase and enstatite inclusions and the FeO content of these ferropericlases, it is concluded that most of these minerals entrapped in diamonds may not represent the lithology of the lower mantle itself as has been suggested by many investigators. Instead, ferropericlases in these diamonds represent most likely the disproportionate product of ferromagnesite [(Mg,Fe)CO3], which underwent a decarbonation reaction to form both diamond and ferropericlase simultaneously in the lower mantle. The wide variation in the Mg# of ferropericlase inclusions in diamonds is attributed to the decarbonation "loop" of the MgCO3-FeCO3 solid solutions. Some of the enstatite inclusions coexisting with these ferropericlases in the same diamond may represent the most abundant mineral species of (Mg,Fe)SiO3-perovskite in the lower mantle. The latter mineral phase experienced a retrogressive transition into enstatite during the transport of diamonds to the Earth's surface. 相似文献
7.
A unique troilite-iron-cohenite-rich lens was found on a ledgein the sediment-contaminated Kitdlit dyke on Disko. The lensdisplays a mineralogical diversity and extreme liquid evolutionnot previously described from any single magmatic sulphide ormetal liquid system. It was deposited at 1200 ?C and fo2 = 10–13as two immiscible liquids–a sulphide liquid and a C-richmetal liquid—which solidified from about 1100? to 300?C as a closed system in a thermal gradient. The lower iron-rich half of the lens formed by crystallizationof iron, cohenite, troilite, schreibersite and w?stite (in thatorder), with trace amounts of the phases found in the upperhalf of the lens. The approximate crystallization order of theupper troilite-rich half of the lens was: iron, cohenite, troilite,chromite, w?stite, fayalite (pseudomorphosed), high-T ‘chalcopyrite’,high-T ‘heazlewoodite’ (beta-(Ni, Fe, Cu, Co)3+?S2)and, finally, lead minerals. The latter comprise native lead,galena, altaite (PbTe), shandite (Ni3Pb2S2) and an unidentifiedphase. Additional immiscible liquids were formed during solidification.Oxysulphide liquid (roughly FeO with a minor FeS-component)exsolved from metal and sulphide liquid in equilibrium withiron and cohenite below 1100 ?C. Later, FeO-rich silicate liquid,and Pb-rich liquid with Ni, Cu, S and Te, exsolved from thesulphide liquid. Segregation of oxysulphide and Pb-rich liquidmay occur during core formation in planetoids, and there arethus important cosmochemical implications. 相似文献
8.
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. 相似文献
9.
O18/O16 and C12/C13 ratios have been determined for carbonaterocks and coexisting minerals from two contact metamorphic aureolesat Birch Creek, California and Marble Canyon, Texas. The peliticmetasediments and granitic intrusions of the Birth Creek localitywere also analyzed for their O18/16 and D/H ratios. Oxygen andcarbon isotope fractionations in coexisting dolomite and calciteare interrelated but show no obvious correlation with sampledistance from the intrusive contact. Small-scale oxygen isotopicexchange effects between rock units are observed within a fewfeet of the intrusive-country rock contacts and the marble-schistcontacts at Birch Creek. Oxygen isotopic temperatures obtainedfrom quartz—biotite fractionations in the biotite schistsof Birch Creek show a systematic decrease with increasing distanceaway from the intrusive contact; the isotopic contact temperatureis calculated to be 535–45°C. Low O18/16 and C13/12ratiosof the contact metamorphic marbles generally correlate wellwith presence of calc-silicate minerals, indicating that theCO2 liberated during metamorphic decarbonation reactions isenriched in both O18and C12 relative to the carbonates. Materialbalance calculations indicate that the liberated CO2 is about5 per mille richer in O18 and about 6 per mille richer in C13than coexisting calcite. 相似文献
10.
Yu. A. Litvin A. V. Spivak D. A. Simonova L. S. Dubrovinsky 《Doklady Earth Sciences》2017,473(2):444-448
Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO2–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied. 相似文献
11.
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
12.
MARKL GREGOR; ABART RAINER; VENNEMANN TORSTEN; SOMMER HOLGER 《Journal of Petrology》2003,44(6):1097-1120
In Central Dronning Maud Land, East Antarctica, rare metre-sizedlenses of spinel peridotite are enclosed in high-grade metamorphicrocks. The rocks experienced a medium-P granulite-facies metamorphismat 575 Ma and a low-P amphibolite-facies overprint at 530 Ma.The latter is probably related to extensive granitoid magmatismbetween 530 and 500 Ma, which produced large volumes (abouthalf of the outcrops today) of granitic to syenitic rocks aswell as abundant K-feldspar–quartz pegmatites. One ofthe spinel peridotite lenses in the Schirmacher Oasis of CentralDronning Maud Land is crosscut by several small (up to 10 cmwide) veins with a characteristic zoned sequence of mineralassemblages, which was formed by reaction of a hydrous, SiO2-saturatedfluid or pegmatitic melt with the peridotite. The zoned sequenceconsists of the following mineral assemblages (from the centreof the vein towards the outer margin): zone 0, plagioclase +quartz; zone 1, green biotite intergrown with zircon + clinoamphibole;zone 2, cummingtonite + dark brown biotite intergrown with rutile+ clinoamphibole; zone 3, cummingtonite + light brown biotite+ spinel; zone 4, olivine + orthopyroxene + spinel ±clinopyroxene (unaltered peridotite). This sequence was investigatedwith respect to its conditions of formation, modal mineralogy,mineral chemistry, fluid inclusions, and oxygen and hydrogenisotope compositions of selected minerals. Based on the stabilityof cummingtonite and on equilibrium calculations in the MgO–SiO2–H2Osystem and on quartz–biotite oxygen isotope thermometry,the reaction vein formed at 650°C, which is in accord withtypical pegmatite crystallization temperatures. The pegmatiteof zone 0 is interpreted to have formed in an open fissure whereas,on textural grounds, zone 3 replaces former peridotite. On thebasis of mass balance constraints, the boundary between zones1 and 2 is interpreted to approximately represent the formerboundary between peridotite and the open fissure before reaction.Oxygen isotope systematics show that the infiltrating fluidhad an isotopic composition of 9–10 SMOW. All mineralsof the reaction vein with the exception of the inherited spineland olivine in the adjacent peridotite are in equilibrium withsuch a fluid. Spinel in the peridotite is depleted in 18O comparedwith coexisting olivine, which suggests isotopic disequilibrium.Spinel in zone 3 has a distinctly different isotopic compositioncompared with that in the peridotite, apparently approachingbut not reaching equilibrium. The combination of mineral chemistryand mass balance constraints of the modal mineralogy constrainsthe volume change during metasomatism and the direction of elementaldiffusion. It is indicated that Mg, Cr and Ni always diffusedtowards the vein, whereas Si, Al, K, Na, H2O and possibly Fediffused into the peridotite. KEY WORDS: peridotite; metasomatism; pegmatite; diffusion; reaction 相似文献
13.
F. Kaminsky O. Zakharchenko R. Davies W. Griffin G. Khachatryan-Blinova A. Shiryaev 《Contributions to Mineralogy and Petrology》2001,140(6):734-753
Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO2. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr2O3<36.5 wt%, and TiO2>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)2Si2O4. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('13C=-7.8 to -2.5), except for the one majorite-bearing diamond ('13C=-11.4). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered. 相似文献
14.
Regional Variation and Petrogenesis of Basalt Glasses from the FAMOUS Area, Mid-Atlantic Ridge 总被引：1，自引：1，他引：1
Basalt glasses and minerals from precisely located dredge andsubmersible stations in the FAMOUS area have been analysed byelectron microprobe, and their regional relationships and petrographicfeatures are described. Samples from the median valley southof fracture zone B, and from the walls of the valleys northand south of fracture zone B, tend to show higher K2O, TiO2,and FeO/MgO than samples from the active volcanic centers inthe northern median valley. Some fracture zone samples are highin K2O relative to median valley basalts. All glasses definean apparent binary cotectic in the normative plagioclase-pyroxene-olivineternary, suggesting compositional control by low-pressure crystal-meltequilibria. Correlations of FeO/MgO between pyroxene and glassand olivine and glass suggest that most crystals are in equilibriumwith their liquids; a few xenocrysts depart significantly fromequilibrium. Plagioclase compositions are much less well correlatedwith liquid composition. 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.
An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa 总被引：10，自引：2，他引：10
Crystallization experiments were performed at 200 MPa in thetemperature range 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn3O4 buffers to assess the role of water andfO2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·35–4·7wt % H2O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer).Residual liquids obtained at low fO2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high fO2 (MnO–Mn3O4buffer) results in a decrease of melt FeO*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H2Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO2 and K2O, but have Ca/Na ratios and FeO* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite 相似文献
17.
Toru Sugawara 《Contributions to Mineralogy and Petrology》2001,141(6):659-686
A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO2-Al2O3-Fe2O3-FeO-MgO-CaO-Na2O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe3+,Al3+)Si2O8-Ca(Fe2+,Mg)Si3O8 system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeOt, MgO content and Mg/(Fet+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ǂ.34 wt% (relative error =9%) and ǂ.08 wt% (14%) and ǂ.7 (8%) respectively. Calculated Fe3+-Al exchange chemical potentials of plagioclase, mFe3 + ( Al )- 1 Pl{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} agree with those calculated using reported thermodynamic models for multicomponent spinel, mFe3 + ( Al )- 1 Sp{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} and clinopyroxene, mFe3 + ( Al )- 1 Cpx{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} . The FeOt content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe3+/(Fe3++Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase-liquid oxygen barometer are scattered, this study found that plagioclase-spinel-clinopyroxene-oxygen and plagioclase-olivine-oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and fO2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies. 相似文献
18.
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 860o?15oC. 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 ao changes from 8–391?0.002 A (magnetic,Fe+2Fe2+2O4OJ to 8.150?0.004 (hercynite, Fe+2Al2O4)by ao?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. 相似文献
19.
A. Zeh 《Journal of Metamorphic Geology》2001,19(4):329-350
Generally, P–T pseudosections for reduced compositional systems, such as K2O–FeO–MgO–Al2O3–SiO2–H2O, Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O and MnO–K2O–FeO–MgO–Al2O3–SiO2–H2O, are well suited for inferring detailed P–T paths, comparing mineral assemblages observed in natural rocks with those calculated. Examples are provided by P–T paths inferred for four metapelitic samples from a 1 m2 wide outcrop of the Herbert Mountains in the Shackleton Range, Antarctica. The method works well if the bulk composition used is reconstituted from average mineral modes and mineral compositions (AMC) or when X‐ray fluorescence (XRF) data are corrected for Al2O3 and FeO. A plagioclase correction is suitable for Al2O3. Correction for FeO is dependent on additional microscopic observations, e.g. the kind and amount of opaque minerals. In some cases, all iron can be treated as FeOtot, whereas in others a magnetite or hematite correction yields much better results. Comparison between calculated and observed mineral modes and mineral compositions shows that the AMC bulk composition is best suited to the interpretation of rock textures using P–T pseudosections, whereas corrected XRF data yield good results only when the investigated sample has few opaque minerals. The results indicate that metapelitic rocks from the Herbert Mountains of the Northern Shackleton Range underwent a prograde P–T evolution from about 600 °C/5.5 kbar to 660 °C/7 kbar, followed by nearly adiabatic cooling to about 600 °C at 4.5 kbar. 相似文献
20.
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