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1.
Pyroxenes of the Bushveld Intrusion, South Africa 总被引:1,自引:1,他引:1
New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca45.4Mg49.6Fe5.0 to Ca42.7Mg0.6Fe56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca2.8Mg85.0Fe12.2)through pigeonites to ferropigeonites (approximately Ca3Mg27Fe55).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system WoEnFs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate 相似文献
2.
Leslie C. Coleman 《Contributions to Mineralogy and Petrology》1978,66(3):221-227
Electron-microprobe analyses of coexisting Ca-rich and Ca-poor pyroxenes from rocks of the Skaergaard intrusion indicate that their compositional relationships are controlled by two types of tie-line in the pyroxene quadrilateral. Solidus tie-lines join bulk compositions of pairs of pyroxenes that crystallized contemporaneously from a melt at equilibrium. Subsolidus tie-lines join the compositions of lamellae and host materials in pyroxene exsolution intergrowths. The solidus tie-line for a pair of pyroxenes in a specimen and their subsolidus tie-lines do not coincide and the subsolidus tie-line for inverted pigeonite is further from the hedenbergite-ferrosilite join of the quadrilateral than that for augite.The orientation of solidus tie-lines within the pyroxene quadrilateral indicates that during the simultaneous crystallization of two pyroxenes from the Skaergaard magma there was similar partitioning of Mg and Fe in the two phases relative to the melt. The relationship of the subsolidus tie-lines of a pair of coexisting pyroxenes to their solidus tie-line indicates that during the formation of exsolution intergrowths, changes in the composition of the pyroxene matrix involved primarily a change in its CaMg+Fe ratio while those of the lamellae involved both a change in their CaMg+Fe ratio and in their MgFe ratio. The MgFe ratio of the augite lamellae in inverted pigeonite progressively increased with cooling while that of the Ca-poor lamellae in augite progressively decreased with cooling. 相似文献
3.
The MD dyke swarm is composed of four generations of large basictholeiite dykes which cut the entire Archaean craton of southernWest Greenland. The four successive generations (MD1, MD2, MD3a,MD3b) are characterized by their orientation and cross-cuttingrelationships and by their mineralogy, texture and progressivelyevolved tholeiitic chemistry. Rare-earth element (REE) abundancessuggest that the dykes may have a fairly complex petrogeneticevolution. The suite varies from early (MD1) heteradcumulatenorites to ophitic and sub-ophitic gabbroic and doleritic rocks(MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyricdolerites. The pyroxene chemistry parallels the geochemical evolution ofthe dykes showing an overall Fe-enrichment trend. However, theclinopyroxenes are enigmatic in that, although they occur predominantlyas part of medium and coarse-grained holocrystalline textures,they are chemically highly variable and calcium-poor, many plottingin the metastable field in the system MgSiO3 (En)-CaSiO3 (Wo)-FeSiO3(Fs). Many individual grains are extremely complex and may beregularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes containpyroxene dendrites poikilitically enclosed by plagioclase. Thedendrites vary compositionally from hypersthene bases to branchesof pigeonite and subcalcic augite and terminate in augite branchtips. The MD2 and MD3a dyke pyroxenes are the most complex.The majority of them are sub-ophitic grains, many with successivezones of orthopyroxene, pigeonite, subcalcic augite, augiteand ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichmentor Fe-depletion and apparently opposing zoning trends can occurin neighbouring grains. Even small interstitial pyroxenes showa very wide range of compositions. Morphologically unusual andcomplex clinopyroxene cylinders occur in someof the MD3a dykes. They are chemically relatively uniform andare normal tholeiitic augites. The MD3b rocks have small concentricallyzoned sub-ophitic pyroxenes which show Ca-enrichment with arelatively constant Fs component (29 to 39 mol. per cent). Themost extremely zoned grains have hypersthene cores with successivecoronas of pigeonite and subcalcic augite and have margins ofaugite or ferroaugite. The present coexistenceof such compositionally widely variable pyroxenes and the extremeand often irregular nature of their chemical zoning make thedetermination of true original coexisting pyroxene phases andthe use of a two pyroxene geothermometer very difficult andof limited significance. The presence of a wide variety of pyroxenes of apparently bothstable and metastable compositions in these holocrystallinedykes suggests that these rocks have undergone a complex andrather unusual cooling history. The principal genetic factorswhich could have influenced their crystallization are (1) supercooling,(2) the evolution of discrete interstitial liquid cells, (3)augite-pigeonite peritectic reactions and (4) plagioclase growthand delay of pyroxene nucleation during supercooling of liquidto below the basalt liquidus. 相似文献
4.
Mineralogy and Petrogenesis of the Deccan Trap Lava Flows around Mahabaleshwar, India 总被引:2,自引:0,他引:2
Electron microprobe analyses of minerals of thirteen DeccanTrap lava flows at Mahabaleshwar have been carried out in thepresent study. All of these flows have tholeiitic bulk compositionsand all, except one (represented by MB-81-17 of Mahoney et al.,1982) contain olivine, plagioclase, two pyroxenes, and Fe-Tioxide minerals. Olivine and plagioclase appear as distinct phenocrystsin all but one flow, and Ca-rich pyroxene joins as a phenocrystphase in the younger flows. Pigeonite and Fe-Ti oxide minerals(titanomagnetite and ilmenite) occur in the groundmass. Olivineoccurs as both groundmass and phenocryst phase in MB-81-17 (whichis the only flow without low-Ca pyroxene phase); in all otherflows olivine appears only as phenocryst phase. In all but one(MB-81-17) flow olivine is completely altered. MB-81-17 olivinegrains are only partly altered, and in this rock the cores ofphenocrysts are rounded and have a composition of Fo77 whereastheir euhedral rims have a composition around Fo67. The groundmassolivine grains in MB-81-17 are Fo4132. Substantial Fe-enrichmentand zoning trends are shown by the pyroxenes in individual rocks.The cores of Ca-rich pyroxene phenocrysts of some of the flowshave as much as 4 wt. per cent A12O3 and may have crystallizedat higher (crustal) pressures. Pigeonite thermometry (Ishii,1975) suggests an average of 1050?C for crystallization of thegroundmass pigeonite (eruption temperature?). Fe-Ti oxide mineralsare mostly altered in the older flows. In the younger flows,coexisting unaltered titanomagnetite and ilmenite yield maximumtemperature estimates for the crystallization of these phaseof about 1025?C and an oxygen fugacity of 1011.5 atm.The T-fo2 path followed by these flows seems to have been consistentlysomewhat lower than that defined by the 1 atm. fayalite-magnetitequartz curve. All of the lavas examined have experienced extensivefractional crystallization of olivine and some clinopyroxeneat relatively higher pressures. These lavas were saturated orclose to being saturated with olivine+plagioclase+clinopyroxeneduring eruption. Plagioclase accumulation, although it appearsto have occurred, has not been significant. It is suggestedthat MB-81-17 magma was contaminated by a calcite-rich rock(limestone?) whereas the lower Group 1 magmas may have beenselectively contaminated by quartz-bearing contaminant. Alternately,parental magma of MB-81-1 (with the highest Mg-number and 8= -16) may have been produced in the upper mantle into whichminor masses of old crust was well mixed. Magma mixing, crystalfractionation, and contamination processes of Mahabaleshwarbasalts and possible genetic relationships with other DeccanTrap lavas are discussed. 相似文献
5.
The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 05 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900l550?C subsolidus range. The drymonzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H2O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.53 wt.% initial H2O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for OpxCpxPlg primocrysts in the dioritescovers c. 1100950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO2near NNO. A solidus temperature of c. 900?C is inferred forthis dry system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050690?C and{small tilde}900680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid. 相似文献
6.
The 160 km2 Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 56 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo86·2 to Fo0·2 with a hiatus between about Fo71and Fo61. Plagioclase ranges from An79·5 to An1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo2.4En66.8Fs30.8-Wo2·0En17·0Fs81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs67 and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo44·7En43·8Fs11·5-Wo47·0En0Fs53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo55 and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing PH2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after removalof the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material. 相似文献
7.
The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000km2 in area and 8 to 9 km thick, in the Pensacola Mountainsof Antarctica. Textures, structures, magmatic stratigraphy,and chemical variation indicate that layered gabbros and relatedrocks of this body developed by accumulation of crystals thatsettled on the floor of a magma chamber. The major cumulus phasesin the exposed part of the intrusion are plagioclase, pyroxene,and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-richand Ca-poor pyroxene coexist as cumulus phases in the lowerexposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugiteseries (Ca36.4Mg48.7Fe14.9-Ca30.0Mg23.5Fe46.5) that extendsup through the 300 m thick capping granophyre. The Ca-poor pyroxenesbelong to a bronzite-inverted pigeonite series (Ca3.5Mg69.1Fe27.4-Ca11.4Mg34.0Fe54.6)that extends only to about 200 m below the granophyre layer.In addition to the cumulus pyroxenes some rocks contain post-cumulusgreen calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphicheight is one of general iron enrichment. Superimposed on thistrend are (1) a 1 km thick section in the lower part of thebody that shows slight to no iron enrichment and (2) a markedreversal in the Fe/(Fe+Mg) ratio about 1 km below the top ofthe body. The variations from the general trend are associatedwith cyclic units and are best explained by convective overturnof the magma. In general, the pyroxene compositional trends are similar tothose of the Skaergaard and Bushveld intrusions. One significantdifference in the Dufek intrusion is the limited iron enrichmentof its Ca-rich pyroxenes, that may relate to a slower decreaseof PO2 during crystallization of the Dufek magma. 相似文献
8.
RONALD FROST B.; FROST CAROL D.; HULSEBOSCH THOMAS P.; SWAPP SUSAN M. 《Journal of Petrology》2000,41(12):1759-1776
The 2·63 Ga Louis Lake batholith, a calc-alkalic plutonexposed in Wind River Range of western Wyoming, consists ofminor diorite, quartz diorite, granodiorite, and granite. Atshallow structural levels the batholith is pyroxene free, butat deeper levels, all units of the batholith contain pyroxenes.On its northern margin the batholith was emplaced at P = 56kbar, T = 775800°C, fO2 at FMQ (fayalitemagnetitequartz)+ 1·5 to FMQ + 1·8, and aH2O 相似文献
9.
Paragenetic types of pyroxenes, selected according to mineralassociations, differ in their average contents of the majorcations. By comparing with the average composition it is seenthat both the Fe/(Mg+Fe) ratio and the Fe+2, Mn, Mg, Fe+3, andCr contents are determined in the main by the composition ofthe host rocks, but the AIz, Aly, Ca, and Na contents in pyroxenesare influenced by the conditions under which the rock was formed.The dependence of the Alz and Aly contents of orthopyroxenesand clinopyroxenes on temperature and pressure is shown withthe help of a new P-T diagram and by comparison with chemicalanalyses of natural pyroxenes. The correlation between the compound cations in pyroxenes isused to test the hypothesis of real isomorphous substitutionsin each paragenetic type of pyroxenes, and to determine rationalmethods of calculating pyroxene analyses into components. The calculated discriminant functions are useful for referring(with 510 per cent error) analyses of pyroxenes to oneof a number of paragenetic types, and in particular for distinguishingpyroxenes from magmatic and metamorphosed gabbros, and fromhigh- and low-temperature two-pyroxene granulites. 相似文献
10.
The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO2relative to the NiNiO solid buffer), between 800 and660°C and for melt H2O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckitearfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H2O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H2O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 5060°C,and that of amphibole by 30°C. Riebeckitearfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO2 lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H2O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H2O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO2 at or below the fayalitequartzmagnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria 相似文献
11.
The stability relations of mineral assemblages consisting ofvarious combinations of Capyroxene, orthopyroxene, olivine,quartz, dolomite, calcite, and graphite are treated by analyticalmeans with emphasis on reactions of the following type: MgSiO3CaCO3SiO2CaMgSi2O6CO2 (1) 3MgSiO3CaCO3 Mg2SiO4CaMgSi2O6CO2 (2) and the associated reactions involving ferrous iron. In particular,reaction (1) is discussed in detail in terms of uncertaintiesinherent in the thermochemical data and the limits placed onits equilibrium constant by the geological conditions of theoccurrence of the observed assemblages. It is shown from theequation of equilibrium of (1) that the composition of orthopyroxeneis a linear function of the carbon dioxide fugacity and thatthe latter is lowered by the presence of the FeSiO3 component.The latter conclusion is compatible with the rarity of the associationcalcite-orthopyroxcne and its apparent confinement to iron-richrocks. An analysis of the same assemblage involved in reaction (1)indicates that when graphite coexists with the iron-rich pyroxene,the fugacity of oxygen falls near the center of the magnetitefield for a wide range of CO2 fugacities and temperatures. It is shown that reaction (2) is also displaced strongly tothe right under most metamorphic conditions, which is compatiblewith fairly common occurrence of the association of Capyroxeneand olivine. Finally, a critique is presented on the possibility of the useof the pyroxene-bearing assemblages as indicators of metamorphicgrade and it is concluded that this is not rigorously possiblesince the systems almost invariably possess too many degreesof freedom. 相似文献
12.
The Red Mountain alpine peridotie forms the basal, dominantlyharzburgitic tectonite portion of an ophiolite suite in SouthIsland, New Zealand. Olivine and pyroxene Mg/Fe compositionsare constant for individual lithologies, but generally increasethrough the series harzburgite, orthopyroxenite, harzburgiticdunite, dunite. An olivine-clinopyroxene dominated transitionalperidotite along the western margin of the mass has more Fe-richsilicates than in the harzburgitic suite. Fe-Mg silicate-spinelrelationships and the distribution of Al between coexistingpyroxene and spinel indicate nearly complete post-layering equilibration.A partial re-equilibration is suggested by narrow compositionalrims on pyroxenes and spinel. Relative to the mineral cores,the rims show enhanced partitioning of Al into spinel relativeto pyroxene. The Fe-Mg relationships between silicates and spinel,and the compositional variations from cores to rims of pyroxenesand spinels indicate that the rims formed at lower temperaturesthan the mineral cores. This conclusion is supported by theapplication of several geothermometers, which give average temperaturesof equilibration and partial re-equilibration of 10001070?C and 9201030 ?C, respectively. Pyroxene overgrowthson olivine probably represent pre-equilibration cooling phenomena.Equilibration pressures cannot be estimated with precision becauseRed Mountain pyroxenes have Al contents that vary as a functionof whole-rock Al2O3, and other compositional variables, as wellas of T and P of equilibration. The lack of plagioclase in theharzburgite tectonites, and the wide range of (Al/Cr)spinelindicate equilibration at fairly high pressures, probably atdepths within the 2580 km range. The transitional peridotiteprobably formed by re-equilibration of residual crystals withbasaltic melt at shallower (<25 km) depths, and is evidencesupporting the conclusion that the ultramafic and mafic partsof the ophiolite suite at Red Mountain represent complementaryparts of the same melting event. 相似文献
13.
Using experimental results at 1·0 GPa for the systemsCaOSiO2, MgOSiO2, CaMgSi2O6SiO2 and CaMgSi2O6Mg2SiO4,and all the currently available phase equilibria and thermodynamicdata at 1 bar, we have optimized the thermodynamic propertiesof the liquid phase at 1·0 GPa. The new optimized thermodynamicparameters indicate that pressure has little effect on the topologyof the CaOSiO2, CaMgSi2O6SiO2, and CaMgSi2O6Mg2SiO4systems but a pronounced one on the MgOSiO2 binary. Themost striking change concerns passage of the MgSiO3 phase fromperitectic melting at 1 bar to eutectic melting at 1·0GPa. This transition is estimated to occur at 0·41 GPa.For the CaMgSi2O6SiO2 and CaMgSi2O6Mg2SiO4 pseudo-binaries,the size of the field clinopyroxene + liquid increases withincreasing pressure. This change is related to the shift ofthe piercing points clinopyroxene + silica + liquid (from 0·375mol fraction SiO2 at 1 bar to 0·414 at 1·0 GPa)and clinopyroxene + olivine + liquid (from 0·191 molfraction SiO2 at 1 bar to 0·331 at 1·0 GPa) thatbound the clinopyroxene + liquid field in the CaMgSi2O6·SiO2and CaMgSi2O6·Mg2SiO4 pseudo-binaries, respectively. KEY WORDS: CaOSiO2; CaMgSi2O6Mg2SiO4; CaMgSi2O6SiO2; experiments; MgOSiO2 相似文献
14.
The bronzitechromite-anorthite assemblage of the Funit(Cameron & Emerson, 1959) from the Critical Zone of theBushveld Igneous Complex, was examined with the aid of an electrolyticcell designed after Sato (1971). The resultant fO2-T data reveala last equilibration at an fO2 value of 1011·82 ±·40 atm and at a temperature of 1091 ± 35 °C.These fO2-T data when compared with: (1) a one atmosphere quenchingtechnique solidus determinationof 1110 ± 5 °C, (2) the Bushveld plagioclase compositional trends (Cameron,1970), (3) Bushveld petrofabric examinations (Cameron, 1969) (4) phase equilibria in the system CaOMgOFeOCaAl2Si2O8SiO2(Roeder & Osborn, 1966), (5) phase equilibria in the system CaAl2Si2O8NaAlSi3O8SiO2MgOFeO2H2OCO2(Eggler, 1974), all support the idea that the Eastern Bushveld magma was notappreciably differentiating in the middle Critical Zone betweenF and the L Horizons, an accumulation of nearly 220 meters. 相似文献
15.
Ca-rich and Ca-poor pyroxenes present in the Bushveld rocksof the Bethal area display well developed exsolution texturestypical of slowly cooled mafic intrusions. This gave rise topoor reproducibility in electron microprobe analyses of thesame pyroxene grain, as well as results which departed fromthe bulk composition of the original homogeneous mineral. EMMA-4was used together with the electron microprobe to establishthe composition of the constituent phases in exsolved pyroxenes.The data showed that microprobe analyses carried out with adefocused beam were equivalent to the bulk composition of thepyroxenes. Microprobe analyses obtained using a focused beamwere found to approach closely the bulk composition of pyroxenesonly when the exsolution density reached 90 lamellae per millimetre. Transmission electron microscope examination of microstructuresin ion-thinned samples of pyroxenes at 100 kV and 1000 kV showedthat the exsolution mechanism in Ca-rich and Ca-poor pyroxeneswas one of heterogeneous nucleation. Subsequent growth tookplace by means of the migration of ledges along the (100) plane.Pigeonite inversion was also shown to occur in iron-rich Ca-poorpyroxene exsolution lamellae in augite. Fractionation trends established for the Bethal pyroxenes frommicroprobe analyses indicated an overall range from Fs14En84Wo2to Fs60En31Wo9 in the Ca-poor pyroxene and Fs7En50Wo43 to Fs36En27Wo37in the Ca-rich pyroxene. Comparison of pyroxene fractionationtrends from the western, eastern and Bethal areas of the Bushveldsuggests that crystallization took place under different conditionsof pressure and temperature. 相似文献
16.
Resolving Sediment Subduction and Crustal Contamination in the Lesser Antilles Island Arc: a Combined He-O-Sr Isotope Approach 总被引:2,自引:0,他引:2
VAN SOEST MATTHIJS C.; HILTON DAVID R.; MACPHERSON COLIN G.; MATTEY DAVID P. 《Journal of Petrology》2002,43(1):143-170
We report an extensive helium isotope survey of basaltic toandesitic lavas from the Lesser Antilles island arcanarc system with well-documented evidence of crustal contamination.Given the sensitivity of helium isotopes as a tracer of theeffects of crustal additions, our aim is to evaluate the relationshipof 3He/4He ratios to other indices of contamination processessuch as oxygen and strontium isotopes. To this end, we havecarried out 53 3He/4He analyses on separated minerals (olivinesand pyroxenes) from throughout the arc, which we compare withwhole-rock strontium and phenocryst oxygen isotope measurements.We show that the three isotopic tracers show coherent patternsthroughout the Lesser Antilles, indicating a regional controlon crustal contamination. The southern section of the arc (Grenadato Martinique) shows clear evidence for major crustal contaminationin all three isotopic systems with results for our samples inthe range 3He/4He(olivine) 3·67·6RA, 相似文献
17.
M. E. Fleet 《Contributions to Mineralogy and Petrology》1974,47(3):207-214
The separate distributions for MgSiO3 and FeSiO3 in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe2+ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe2+ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO3 content but the corresponding Mg site occupancy appears largely independent of MgSiO3. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe2+ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K D with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K D between igneous and metamorphic pyroxenes. The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer. 相似文献
18.
Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation
where Gij* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. Xio = Xi/(Xi+Xj))and Xi is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (Wij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are 1416?8kJ and 84?88 J K1 mol 1 respectively. The heatcapacity is given by 114?67+17?09E-3T31?40E5T2.The Wij data are: Opx: W12 = W21 = 25 W13 = (13?10-015T),W31 = (3?370?005T), W23 = 20, W32 = 16, W24 = 5, W42= 7, W34 = 15, W43 = 15; Cpx: W12 = (25?484+0?0812P), W21 =(31?2160?0061P),W31 = W13 = 0W14 = (93?30?045T), W41 = (20?0+0?028T),W23 = 24, W32 = 15, W24 = 12, W42 = 12, W34 = (16?941+0?00592P),W43 = (20?6970?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed. 相似文献
19.
Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca6Fe82Mg12; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca7Fe72Mg21), ferroaugite, fayalite (near Fo9), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca24Fe58Mg18.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca2Fe78Mg20) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca2Fe79M19) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases. 相似文献
20.
The Plagioclase-Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites 总被引:4,自引:1,他引:3
Intermediate-composition plagioclase (An4060) is typicallyless dense than the relatively evolved basaltic magmas fromwhich it crystallizes and the crystallization of plagioclaseproduces a dense residual liquid, thus plagioclase should havea tendency to float in these magmatic systems. There is, however,little direct evidence for plagioclase flotation cumulates eitherin layered intrusions or in Proterozoic anorthosite complexes.The layered series of the Poe Mountain anorthosite, southeastWyoming, contains numerous anorthositeleucogabbro blocksthat constrain density relations during differentiation. Allblocks are more mafic than their hosting anorthositic cumulates,their plagioclase compositions are more calcic, and each blockis in strong Sr isotopic disequilibrium with its host cumulate.Associated structuresdisrupted and deformed layeringindicatethat (1) a floor was present during crystallization and thatplagioclase was accumulating and/or crystallizing on the floor,(2) compositional layering and plagioclase lamination formeddirectly at the magmacrystal pile interface, and (3)the upper portions of the crystal pile contained significantamounts of interstitial melt. Liquid densities are calculatedfor proposed high-Al olivine gabbroic parental magmas and Fe-enrichedferrodioritic and monzodioritic residual magmas of the anorthositestaking into account pressure, oxygen fugacity, P2O5, estimatedvolatile contents, and variable temperatures of crystallization.For all reasonable conditions, calculated block densities aregreater than those of the associated melt. The liquid densities,however, are greater than those for An4060 plagioclase,which cannot have settled to the floor. Plagioclase must eitherhave been carried to the floor in relatively dense packets ofcooled liquid plus crystals or have crystallized in situ. Asloping floor, possibly produced by diapiric ascent of relativelylight plagioclase-rich cumulates, is required to allow for drainingand removal of the dense interstitial liquid produced in thecrystal pile and may be a characteristic feature during thecrystallization of many Proterozoic anorthosites and layeredintrusions. KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase 相似文献