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1.
Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-fo2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An17·3Ab82·3Or0·4-An20·0Ab78·1Or1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H2O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO2 between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na2O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi3O8 component accompanied by a simultaneous increasein CaAl2Si2O8 (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H2O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO2) and sillimanite(49 mol. per cent SiO2) are Fe2O3 and TiO2 rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al2O3) occurs with corundumwhich has high Fe2O3 and TiO2 (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO2 contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An1·9Ab26·0Or72·1-An1·3Ab15·9Or82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K2O from 8·6 to less than 1 wt. per cent and oxidetotals (less H2O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An2·0Ab34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO(total iroq) and up to 10 percent normative corundum. Near liquidus biotite with higher Al2O3and TiO2 than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO2) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl2O4—66·6–45·0;MgAl2O4—53·0–18·7; Fe3O4—6·9–28·1;MnAl2O4—1·2–1·5; Fe2TiO4—0·6–6·2.Rims are generally enriched in the Fe3O4 component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe3+, Al and decreasing Ti4+, Fe2+ inilmenite, and increasing Fe2+, Ti4+ and decreasing Fe3+ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al2O3, TiO2, FeO, Fe2O3, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-fo2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. fo2 values of metamorphismand melting were between the Ni-NiO and Fe2O3-Fe3O4 buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb Pload = Pfluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.  相似文献   

2.
Recent basaltic andesite lavas from Merapi volcano contain abundant,complexly zoned, plagioclase phenocrysts, analysed here fortheir petrographic textures, major element composition and Srisotope composition. Anorthite (An) content in individual crystalscan vary by as much as 55 mol% (An40–95) across internalresorption surfaces with a negative correlation between highAn mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analysesof zoned plagioclase phenocrysts show that the 87Sr/86Sr ratiosof individual zones range from 0·70568 to 0·70627.The upper end of this range is notably more radiogenic thanthe host basaltic andesite whole-rocks (< 0·70574).Crystal zones with the highest An content have the highest 87Sr/86Srvalues, requiring a source or melt with elevated radiogenicSr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptiverocks contain abundant xenoliths, including metamorphosed volcanoclasticsediment and carbonate country rock (calc-silicate skarns) analysedhere for petrographic textures, mineralogy, major element compositionand Sr isotope composition. The xenoliths contain extremelycalcic plagioclase (up to An100) and have whole-rock 87Sr/86Srratios of 0·70584 to 0·70786. The presence ofthese xenoliths and their mineralogy and geochemistry, coupledwith the 87Sr/86Sr ratios observed in different zones of individualphenocrysts, indicate that magma–crust interaction atMerapi is potentially more significant than previously thought,as numerous crystal cores in the phenocrysts appear to be inheritedfrom a metamorphosed sedimentary crustal source. This has potentiallysignificant consequences for geochemical mass-balance calculations,volatile saturation and flux and eruptive behaviour at Merapiand similar island arc volcanic systems elsewhere. KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate  相似文献   

3.
High-temperature–pressure experiments were carried outto determine the chlorine–hydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 – 4·6[Mg/(Mg + Fe)]hb at 2–3kbar and 800–850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 1991–1995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·2–1·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano  相似文献   

4.
A localized dehydration zone, Söndrum stone quarry, Halmstad,SW Sweden, consists of a central, 1 m wide granitic pegmatoiddyke, on either side of which extends a 2·5–3 mwide dehydration zone (650–700°C; 800 MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet)overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet).Whole-rock chemistry indicates that dehydration of the graniticgneiss was predominantly isochemical. Exceptions include [Y+ heavy rare earth elements (HREE)], Ba, Sr, and F, which aremarkedly depleted throughout the dehydration zone. Systematictrends in the silicate and fluorapatite mineral chemistry acrossthe dehydration zone include depletion in Fe, (Y + HREE), Na,K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presenceof a fluid containing CO2, NaCl, and H2O components. Water activitiesin the dehydration zone average 0·36, or XH2O = 0·25.All lines of evidence suggest that the formation of the dehydrationzone was due to advective transport of a CO2-rich fluid witha minor NaCl brine component originating from a tectonic fracture.Fluid infiltration resulted in the localized partial breakdownof biotite and amphiboles to pyroxenes releasing Ti and Ca,which were partitioned into the remaining biotite and amphibole,as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, andF. At some later stage, H2O-rich fluids (H2O activity >0·8)gave rise to localized partial melting and the probable injectionof a granitic melt into the tectonic fracture, which resultedin the biotite and amphibole recording a diffusion profile forF across the dehydration zone into the granitic gneiss as wellas a diffusion profile in Fe, Mn, and Mg for all Fe–Mgsilicate minerals within 100 cm of the pegmatoid dyke. KEY WORDS: charnockite; fluids; CO2; brines; localized dehydration; Söndrum  相似文献   

5.
The Laki eruption involved 10 fissure-opening episodes thatproduced 15·1 km3 of homogeneous quartz-tholeiite magma.This study focuses on the texture and chemistry of samples fromthe first five episodes, the most productive period of the eruption.The samples comprise pumiceous tephra clasts from early falloutdeposits and lava surface samples from fire-fountaining andcone-building activity. The fluid lava core was periodicallyexposed at the surface upon lobe breakout, and its characteristicsare preserved in glassy selvages from the lava surface. In allsamples, plagioclase is the dominant mineral phase, followedby clinopyroxene and then olivine. Samples contain <7 vol.% of euhedral phenocrysts (>100 µm) with primitivecores [An* = 100 x Ca/(Ca + Na) >70; Fo > 75; En* = 100x Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol.% of skeletal, densely distributed groundmass crystals (<100µm), which are similar in composition to phenocryst rims(tephra: An*58–67, Fo72–78, En*72–81; lava:An*49–70, Fo63–78, En57–78). Tephra and lavahave distinct vesicularity (tephra: >40 vol. %; lava: <40vol. %), groundmass crystal content (tephra: <10 vol. %;lava: 20–30 vol. %), and matrix glass composition (tephra:5·4–5·6 wt % MgO; lava: 4·3–5·0wt % MgO). Whole-rock and matrix glass compositions define atrend consistent with liquid evolution during in situ crystallizationof groundmass phases. Plagioclase–glass and olivine–glassthermometers place the formation of phenocryst cores at 10 kmdepth in a melt with 1 wt % H2O, at near-liquidus temperatures(1150°C). Phenocryst rims and groundmass crystals formedclose to the surface, at 10–40°C melt undercoolingand in an 10–20°C cooler drier magma (0–0·1wt % H2O), causing an 10 mol % drop in An content in plagioclase.The shape, internal zoning and number density of groundmasscrystals indicate that they formed under supersaturated conditions.Based on this information, we propose that degassing duringascent had a major role in rapidly undercooling the melt, promptingintensive shallow groundmass crystallization that affected themagma and lava rheology. Petrological and textural differencesbetween tephra and lava reflect variations in the rates of magmaascent and the timing of surface quenching during each eruptiveepisode. That in turn affected the time available for crystallizationand subsequent re-equilibration of the melt to surface (degassed)conditions. During the explosive phases, the rates of magmaascent were high enough to inhibit crystallization, yieldingcrystal-poor tephra. In contrast, pervasive groundmass crystallizationoccurred in the lava, increasing its yield strength and causinga thick rubbly layer to form during flow emplacement. Lava selvagescollected across the flow-field have strikingly homogeneousglass compositions, demonstrating the high thermal efficiencyof fluid lava transport. Cooling is estimated as 0·3°C/km,showing that rubbly surfaced flows can be as thermally efficientas tube-fed phoehoe lavas. KEY WORDS: lava; crystallization; basalt; cooling rate; pressure; geobarometry; P–T conditions; plagioclase; degassing; Laki, Iceland  相似文献   

6.
The 160 km2 Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 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 ‘removal’of 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.
A mantle xenolith suite from two Late Tertiary necks on SalIsland (Cape Verde Archipelago) consists of nearly equivalentamounts of anhydrous spinel-bearing lherzolites and harzburgites,in which secondary metasomatic textural domains are superimposedon the original protogranular textures. Detailed petrographicstudies, coupled with in situ major and trace element analysesof the constituent minerals and interstitial glasses, revealthe complex evolutionary history of the Cape Verde lithosphericmantle, from depletion in the garnet facies to re-equilibrationand re-enrichment in the spinel stability field. Low CaO (16·4–18·0wt %) and heavy rare earth element (HREE; Ybn = 2·4–4·8),and high Cr2O3 (1·06–1·84 wt %) contentsin the clinopyroxenes of the lherzolites can be quantitativelyaccounted for by (1) low-degree (4%) partial melting of a PrimitiveMantle-like garnet lherzolite followed by (2) partial re-equilibrationof the melting residuum from the garnet to the spinel stabilityfield. This model is further supported by thermobarometric estimates(T = 975–1210°C; P = 1·3–2·1 GPa),which cluster around the spinel–garnet boundary in theperidotite system. Secondary parageneses, regardless of theprimary lithologies, are characterized by (1) two clinopyroxenes,cpx2-O and cpx2-C, respectively related to orthopyroxene andclinopyroxene destabilization after reaction with metasomaticfluids, and (2) glasses with anomalously high, even for continentalsettings, K2O contents (up to 8·78 wt %), together withK-feldspar. Major and trace element mass balance calculationsbetween the primary and secondary parageneses suggest infiltrationof a kimberlite-like metasomatizing agent (on volatile-freebasis, MgO 17–27 wt %; K2O/Na2O 1·6–3·2molar; (K2O + Na2O)/Al2O3 1·1–3·0 molar;Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-likemetasomatism in the Cape Verde lithospheric mantle, togetherwith the presence of lherzolitic domains, partially re-equilibratedfrom the garnet to the spinel stability field, may suggest thepresence of subcontinental mantle lithosphere relicts left behindby drifting of the African Plate during the opening of the CentralAtlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites  相似文献   

8.
The spinel–garnet transition in Cr/Al-enriched peridotiticbulk compositions is known from experimental investigationsto occur at 20–70 kbar, within the pressure range sampledby kimberlites. We show that the Cr2O3–CaO compositionsof concentrate garnets from kimberlite have maximum Cr/Ca arrayscharacterized by Cr2O3/CaO 0·96–0·81, andinterpret the arrays as primary evidence of chromite–garnetcoexistence in Cr-rich harzburgitic or lherzolitic bulk compositionsderived from depth within the lithosphere. Under Cr-saturatedconditions on a known geotherm, each Cr/Ca array implicitlydelineates an isobar inside a garnet Cr2O3–CaO diagram.This simplification invites a graphical approach to calibratean empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite–garnetharzburgite xenoliths from the Roberts Victor kimberlite tightlybracket a graphite–diamond constraint (GDC) located atCr2O3 = 0·94CaO + 5·0 (wt %), representing a pivotalcalibration corresponding to 43 kbar on a 38 mW/m2 conductivegeotherm. Additional calibration points are established at 14,17·4 and 59·1 kbar by judiciously projecting garnetcompositions from simple-system experiments onto the same geotherm.The garnet Cr/Ca barometer is then simply formulated as follows(in wt %):
if Cr2O3 0·94CaO + 5, then P38 (kbar) = 26·9+ 3·22Cr2O3 – 3·03CaO, or
if Cr2O3 <0·94CaO + 5, then P38 (kbar) = 9·2+ 36[(Cr2O3+ 1·6)/(CaO + 7·02)].
A small correction to P38 values, applicable for 35–48mW/m2 conductive geotherms, is derived empirically by requiringconventional thermobarometry results and garnet concentratecompositions to be consistent with the presence of diamondsin the Kyle Lake kimberlite and their absence in the Zero kimberlite.We discuss application of the P38 barometer to estimate (1)real pressures in the special case where chromite–garnetcoexistence is known, (2) minimum pressures in the general casewhere Cr saturation is unknown, and (3) the maximum depth ofdepleted lithospheres, particularly those underlying Archaeancratons. A comparison with the PCr barometer of Ryan et al.(1996, Journal of Geophysical Research 101, 5611–5625)shows agreement with P38 at 55 ± 2 kbar, and 6–12%higher PCr values at lower P38. Because the PCr formulationsystematically overestimates the 43 kbar value of the GDC by2–6 kbar, we conclude that the empirical Cr/Ca-in-garnetbarometer is preferred for all situations where conductive geothermsintersect the graphite–diamond equilibrium. KEY WORDS: Cr-pyrope; chromite; P38 barometer; mantle petrology; lithosphere thickness  相似文献   

9.
Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (XMg =0·83) to rim (XMg = 0·6–0·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py23–27, Alm41–43,Grs29–33) has two different compositional domains, onerelatively rich in Mg (Py27–30) and the other rich inCa (Grs35–38) with a low Mg (Py20–25) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs40–57,Py2–7, Alm46–51). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached P–Tconditions of 550–583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps  相似文献   

10.
Low-Pressure Experimental Constraints on the Evolution of Komatiites   总被引：1，自引：0，他引：1  
THY  P. 《Journal of Petrology》1995,36(6):1529-1548
Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo90 to Fo74 and displaysan average KFe/MgD (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe2+) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average KFe/MgD (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An82 and An72 Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo94 olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario  相似文献   

11.
The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al2O3 in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al2O3—SiO2(FMAS). These results have been extended into the CaO—FeO—MgO—Al2O3—SiO2(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al2O3 contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters Goy= –5436 + 2.45T cal mol–1,and WM1FeA1= –920 cal mol–1. The volume change ofreaction, Vo, the entropy change, So970 and the enthalpy changeHo1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term WgaCaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al2O3 contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.  相似文献   

12.
Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO2 and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. 87Sr/ 86Sr(i) = 0· 70298–0· 70316,Nd(t) = +3· 8 to +6· 3 and 206Pb/ 204Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China  相似文献   

13.
Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]–1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain  相似文献   

14.
The occurrence of actinolite in magnetite deposits of possiblemagmatic origin has prompted an experimental investigation ofthe upper thermal stability of Mg-rich actinolite to determinehow the stability of actinolite changes with increasing Fe content.Experiments were carried out primarily on the compositionalre-equilibration of natural tremolite [molar Fe/(Fe + Mg) =Fe-number = 0·014] in the presence of synthetic clinopyroxene(Ca0·80Fe0·67Mg0·54Si2·00O6), syntheticpigeonite/orthopyroxene (Ca0·08Fe1·19Mg0·70Si2·02O6),quartz, and water to a more Fe-rich actinolite over the rangeof 600–880°C, 1 and 4 kbar, at the Ni–NiO oxygenbuffer, for durations of 1–2 weeks. The bulk compositionof the mineral mixture is close to actinolite with Fe-number= 0·5. These experiments constitute a half-reversal ofthe amphibole composition, which, when approached from a Mg-richstarting composition, provides information on the minimum Fecontent of actinolite at a given temperature. Compositionalchanges were monitored by electron microprobe analysis of amphibolerim compositions and/or overgrowths on the original tremolite.At 4 kbar and 880–800°C, tremolite shows strong re-equilibrationwith overgrowths of an Fe-rich but low-Ca (1·7 > Ca> 1·4) actinolite; Fe-rich cummingtonite (Ca <0·7)begins to nucleate at 860°C. At 800–700°C, tremoliteshows weak compositional re-equilibration but strong nucleationof Fe-rich cummingtonite. Similar results were observed at 1kbar, with tremolite showing strong re-equilibration to low-Caactinolite at 790–600°C with cummingtonite nucleationat 800°C and below. The wide variation in Ca contents ofthe re-equilibrated amphiboles was unexpected. Additional univariantreversal experiments were carried out on the thermal decompositionof a natural actinolite (Fe-number = 0·22) from PleitoMelón, Chile, indicating the breakdown of actinoliteto clinopyroxene, orthopyroxene, quartz, and water at 780°Cand 1 kbar, and 850°C and 4 kbar. Considering only amphiboleswith Ca >1·7 a.p.f.u., the thermal stability of actinoliteis observed to decrease in a linear manner over the P–Trange investigated with a dT/dFe-number slope of –372°C/Fe-numberat 1 kbar and –546°C/Fe-number at 4 kbar. The highthermal stabilities (750–900°C) of actinolites withFe-numbers in the range of 0–0·4 overlap with therange of water-saturated melting for a typical andesite or tonalite.These conditions also overlap the field of experimental Fe–P-richmelt formation, suggesting that actinolite may have an igneousorigin in Kiruna-type ore deposits. KEY WORDS: actinolite; mineral stability; Kiruna deposits, thermodynamic values; cummingonite  相似文献   

15.
The Diahot terrane of NE New Caledonia contains an interbeddedsequence of Cretaceous to Eocene metasediments, felsic and maficmetavolcanics that experienced c. 40 Ma high-P/T metamorphism.Metabasaltic assemblages define two prograde events (M1 andM2) and a tectonically disrupted crustal profile that extendsfrom lawsonite–blueschist conditions in the SW to paragonite–eclogiteconditions in the NE. Weakly deformed metabasalts from lowest-gradeparts of the Diahot terrane contain M1 omphacite, chlorite,lawsonite and glaucophane-bearing assemblages that partiallypseudomorph igneous plagioclase and augite, and reflect P =0·7–1·0 GPa and T = 350–400°C.M1 assemblages are enveloped by a steeply SW-dipping S2 foliationthat becomes progressively more intense towards the NE overa distance of c. 15 km. S2 assemblages are divided into fourzones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine;(3) clinozoisite–hornblende–almandine; (4) almandine–omphacite.S2 assemblages reflect a P–T gradient that spans the exposed15 km of the Diahot terrane from P = 0·8–1·0GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7GPa and T = 550–600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a P–T array between 1·0and 1·7 GPa that was extensively disrupted by tectonicthinning during exhumation. KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection  相似文献   

16.
Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and XSpa, (from 0•2 to 0•05), XGra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with XAn increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O.Plagioclase is consumed to provide Ca for growing garnet, andXAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. * Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487  相似文献   

17.
ARAI  SHOJI 《Journal of Petrology》1980,21(1):141-165
Dunite, harzburgite and chromitite of alpine-type ultramaficcomplexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchizone, western Japan, are massive and almost lacking in layeredstructure. Constituent minerals are more or less deformed andequilibrated at a relatively low temperature, about 700 °Cor lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromititecomplexes in the Sangun—Yamaguchi zone are characterizedby the uniformity of the Cr/Cr + Al ratio, around 0.5, regardlessof locality and rock type, which is in contrast to the widevariation of the ratio of chromian spinel from the ordinaryalpine-type dunite—harzburgite complex. Mg/Mg + Fe" ratioof chromian spinel, on the other hand, is variable in parallelto the volume per cent of chromian spinel in ultramafic rocks.Olivine in ultramafic rocks is uniform in chemical composition,from Fo88 to Fo92, except for that in chromitite, of which itis Fo95 to Fo97. Primary chemical compositions (especially the Mg/Mg + Fe" ratio)of the constituent minerals have been modified to some extentby element redistribution at low temperature (700 °C orlower), the degree of modification depending on the volume ratioof the minerals. For example, the Mg/Mg + Fe" ratio of chromianspinel in peridotitic rocks has been lowered substantially,and inversely, that of olivine in chromitite has been raised.Primary Mg/Mg + Fe" ratios of olivine and chromian spinel canbe estimated, assuming a certain value of high temperature,e.g. 1200 °C, and the partition coefficient between olivineand chromian spinel at that temperature. As a result, the Mg/Mg+ Fe" ratio of olivine (0.88 to 0.92), and that of chromianspinel (0.78 to 0.80) were uniform, irrespective of rock typeand locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchizone were accumulated as refractory residue after the relativelylarge-scaled partial fusion of some primordial peridotites,which resulted in the chemical uniformity of the residual minerals.They may have initially constituted the lowest part of an ophioliticsuite as ultramafic tectonite and been emplaced as dismemberedportions after the disruption of the parent body.  相似文献   

18.
Kistufell: Primitive Melt from the Iceland Mantle Plume   总被引：5，自引：2，他引：5  
This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (10–15 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (Lan/Ybn<1, Ban/Zrn 0·55–0·58) indicative ofa trace element depleted source, and the Sr–Nd–Pbisotopic ratios (87Sr/86Sr 0·70304–0·70308,143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb18·343–18·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·3–16·8 R/Ra) combinedwith low 207Pb/204Pb (15·42–15·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Srn/Ndn = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The  相似文献   

19.
The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria,enclose abundant xenoliths of spinel lherzolites, many of whichcontain amphibole ± apatite ± phlogopite. Thexenolith suite also includes cumulate wehrlites, spinel metapyroxenitesand garnet metapyroxenites. All xenolith types contain abundantlarge CO2-rich fluid inclusions. Microstructural evidence forthe exsolution of spinel, orthopyroxene, garnet and rare plagioclasefrom complex clinopyroxenes suggests that all of the metapyroxeniteshave formed from clinopyroxene (± spinel ± orthopyroxene)cumulates by exsolution and recrystallization during coolingto the ambient geotherm. Pyroxene chemistry implies that a rangeof parental magma types was involved. Garnet pyroxenites showa series of reactions to successively finer-grained, lower-Pmineral assemblages, which imply a relatively slow initial upwardtransport of the xenoliths in the magma, prior to explosiveeruption. The same process has allowed crystallization of phenocrystsfrom small patches of interstitial melt within xenoliths oflherzolite, wehrlite and metapyroxenite. Critically selected P-T estimates for 16 garnet websteritesare consistent with published experimental studies of the spinel/garnetpyroxenite transition, and define a geotherm from 900 °C,11 kb to 1100 °C, 16 kb. Other published data extend thecurve down to c. 7 kb and up to 25 kb. This elevated geothermsuggests that the high regional heat flow is related to convectiveheat transfer by dike injection accompanying the vulcanism.T estimates for the lherzolites range from 850–1050 °C;comparison with the derived geotherm implies that the spinellherzolites are derived from depths of 30–55 km. Thiszone has low seismic velocities (Vp = 6.8–7.8 km/sec)and has thus previously been regarded as a thick, largely maficlower crust. The xenolith data show that this Mower crust' isdominantly ultramafic, with layers, dikes and some large bodiesof pyroxenites and mafic granulites. The anomalously low Vpmay be due to the high T, the high proportion of fluid-filledpore volume, and the magnesian composition of the lherzolites.The seismically defined Moho (Vp >8.0 km/sec) coincides withthe experimentally determined position of the spinel lherzolite-garnetlherzolite transition.  相似文献   

20.
Vesicular interstitial glass in four kaersutite-bearing spinel–wehrlitexenoliths from Foster Crater, Antarctica has reacted with hostolivine (Fo75–79) and clinopyroxene (Ca47 Mg45 Fe8) andcontains a microphenocryst assemblage of spinel, olivine, andclinopyroxene together with later rhonite and plagioclase. Electronmicroprobe analyses of the glasses have low SiO2 (46–49wt. per cent) and MgO (2.2–3.7) contents and high contentsof alkalis, TiO2 (3.4–4.8), Al2O3 (18.1–20.6) andP2O5 (1.1–1.3). Olivine microphenocryst cores in glassare magnesian (up to Fo88) and must have precipitated from moreprimitive liquids; rim compositions are Fe-rich (Fo75) and inequilibrium with glass. Continuous core to rim zonation in theolivine microphenocrysts indicate that glass compositions havefractionated due to crystallization of the enclosed mineralassemblage. Mass balance addition calculations, using the compositionsand proportions of the crystals in glass, produce melt compositionsappropriate to primary alkali basaltic magmas. Glasses showlight rare earth element (REE) enrichment relative to chondrites(Ce/YbN = 10.5) and, together with Ba, Rb, Cr, Hf, Ta, and Thare similar to many of the basanites from the Erebus VolcanicProvince. Textural relationships of the kaersutite are complex owing tothe instability of kaersutite in the presence of melt. However,in the association with glass we observe textural evidence suchas olivine and clinopyroxene microphenocrysts, identical tothe liquidus phases of the glasses, enclosed by kaersutite crystals.We believe that relationships such as this link the crystallizationof kaersutite to mafic melt which infiltrated and reacted withthe host wehrlite. Thus, the melt did not form in situ withinthe xenolith but originated elsewhere in the upper mantle.  相似文献   

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