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1.
NEUMANN ELSE-RAGNHILD; GRIFFIN WILLIAM LINDSEY; PEARSON NORMAN J.; O'REILLY SUZANNE YVONNE 《Journal of Petrology》2004,45(12):2573-2612
Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (87Sr/86Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 2530% fractional melting of primordialmantle material; it is thus significantly more refractory thannormal mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the SrNd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and meltwall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in ZrHf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. TiAlFe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, meltwall-rock reactionsand chromatographic fractionation either from a CO2-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism 相似文献
2.
Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps) 总被引:6,自引:1,他引:6
Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps),occur as small bodies within lower-crustal rocks (gneisses andmigmatites) subducted at eclogite-facies conditions during theVariscan orogeny. They record a complex metamorphic and deformationevolution as indicated by the transition from coarse-grainedspinel-bearing peridotites to fine-grained garnet + amphibole-bearingperidotites, and are interpreted as portions of mantle wedgethat were incorporated in a downgoing slab of cold continentalcrust. The transition from spinel- to garnet-bearing assemblagewas accompanied by significant input of metasomatic agents,as shown by the crystallization of abundant amphibole. Herewe present trace-element mineral chemistry data for selectedUlten peridotites, with the aim of unravelling the nature ofthe metasomatic processes. Amphiboles display significant lightrare earth element (LREE) enrichment [CeN/YbN = 3·9011·50;LREE up to (2050) x C1], high Sr (150250 ppm),K (19107280 ppm) and Ba (280800 ppm) contents,and low concentrations of high field strength elements (HFSE)(Zr = 1425 ppm, Y = 6·716 ppm, Ti = 11502500ppm, Nb = 27 ppm). On the basis of (1) the evidence formodal orthopyroxene decrease as a result of the garnet-formingreaction rather than abundant orthopyroxene crystallization,(2) the high modal amounts of amphibole (up to 23%) in the mostmetasomatized peridotites and (3) the strong large ion lithophileelement (LILE)/HFSE fractionation in amphiboles, we infer thatthe metasomatic agent was an H2OCO2 fluid with a lowCO2/H2O ratio. Petrological investigations and geochronologicaldata indicate that the host metapelites experienced in situpartial melting and migmatization concomitantly with the garnet+ amphibole-facies recrystallization in the enclosed peridotites.We infer that the metasomatizing hydrous fluids could representthe residual fluids left after the crystallization of leucosomes,starting from water-undersaturated melts produced during migmatizationof the host gneisses. KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids 相似文献
3.
Metasomatism and Melting in Carbonated Peridotite Xenoliths from the Mantle Wedge: The Gobernador Gregores Case (Southern Patagonia) 总被引:8,自引:5,他引:8
LAURORA ANGELA; MAZZUCCHELLI MAURIZIO; RIVALENTI GIORGIO; VANNUCCI RICCARDO; ZANETTI ALBERTO; BARBIERI MARIA ADELAIDE; CINGOLANI CARLOS A. 《Journal of Petrology》2001,42(1):69-87
Spinel-facies mantle xenoliths occur in a diatreme cutting throughthe Neogene Southern Patagonia Plateau at Gobernador Gregores(Santa Cruz Province, Argentina). This plateau is in a back-arcposition with respect to the Chile trench. Xenoliths differin their whole-rock composition from other South America occurrences,having higher CaO/Al2O3 ratios and, in some samples, TiO2 enrichment,whereas the Na2O/Al2O3 variation range is similar. Three assemblagescan be distinguished. Assemblage 1, in anhydrous protogranularlherzolites and harzburgites, contains clinopyroxene with adepleted major and trace element composition, indicating pre-metasomaticdepletion processes. This assemblage fully recrystallized toAssemblage 2 (amphibole ± phlogopite ± Cl-apatite-bearing)during a metasomatic episode. This causes clinopyroxene to acquiregeochemical characteristics often attributed to carbonate-meltmetasomatism. Noticeably, amphibole is markedly enriched inNb (up to 298 ppm), especially when depleted in Ti. A furtherevent, related to decompression during xenolith uplift to thesurface, induces closed-system (perhaps with the exception ofCO2 addition) disequilibrium melting of Assemblage 2, dominantlyof amphibole. It is found in pockets (where amphibole is a residualphase) consisting of NaSi-rich glass and carbonate (Mg-richcalcite) drops, and in veins originating from the pockets (Assemblage3). Euhedral olivine, clinopyroxene and spinel crystallize onlyin the silicate glass. So do new, euhedral apatite crystalswhen glass is in contact with previous Assemblage 2 apatite.Textural evidence and comparison with experimental work suggestthat silicate glass and carbonates are the result of unmixingof a former homogeneous melt. Because of the different flowrates of carbonate and silicate melt, the xenoliths become enrichedin carbonate, which is found in the veins during their migration.Thus, the high CaO/Al2O3 ratio of whole rocks provides inconclusiveevidence of carbonatite metasomatism. This factor, and otherminor deviations from the expected results of carbonatite metasomatism,lead us to hypothesize an aqueous, Cl-rich fluid, possibly slabderived, as an alternative agent. Amphibole, resulting fromreactive porous flow of this agent in the mantle, could fullyexplain the observed geochemical features, as indicated by estimatesof its partition coefficients. KEY WORDS: carbonated xenoliths; Gobernador Gregores; LAMICP-MS; mantle metasomatism; silicate glass 相似文献
4.
Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya) 总被引:1,自引:0,他引:1
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass 相似文献
5.
Several spinel peridotite xenoliths from Spitsbergen have SrNdisotopic compositions that plot to the right of the mantlearray defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to SrNd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. Chromatographiceffects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing SrNd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, SrNd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling 相似文献
6.
The phase diagram for lherzoliteCO2H2O providesa framework for interpreting the distribution of phase assemblagesin the upper mantle with various thermal structures, in differenttectonic settings. Experiments show that at depths >80 km,the near-solidus partial melts from lherzoliteCO2H2Oare dolomitic, changing through carbonatesilicate liquidswith rising temperatures to mafic liquids; vapor, if it coexists,is aqueous. Experimental data from simple systems suggest thata critical end-point (K) occurs on the mantle solidus at anundetermined depth. Isobaric (TX) phase diagrams forvolatile-bearing systems with K elucidate the contrasting phaserelationships for lherzoliteCO2H2O at depths belowand above a critical end-point, arbitrarily placed at 250 km.At levels deeper than K, lherzolite can exist with dolomiticmelt, aqueous vapor, or with critical fluids varying continuouslybetween these end-members. Analyses of fluids in microinclusionsof fibrous diamonds reveal this same range of compositions,supporting the occurrence of a critical end-point. Other evidencefrom diamonds indicates that the minimum depth for this end-pointis 125 km; maximum depth is not constrained. Constructed cross-sectionsshowing diagrammatically the phase fields intersected by upwellingmantle indicate how rising trace melts may influence trace elementconcentrations within a mantle plume. KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids 相似文献
7.
The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms 总被引:21,自引:0,他引:21
Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al2O3 of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al2O3 decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to PT estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (3766 km), spinel/garnet lherzolite from 19to 24 kb (6280 km), and garnet lherzolite from 24 to25 kb (7983 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior. 相似文献
8.
Ishimaru Satoko; Arai Shoji; Ishida Yoshito; Shirasaka Miki; Okrugin Victor M. 《Journal of Petrology》2007,48(2):395-433
9.
MATVEEV S.; O'NEILL H. ST. C.; BALLHAUS C.; TAYLOR W. R.; GREEN D. H. 《Journal of Petrology》2001,42(4):721-729
Hydrogen solubility and hydroxyl substitution mechanism in olivineat upper-mantle conditions are not only a function of pressure,temperature, water fugacity and hydrogen fugacity, but are alsoinfluenced by silica activity. Olivine synthesized in equilibriumwith magnesiowüstite displays hydroxyl stretching bandsin the wavenumber range from 3640 to 3430 cm1. In contrast,olivine in equilibrium with orthopyroxene shows absorption bandsin a narrower wavenumber range from 3380 to 3285 cm1.The two fundamentally different spectra are assigned to hydroxylin tetrahedral and octahedral sublattices, respectively. Olivinein equilibrium with orthopyroxene is also less capable of incorporatinghydroxyl, relative to olivines in equilibrium with magnesiowüstite,by about a factor of ten. A comparison of spectra obtained aspart of this study with hydroxyl spectra of natural mantle olivinesshows that the latter display hydroxyl stretching patterns reminiscentof equilibrium with magnesiowüstite, although undoubtedlyolivine in the Earths mantle coexists with orthopyroxene.This may be attributed to a metasomatic overprint by a low-silicafluid and/or melt that was in reaction relationship with orthopyroxene.A likely metasomatic agent is a carbonatitic melt. When carbonatiticmelts decompose to oxides and CO2, they may temporarily imposea low-aSiO2 environment inherited by the olivine structure.If this suggestion proves true, Fourier transform IR spectroscopymay be used to fingerprint metasomatic episodes in the lithosphericmantle. KEY WORDS: FTIR spectrometry; olivine; mantle; metasomatism; water 相似文献
10.
WITT-EICKSCHEN G.; SECK H. A.; MEZGER K.; EGGINS S. M.; ALTHERR R. 《Journal of Petrology》2003,44(6):1077-1095
The Pb isotope compositions of amphiboles and clinopyroxenesin spinel peridotite and pyroxenite mantle xenoliths from theintra-plate Quaternary volcanic fields of the Eifel province(Germany) are strongly correlated with their SrNd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and SrNdPbisotope compositions similar to the depleted source of mid-oceanridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearingxenoliths from shallower depths (<45 km) provide evidencefor three temporally distinct episodes of mantle metasomatismin the subcontinental lithosphere: (1) aqueous fluids from anisotopically enriched (EM-like) mantle reservoir caused amphiboleformation during deformation in the shallow continental lithosphericmantle and may be subduction related, probably associated withthe last major tectonic event that influenced the area (Hercynianorogeny). (2) During a second phase of mantle metasomatism theEM-like lithospheric mantle was affected by melts from an ancient,HIMU-like (high time-integrated µ = 238U/204Pb) mantlesource. The HIMU-like component introduced by these fluids hada much more radiogenic Pb isotope composition than the asthenosphericsource of the widespread Cenozoic magmatism in Europe and maybe linked to reactivation of ancient subducted crustal domainsduring the Hercynian orogeny or to early Cretaceous deep-sourcedmantle plumes. (3) During a brief final stage the heterogeneouslyenriched EMHIMU subcontinental lithosphere was locallymodified by basaltic melts migrating along fractures and veinsthrough the upper mantle as a consequence of the Cenozoic Eifelvolcanism. Although a DMM component is completely lacking inthe metasomatic fluids of the metasomatic episodes 1 and 2,the vein melts of episode 3 and the Cenozoic Eifel lavas requiremantle sources containing three end-member components (DMMHIMUEM).Thus, mobilization of the more depleted mantle material occurredat the earliest in the Tertiary, contemporaneously with thedevelopment of the extensive rift system and main melt generationin Europe. Alternatively, the variety of SrNdPbisotope signatures of the metasomatic agents may have been producedby melting of isotopically distinct mantle domains in a heterogeneousuprising mantle plume. KEY WORDS: Eifel; Europe; mantle xenoliths; metasomatism; Pb isotopes 相似文献
11.
K-feldsparplagioclasequartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H2O-rich, hornblende-bearingzone to an H2O-poor, hornblende-free, orthopyroxene-bearing,granulite facies zone at 700800°C and78 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO2-rich fluid, whereas for the Val Strona diOmegna traverse, IvreaVerbano Zone, northern Italy, dehydrationtook place over a 34 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KClNaCl brines; metasomatism; granulite facies metamorphism; charnockiteenderbite; orthopyroxene; K-feldspar; biotite; hornblende 相似文献
12.
Calculation of Phase Relations Involving Haplogranitic Melts Using an Internally Consistent Thermodynamic Dataset 总被引:5,自引:4,他引:5
A simple thermodynamic model is developed for silicate meltsin the system CaONa2OK2OAl2O3SiO2H2O(CNKASH). The Holland & Powell (Journal of Metamorphic Geology,16, 289302, 1998) internally consistent thermodynamicdataset is extended via the incorporation of the experimentallydetermined melting relationships in unary and binary subsystemsof CNKASH. The predictive capability of the model is evaluatedvia the experimental data in ternary and quaternary subsystems.The resulting dataset, with the software THERMOCALC, is thenused to calculate melting relationships for haplogranitic compositions.Predictions of the PT stabilities of assemblages in water-saturatedand -undersaturated bulk compositions are illustrated. It isnow possible to make useful calculations of the melting behaviourof appropriate composition rocks under crustal conditions. KEY WORDS: thermodynamics; melts; granite; dataset 相似文献
13.
Two types of sapphirine occurrences were found in the Lewisiancomplex in South Harris, NW Scotland: (1) inclusions withinporphyroblasts; (2) symplectic grains together with secondarycordierite, plagioclase and orthopyroxene. The presence of sapphirineinclusions implies that sapphirine was stable at the early stageof ultra-high-temperature metamorphism, whereas symplectic sapphirinegrains were formed during decompressional retrograde metamorphism.The sapphirine occurrences and compositions of associated mineralsdepend on the host rock composition. Sapphirine inclusions occuronly in rocks with high bulk-XMg, and sapphirine is never presentas porphyroblastic grains because of its breakdown in responseto pressure increase. Sapphirine symplectites can be seen inthe relatively low bulk-XMg rocks, and the texture suggestslocal equilibrium in the MgAl-rich domain that is formedby metamorphic segregation of the MgAl-rich mineralsin response to partial melting. The various sapphirine occurrencesobserved in South Harris were controlled by not only protolithcomposition but also local, mineral-scale, composition in acontinuous metamorphic history. KEY WORDS: sapphirine; Lewisian complex; high-Mg garnet; partial melting 相似文献
14.
MOINE B. N.; GREGOIRE M.; O'REILLY S. Y.; SHEPPARD S. M. F.; COTTIN J. Y. 《Journal of Petrology》2001,42(11):2145-2167
A basanite dyke in the Kerguelen Archipelago contains abundantcomposite mantle xenoliths consisting of spinel-bearing dunitescross-cut by amphibole-rich veins. Two types of veins (thickand thin) have been distinguished: the thick veins representalmost complete crystallization products of highly alkalinemelts similar to the host basanites, whereas thin veins areprecipitates from fractionates of the parental melts to thethick veins. These fractionated fluids are enriched in H2O relativeto the parental melts. The amphiboles in the thin veins arelower in Ti and higher in Nb, Ta, Zr and Hf than amphibolesin the thick veins. This fractionation of high field strengthelements (HFSE) is consistent with a combination of the changingcomposition of the fractionated fluids and the change in intrinsicamphibolefluid partition coefficients for HFSE in fluidswith higher aH2O and lower aTiO2. The trace element contentof amphiboles disseminated in dunitic wall-rocks is closelyrelated to the composition of adjacent veins and thus theseamphiboles are precipitates from fluids percolating into thedunite from the veins. Disseminated amphibole reflects the compositionof the percolating melt, which is similar to that of the associatedveins. KEY WORDS: mantle amphibole; Kerguelen; HFSE fractionation; mantle HFSE; mantle xenoliths 相似文献
15.
Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study 总被引:9,自引:5,他引:4
Xenoliths from Engeln–Kempenich in the East Eifel volcanicfield (Germany) comprise gabbroic to ultramafic cumulates, andmeta-igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas.The metamorphic xenoliths were divided into three types: (1)primitive type P, which are little affected by metasomatic replacementstructures; (2) enriched type E1 defined by metasomatic replacementof primary pyroxene and garnet by pargasitic amphibole and biotite;(3) enriched type E2 defined by breakdown of hydrous phases.Type E rocks are geochemically related to type P and cumulatexenoliths by compositional trends. During modal metasomatism,type E rocks were oxidized. Type E1 rocks were typically enrichedin Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr,and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relativeto type P rocks. Formation of the hydrous, chlorine-bearingphases amphibole and scapolite containing glass and fluid inclusionsin the E1 rocks provides evidence for a water and Cl-bearingfluid phase coexisting with silicate melt. Accordingly, we calculated10 mol % H2O back into the CO2-dominated fluid inclusions, inagreement with experimental data on the composition of a fluidphase coexisting with mafic alkaline melts at elevated pressure.Primary CO2-dominated fluid inclusions coexisting with glassinclusions in metamorphic corona phases and neoblasts, and incumulate xenoliths, have overlapping densities. Fluid inclusionbarometry using the corrected densities indicates that bothcumulates and metamorphic xenoliths originated from the samedepth at 22–25 km (650 ± 50 MPa). This is interpretedas being a main magma reservoir level within the upper partof the lower crust close to the Conrad discontinuity, wherethe xenoliths represent wall-rocks. The Conrad discontinuityseparates an upper-crustal layer, consisting of preferentiallyductile granodioritic and tonalitic gneisses, and more brittlelower-crustal mafic granulites. The brittle–ductile transitionappears to be a preferred level of magma stagnation. KEY WORDS: continental lower crust; fluids; magma chamber; metasomatism; xenoliths 相似文献
16.
An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures 总被引:4,自引:0,他引:4
Experiments in the quartz-saturated part of the system KFMASHunder fO2 conditions of the haematitemagnetite bufferand using bulk compositions with XMg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the PT field 912 kbar, 8501100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofPT and TX pseudosections. Sapphirine did not appearin the bulk composition of XMg = 0·53. A partial petrogeneticgrid applicable to high MgAl granulites metamorphosedat high fO2, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate PTconditions and retrograde PT trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO2 相似文献
17.
GREGOIRE M.; MOINE B. N.; O'REILLY SUZANNE Y.; COTTIN J. Y.; GIRET A. 《Journal of Petrology》2000,41(4):477-509
Mantle xenoliths in alkaline lavas of the Kerguelen Islandsconsist of: (1) protogranular, Cr-diopside-bearing harzburgite;(2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite;(3) dunite that contains clinopyroxene, spinel phlogopite, andrarely amphibole. Trace element data for rocks and mineralsidentify distinctive signatures for the different rock typesand record upper-mantle processes. The harzburgites reflectan initial partial melting event followed by metasomatism bymafic alkaline to carbonatitic melts. The dunites were firstformed by reaction of a harzburgite protolith with tholeiiticto transitional basaltic melts, and subsequently developed metasomaticassemblages of clinopyroxene + phlogopite ± amphiboleby reaction with lamprophyric or carbonatitic melts. We measuredtwo-mineral partition coefficients and calculated mineralmeltpartition coefficients for 27 trace elements. In most samples,calculated budgets indicate that trace elements reside in theconstituent minerals. Clinopyroxene is the major host for REE,Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxenefor Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc. KEY WORDS: mantle xenoliths; mantle metasomatism; partition coefficients; Kerguelen Islands; trace elements 相似文献
18.
High-pressure Partial Melting of Mafic Lithologies in the Mantle 总被引:17,自引:2,他引:15
We review experimental phase equilibria associated with partialmelting of mafic lithologies (pyroxenites) at high pressuresto reveal systematic relationships between bulk compositionsof pyroxenite and their melting relations. An important aspectof pyroxenite phase equilibria is the existence of the garnetpyroxenethermal divide, defined by the enstatiteCa-Tschermakspyroxenediopside plane in CaOMgOAl2O3SiO2projections. This divide appears at pressures above 2 GPa inthe natural system where garnet and pyroxenes are the principalresidual phases in pyroxenites. Bulk compositions that resideon either side of the divide have distinct phase assemblagesfrom subsolidus to liquidus and produce distinct types of partialmelt ranging from strongly nepheline-normative to quartz-normativecompositions. Solidus and liquidus locations are little affectedby the location of natural pyroxenite compositions relativeto the thermal divide and are instead controlled chiefly bybulk alkali contents and Mg-numbers. Changes in phase volumesof residual minerals also influence partial melt compositions.If olivine is absent during partial melting, expansion of thephase volume of garnet relative to clinopyroxene with increasingpressure produces liquids with high Ca/Al and low MgO comparedwith garnet peridotite-derived partial melts. KEY WORDS: experimental petrology; mantle heterogeneity; partial melting; phase equilibrium; pyroxenite 相似文献
19.
FeMg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaOMgOAl2O3SiO2FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·72·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about35°C. Relative to the solidus of model lherzolitein the CaOMgOAl2O3SiO2 system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in PT spaceto the comparable transition in the CaOMgOAl2O3SiO2Na2O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·52°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for FeMgexchange between the coexisting crystalline phases and melt,KdFeMgxlliq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na2O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting 相似文献
20.
Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions 总被引:8,自引:0,他引:8
AEOLUS LEE CIN-TY; LEEMAN WILLIAM P.; CANIL DANTE; LI ZHENG-XUE A 《Journal of Petrology》2005,46(11):2313-2336
V/Sc systematics in peridotites, mid-ocean ridge basalts andarc basalts are investigated to constrain the variation of fO2in the asthenospheric mantle. V/Sc ratios are used here to seethrough those processes that can modify barometric fO2determinations in mantle rocks and/or magmas: early fractionalcrystallization, degassing, crustal assimilation and mantlemetasomatism. Melting models are combined here with a literaturedatabase on peridotites, arc lavas and mid-ocean ridge basalts,along with new, more precise data on peridotites and selectedarc lavas. V/Sc ratios in primitive arc lavas from the Cascadesmagmatic arc are correlated with fluid-mobile elements (e.g.Ba and K), indicating that fluids may subtly influence fO2 duringmelting. However, for the most part, the average V/Sc-inferredfO2s of arc basalts, MORB and peridotites are remarkably similar(1·25 to +0·5 log units from the FMQ buffer)and disagree with the observation that the barometric fO2s ofarc lavas are several orders of magnitude higher. These observationssuggest that the upper part of the Earth's mantle may be stronglybuffered in terms of fO2. The higher barometric fO2s of arclavas and some arc-related xenoliths may be due respectivelyto magmatic differentiation processes and to exposure to large,time-integrated fluid fluxes incurred during the long-term stabilityof the lithospheric mantle. KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs 相似文献