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1.
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
2.
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
3.
NEUMANN E.-R.; WULFF-PEDERSEN E.; PEARSON N. J.; SPENCER E. A. 《Journal of Petrology》2002,43(5):825-857
Mantle xenoliths from Tenerife show evidence of metasomatismand recrystallization overprinting the effects of extensivepartial melting. The evidence includes: recrystallization ofexsolved orthopyroxene porphyroclasts highly depleted in incompatibletrace elements into incompatible-trace-element-enriched, poikiliticorthopyroxene with no visible exsolution lamellae; formationof olivine and REECr-rich, strongly ZrHfTi-depletedclinopyroxene at the expense of orthopyroxene; the presenceof phlogopite; whole-rock CaO/Al2O3 >> 1 (Ca metasomatism) inrecrystallized rocks; and enrichment in incompatible elementsin recrystallized rocks, relative to rocks showing little evidenceof recrystallization. The higher-than-normal degreeof partial melting that preceded the metasomatism probably resultsfrom plume activity during the opening of the Central AtlanticOcean. SrNd isotopic compositions are closely similarto those of Tenerife basalts, indicating resetting from theexpected original mid-ocean ridge basalt composition by themetasomatizing fluids. Metasomatism was caused by silicic carbonatitemelts, and involved open-system processes, such as trappingof elements compatible with newly formed acceptor minerals,leaving residual fluids moving to shallower levels. The compositionsof the metasomatizing fluids changed with time, probably asa result of changing compositions of the melts produced in theCanary Islands plume. Spinel dunites and wehrlites representrocks where all, or most, orthopyroxene has been consumed throughthe metasomatic reactions. KEY WORDS: Canary Islands; Tenerife; mantle xenoliths; geochemistry; Ca metasomatism; open-system processes; lithosphere; ocean islands 相似文献
4.
The Oligo-Miocene Somuncura province is the largest (55 000km2) back-arc mafic volcanic field in Patagonia, and one ofEarth's largest with no clear link to a hotspot or major extension.Major and trace element and SrNdPb isotopic datasuggest involvement of a plume-like component in the mantlemagma source mixed with hydrous, but not high field strengthelement (HFSE)-depleted components, from a disintegrating subductingplate. Magmatism is attributed to mantle upwelling related todisturbances during plate reorganization, possibly at a timewhen the South America plate was nearly stationary over theunderlying mantle. Melting was enhanced by hydration of themantle during Paleogene subduction. Crustal contamination wasminimal in a refractory crust that had been extensively meltedin the Jurassic. Eruption began with low-volume intraplate alkalinemafic flows with depleted NdSr isotopic signatures. Thesewere followed by voluminous 2925 Ma tholeiitic maficflows with flat light and steep heavy rare earth element (REE)patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratiosand enriched SrNd isotopic signatures. Their source canbe explained by mixing EM1Tristan da Cunha-like and depletedmantle components with subduction-related components. Post-plateau2417 Ma alkaline flows with steep REE patterns, highincompatible element abundances, and depleted SrNd isotopicsignatures mark the ebbing of the mantle upwelling. KEY WORDS: Somuncura plateau; slab interaction; Patagonia; large igneous province (LIP); plume-like upwelling 相似文献
5.
LEE DER-CHUEN; HALLIDAY ALEX N.; DAVIES GARETH R.; ESSENE ERIC J.; FITTON J. GODFREY; TEMDJIM ROBERT 《Journal of Petrology》1996,37(2):415-441
Major element, trace element and SrNdPb isotopiccompositions of ultramafic xenoliths and megacrysts from thecontinental Cameroon line provide evidence for metasomatismof the upper most lithospheric mantle by enriched melts duringthe Mesozoic The megacrysts probably crystallized within thelower continental crust from melts similar to the host magmas.All the xenoliths originated as depleted residues after theextraction of basaltic melts, but some indicate evidence ofinteraction with enriched partial melts before entrainment.The UPb isotopic data on garnet are consistent with coolingthrough >900C at >300 Ma. The SmNd isotope systematicsin constituent phases appear to have been in equilibrium ona xenolith scale at the time of entrainment, indicating derivationfrom mantle that remained at temperatures >600C until eruption.Spinel therzolies that show simple light rare earth element(LREE) depletions are characterized by isotopic compositionsthat are comparable with, but slightly more depleted than AtlanticN-MORB, suggesting that the unmetasomatized sub-continentallithosphere of the Cameroon line may be isotopically similarto that of sub-oceanic lithosphere. The Nd-depleted mantle modelages of these xenoliths are consistent with late Proterozoicdepletion, similar in age to much of the overlying continentalcrust. In contrast, samples that have LREE-enriched clinopyr-oxenes(La/Yb =4.79.4) contain trace amounts of amphibole, areenriched in U and have more radiogenic Pb and Sr. These xenolithsyield UPb and SmNd model ages consistent withMesozoic enrichment, in agreement with the age of enrichmentof the source regions of the basalts, as deduced from Pb isotopiccompositions. Clinopyroxenes record three orders of magnitudeenrichment in U and LREE accompanied by progressive K depletionassociated with the growth of trace amphibole, with K/U ratiosthat range from 12000 to 1. The ratios of the trace elementsthought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rband Nd/Sr ratios, display regional variations related to thetime integrated history of enrichments indicated by Nd isotopiccompositions. Mass balance calculations suggest that the meltsresponsible for the most recent enrichment of the lithospherehad higher La/Yb and U/Pb than Cameroon line host magmas, andwere probably the product of small degrees of partial meltingassociated with the earliest stages of the breakup of Pangea. KEY WORDS: Cameroon line; mantle xenoliths; megacrysts; REE; isotopic composition; trace element 相似文献
6.
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
7.
The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite Nd (+0.7 to +1.8) and Sr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite. 相似文献
8.
Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO2, from 9 to 21 wt % Al2O3, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism. 相似文献
9.
DOWNES HILARY; MACDONALD RAY; UPTON BRIAN G. J.; COX KEITH G.; BODINIER JEAN-LOUIS; MASON PAUL R. D.; JAMES DODIE; HILL PETER G.; HEARN B. CARTER JR 《Journal of Petrology》2004,45(8):1631-1662
Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al2O3contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al2O3 and TiO2) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have 143Nd/144Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low 143Nd/144Nd values (0·5113)and extremely high 87Sr/86Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their SrNdisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette 相似文献
10.
L. Melluso S. F. Sethna M. D’Antonio P. Javeri L. Bennio 《Mineralogy and Petrology》2002,74(2-4):323-342
Summary Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres,
tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira
area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12–2 wt%). The
lamprophyres have high content of incompatible elements (e.g., TiO2 > 3.8 wt%, Nb > 130 ppm, Zr > 380 ppm, Ba > 1200 ppm), and relatively high initial (at 65 Ma) 143Nd/144Nd (0.5128) and low 87Sr/86Sr (0.7038–0.7042). They are likely to be small-degree melts (2–3%) of volatile- and incompatible element-enriched mantle
sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks
have highly porphyritic textures (up to 57% phenocrysts of diopside ± olivine), and anomalously low contents of incompatible
elements (e.g., TiO2 < 1.3 wt%, Nb < 24 ppm, Zr < 100 ppm) indicating that they could not represent liquid compositions. Moreover, their very
low initial 143Nd/144Nd ratios (0.5116–0.5120), at 87Sr/86Sr = 0.7045–0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that
could be identified as “Lewisian-type” lower crust and/or enriched mantle. The melting episode that generated these alkaline
rocks likely occurred close to the base of the ca. 100 km-thick Indian lithosphere, very shortly after the main eruption of
the Deccan tholeiites.
Received January 14, 2000; revised version accepted September 28, 2001 相似文献
11.
Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: A review 总被引:1,自引:0,他引:1
Hilary Downes Elena Balaganskaya Andrew Beard Ruslan Liferovich Daniel Demaiffe 《Lithos》2005,85(1-4):48-75
Igneous rocks of the Devonian Kola Alkaline Carbonatite Province (KACP) in NW Russia and eastern Finland can be classified into four groups: (a) primitive mantle-derived silica-undersaturated silicate magmas; (b) evolved alkaline and nepheline syenites; (c) cumulate rocks; (d) carbonatites and phoscorites, some of which may also be cumulates. There is no obvious age difference between these various groups, so all of the magma-types were formed at the same time in a relatively restricted area and must therefore be petrogenetically related. Both sodic and potassic varieties of primitive silicate magmas are present. On major element variation diagrams, the cumulate rocks plot as simple mixtures of their constituent minerals (olivine, clinopyroxene, calcite, etc). There are complete compositional trends between carbonatites, phoscorites and silicate cumulates, which suggests that many carbonatites and phoscorites are also cumulates. CaO / Al2O3 ratios for ultramafic and mafic silicate rocks in dykes and pipes range up to 5, indicating a very small degree of melting of a carbonated mantle at depth. Damkjernites appear to be transitional to carbonatites. Trace element modelling indicates that all the mafic silicate magmas are related to small degrees of melting of a metasomatised garnet peridotite source. Similarities of the REE patterns and initial Sr and Nd isotope compositions for ultramafic alkaline silicate rocks and carbonatites indicate that there is a strong relationship between the two magma-types. There is also a strong petrogenetic link between carbonatites, kimberlites and alkaline ultramafic lamprophyres. Fractional crystallisation of olivine, diopside, melilite and nepheline gave rise to the evolved nepheline syenites, and formed the ultramafic cumulates. All magmas in the KACP appear to have originated in a single event, possibly triggered by the arrival of hot material (mantle plume?) beneath the Archaean/Proterozoic lithosphere of the northern Baltic Shield that had been recently metasomatised. Melting of the carbonated garnet peridotite mantle formed a spectrum of magmas including carbonatite, damkjernite, melilitite, melanephelinite and ultramafic lamprophyre. Pockets of phlogopite metasomatised lithospheric mantle also melted to form potassic magmas including kimberlite. Depth of melting, degree of melting and presence of metasomatic phases are probably the major factors controlling the precise composition of the primary melts formed. 相似文献
12.
This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of
the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58
Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea
Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites,
ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization
of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from
the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered
by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very
low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high
Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of
their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies
on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and
the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics
was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material
(HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere,
whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt
assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated
into the oceanic lithospheric mantle during rifting and the breakup of Pangea. 相似文献
13.
Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity 总被引:14,自引:8,他引:14
New Pb isotopic data are presented for 10 young Mesozoic toCenozoic (0116 Ma) carbonatites from a 1400 km long segmentof the East African Rift. Patterns observed in Pb vs Pb, Srvs Pb and Nd vs Pb isotope diagrams define unusual, nearly linear,trends that are interpreted as mixing between two componentsthat are broadly similar to the two mantle end-member components,HIMU and EM1, which were first recognized from ocean-islandbasalts. The two plutons with isotope signatures closest toHIMU and EM1 crop out within 140 km of each other. From thesedata, EM1 and HIMU are now known to occur in both continentaland oceanic settings that are associated with plumes or rifts.Moreover, these isotopic signatures tend to occur in regionswhere seismic tomography indicates prominent low-velocity zonesin the lower mantle. For these reasons, we favour a model forthe origin of the East African Rift carbonatites that involvesmelting and mixing of HIMU and EM1 components contained withinan isotopically heterogeneous mantle plume. We consider theHIMU and EM1 sources to be stored within the deep (lower 1000km) mantle, possibly the coremantle boundary. The rolethat continental lithosphere plays in carbonatite generationis probably one of concentrating volatiles at the upper levelsof an ascending mantle plume. KEY WORDS: carbonatites; isotopes; rifts; plumes; FOZO 相似文献
14.
Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism 总被引:10,自引:0,他引:10
The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths
from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental
lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite,
and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to
1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the
xenoliths are LILE- and LREE-enriched, but have relatively low 87Sr/86Sr (0.70294 to 0.70342) and high ɛNd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low 87Sr/86Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite
xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases
(amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive
textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history
involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier
metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is
attributed to CO2-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios
and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither
the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism
associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite
metasomatism in an active continental backarc setting.
Received: 26 January 2000 / Accepted: 1 March 2000 相似文献
15.
The Lithospheric Mantle beneath Continental Margins: Melting and Melt-Rock Reaction in Canadian Cordillera Xenoliths 总被引:3,自引:0,他引:3
Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 510%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilationfractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Meltrock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; meltrock reaction 相似文献
16.
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
17.
At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5–150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2–20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO2 bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8–5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr–Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate–carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation. 相似文献
18.
Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite
xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic
micas in kamafugites and carbonatites are aluminous (> 12 wt% Al2O3), typically contain significant levels of Cr (up to 1.1 wt% Cr2O3), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted
in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al2O3 and TiO2 compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised
at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values
found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts
are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle.
Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and
mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al2O3 distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo
olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of
the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite.
Received January 24, 2000; revised version accepted September 27, 2001 相似文献
19.
The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism? 总被引:2,自引:0,他引:2
Shaw J. E.; Baker J. A.; Kent A. J. R.; Ibrahim K. M.; Menzies M. A. 《Journal of Petrology》2007,48(8):1495-1512
We present trace element and SrNdHfPb isotopecompositions for clinopyroxenes from anhydrous spinel peridotiteand garnet ± spinel pyroxenite xenoliths of Pan-Africanlithospheric mantle from Jordan, including the first high-precisiondouble-spike Pb isotope measurements of mantle clinopyroxene.Clinopyroxenes from the peridotites are variably ThULILELREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREEHREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 510% melt, whereas theenrichments in ThULILELREE require a Pan-Africanor later metasomatic event. The large range of Nd, Sr, Pb andHf isotope ratios in both peridotites and pyroxenites (e.g.Nd 1·417·5; 206Pb/204Pb 17·220·4;Hf 0·6164·6) encompasses compositionsmore radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopescover almost the entire range of oceanic basalt values. Hf valuesare some of the highest ever recorded in mantle samples andare decoupled from Nd in the same samples. Marked correlationsbetween SrNdPb isotopes, LILELREE enrichmentsand HFSE depletion suggest that the metasomatizing agent wasa carbonatitic-rich melt and isotopic data suggest that metasomatismmay have been related to Pan-African subduction. The metasomaticmelt permeated depleted upper mantle (<16 kbar) during Pan-Africansubduction at 600900 Ma, and the variably metasomatizedmaterial was then incorporated into the Arabian lithosphericmantle. There is no evidence for recent metasomatism (<30Ma) related to the Afar plume like that postulated to have affectedsouthern Arabian lithospheric mantle. Hf isotopes in the mantleclinopyroxenes are unaffected by metasomatism, and even somestrongly overprinted lithologies record ancient (>1·2Ga) pre-metasomatic LuHf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The resistance of the LuHf isotopic systemto later metasomatic events resulted in the development of extremelyheterogeneous Hf isotopic signatures over time that are decoupledfrom other isotopic systems. No mantle sample in this studyexactly matches the chemical and isotopic signature of the sourceof Jordanian intraplate basalts. However, the xenolith compositionsare broadly similar to those of the source of Arabian intraplatebasalts, suggesting that the numerous Cenozoic intraplate volcanicfields throughout Arabia may be the product of melting uppermantle wedge material fertilized during Pan-African subductionand incorporated into the Arabian lithospheric mantle. We proposea model whereby the proto-Arabian lithospheric mantle underwenta major melting event in early Proterozoiclate Archeantimes (at the earliest at 1·2 Ga). Island-arc volcanismand major crust formation occurred during the Pan-African orogeny,which liberated fluids and possibly small-degree melts thatmigrated through the mantle creating zones of enrichment forcertain elements depending upon their compatibility. Immobileelements, such as Nb, were concentrated near the base of themantle wedge providing the source of the Nb-rich Jordanian volcanicrocks. More mobile elements, such as LILE and LREE, were transportedup through the mantle and fertilized the shallow mantle sourceof the Jordanian xenoliths. Following subduction, the mantlewedge became fossilized and preserved distinct enriched anddepleted zones. Lithospheric rifting in the Miocene triggeredpartial melting of spinel-facies mantle in the lower lithosphere,which mixed with deeper asthenospheric garnet-facies melts asrifting evolved. These melts entrained segments of variablycarbonatite-metasomatized shallow lithospheric mantle en routeto the surface. KEY WORDS: Arabian lithospheric mantle; Jordan; mantle xenoliths; SrNdHfPb isotopes 相似文献
20.
A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif
(380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types
of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory
minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier
phoscorites and carbonatites (group 1) are characterized by relatively low87Sr/86Sr (0.70330–0.70349) and143Nd/144Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from
the second group has higher87Sr/86Sr (0.70350–0.70363) and143Nd/144Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6
ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from
the earlier group fits into neither of the two groups and is characterized by the highest initial87Sr/86Sr (0.70385) and lowest143Nd/144Nd (0.51229) of any of the apatites. Within both groups initial87Sr/86Sr and143Nd/144Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from
the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship
between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate
several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ18O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and
are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted
quadrant of an εNd versus εSr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the
later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation
assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are
similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites
and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study
shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring
the Sr−Nd evolution of carbonatites.
Editorial responsibility: W. Schreyer 相似文献