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1.
We present trace element and Sr–Nd–Hf–Pb 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 Th–U–LILE–LREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREE–HREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 5–10% melt, whereas theenrichments in Th–U–LILE–LREE 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 1·4–17·5; 206Pb/204Pb 17·2–20·4;Hf 0·6–164·6) 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 Sr–Nd–Pb isotopes, LILE–LREE 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 600–900 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 Lu–Hf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The ‘resistance’ of the Lu–Hf 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 Proterozoic–late 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; Sr–Nd–Hf–Pb isotopes  相似文献   

2.
The effects of source composition and source evolution duringprogressive partial melting on the chemistry of mantle-derivedmid-ocean ridge basalt (MORB) melts were tested using a comprehensivegeochemical and Sr–Nd–Pb isotopic dataset for fresh,magnesian basaltic glasses from the Miocene Macquarie Islandophiolite, SW Pacific. These glasses: (1) exhibit clear parent–daughterrelationships; (2) allow simple reconstruction of primary meltcompositions; (3) show exceptional compositional diversity (e.g.K2O/TiO2 0·09–0·9; La/Yb 1·5–22;206Pb/204Pb 18·70–19·52); (4) preserve changesin major element and isotope compositions, which are correlatedwith the degree of trace element enrichment (e.g. La/Sm). Conventionalmodels for MORB genesis invoke melting of mantle that is heterogeneouson a small scale, followed by binary mixing of variably lithophileelement-enriched melt batches. This type of model fails to explainthe compositions of the Macquarie Island glasses, principallybecause incompatible element ratios (e.g. Nb/U, Sr/Nd) and Pbisotope ratios vary non-systematically with the degree of enrichment.We propose that individual melt batches are produced from instantaneous‘parental’ mantle parageneses, which change continuouslyas melting and melt extraction proceeds. This concept of a ‘dynamicsource’ combines the models of small-scale mantle heterogeneitiesand fractional melting. A dynamic source is an assemblage oflocally equilibrated mantle solids and a related melt fraction.Common MORB magmas that integrate the characteristics of numerousmelt batches therefore tend to conceal the chemical and isotopicidentity of a dynamic source. This study shows that isotoperatios of poorly mixed MORB melts are a complex function ofthe dynamic source evolution, and that the range in isotoperatios within a single MORB suite does not necessarily requiremixing of diverse components. KEY WORDS: mid-ocean ridge basalt; Macquarie Island; radiogenic isotopes; mantle; geochemistry  相似文献   

3.
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 Sr–Nd isotopeand trace element compositions. High-temperature anhydrous xenolithsfrom a depth of around 60 km have trace element and Sr–Nd–Pbisotope 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 EM–HIMU 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 (DMM–HIMU–EM).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 Sr–Nd–Pbisotope 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   

4.
The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283,87Sr/86Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar  相似文献   

5.
Basanite intrusions from the Early Cretaceous Erongo complex,Namibia, have compositions consistent with near-primary mantlemelts derived from a depth of at least 100 km. These rocks providea key reference for the mantle component(s) involved in breakup-relatedmagmatism in this region. Initial Sr–Nd–Pb isotoperatios of the Erongo basanites and associated tephrites andphonotephrites (87Sr/86Sr = 0·70425–0·70465;  相似文献   

6.
We present the results of a comprehensive major element, traceelement and Sr–Nd–Pb–O isotopic study of post-glacialvolcanic rocks from the Neovolcanic zones on Iceland. The rocksstudied range in composition from picrites and tholeiites, whichdominate in the main rift systems, to transitional and alkalicbasalts confined to the off-rift and propagating rift systems.There are good correlations of rock types with geochemical enrichmentparameters, such as La/Sm and La/Yb ratios, and with long-termradiogenic tracers, such as Sr–Nd–Pb isotope ratios,indicating a long-lived enrichment/depletion history of thesource region. 87Sr/86Sr vs 143Nd/144Nd defines a negative array.Pb isotopes define well-correlated positive arrays on both 206Pb/204Pbvs 207Pb/204Pb and 208Pb/204Pb diagrams, indicating mixing ofat least two major components: an enriched component representedby the alkali basalts and a depleted component represented bythe picrites. In combined Sr–Nd–Pb isotopic spacethe individual rift systems define coherent mixing arrays withslightly different compositions. The enriched component hasradiogenic Pb (206Pb/204Pb > 19·3) and very similargeochemistry to HIMU-type ocean island basalts (OIB). We ascribethis endmember to recycling of hydrothermally altered upperbasaltic oceanic crust. The depleted component that is sampledby the picrites has unradiogenic Pb (206Pb/204Pb < 17·8),but geochemical signatures distinct from that of normal mid-oceanridge basalt (N-MORB). Highly depleted tholeiites and picriteshave positive anomalies in mantle-normalized trace element diagramsfor Ba, Sr, and Eu (and in some cases also for K, Ti and P),negative anomalies for Hf and Zr, and low 18Oolivine values(4·6–5·0) below the normal mantle range.All of these features are internally correlated, and we, therefore,interpret them to reflect source characteristics and attributethem to recycled lower gabbroic oceanic crust. Regional compositionaldifferences exist for the depleted component. In SW Icelandit has distinctly higher Nb/U (68) and more radiogenic 206Pb/204Pbratios (18·28–18·88) compared with the NErift (Nb/U 47; 206Pb/204Pb = 18·07–18·47).These geochemical differences suggest that different packagesof recycled oceanic lithosphere exist beneath each rift. A thirdand minor component with relatively high 87Sr/86Sr and 207Pb/204Pbis found in a single volcano in SE Iceland (Öræfajökullvolcano), indicating the involvement of recycled sediments inthe source locally. The three plume components form an integralpart of ancient recycled oceanic lithosphere. The slope in theuranogenic Pb diagram indicates a recycling age of about 1·5Ga with time-integrated Th/U ratios of 3·01. Surprisingly,there is little evidence for the involvement of North AtlanticN-MORB source mantle, as would be expected from the interactionof the Iceland plume and the surrounding asthenosphere in formof plume–ridge interaction. The preferential samplingof the enriched and depleted components in the off-rift andmain rift systems, respectively, can be explained by differencesin the geometry of the melting regions. In the off-rift areas,melting columns are truncated deeper and thus are shorter, whichleads to preferential melting of the enriched component, asthis starts melting deeper than the depleted component. In contrast,melting proceeds to shallower depths beneath the main rifts.The longer melting columns also produce significant amountsof melt from the more refractory (lower crustal/lithospheric)component. KEY WORDS: basalts; trace element and Sr, Nd, Pb, O isotope geochemistry; Iceland plume; isotope ratios; oceanic crustal recycling; partial melting; plume–ridge interaction  相似文献   

7.
Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany   总被引：5，自引：0，他引：5  
Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, Fe–Ti oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSr–Nd–Pb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The Nd–Sr–Pbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotite–spinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany  相似文献   

8.
Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic 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 Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects 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 Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd 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   

9.
Petrogenetic models for the origin of lamproites are evaluatedusing new major element, trace element, and Sr, Nd, and Pb isotopedata for Holocene lamproites from the Gaussberg volcano in theEast Antarctic Shield. Gaussberg lamproites exhibit very unusualPb isotope compositions (206Pb/204Pb = 17·44–17·55and 207Pb/204Pb = 15·56–15·63), which incommon Pb isotope space plot above mantle evolution lines andto the left of the meteorite isochron. Combined with very unradiogenicNd, such compositions are shown to be inconsistent with an originby melting of sub-continental lithospheric mantle. Instead,a model is proposed in which late Archaean continent-derivedsediment is subducted as K-hollandite and other ultra-high-pressurephases and sequestered in the Transition Zone (or lower mantle)where it is effectively isolated for 2–3 Gyr. The high207Pb/204Pb ratio is thus inherited from ancient continent-derivedsediment, and the relatively low 206Pb/204Pb ratio is the resultof a single stage of U/Pb fractionation by subduction-relatedU loss during slab dehydration. Sr and Nd isotope ratios, andtrace element characteristics (e.g. Nb/Ta ratios) are consistentwith sediment subduction and dehydration-related fractionation.Similar models that use variable time of isolation of subductedsediment can be derived for all lamproites. Our interpretationof lamproite sources has important implications for ocean islandbasalt petrogenesis as well as the preservation of geochemicallyanomalous reservoirs in the mantle. KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted sediment; anomalous mantle reservoirs  相似文献   

10.
A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± Fe–Ti oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na2O and SiO2 variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 30–40% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: 40Ar/39Ar ages; intraplate volcanism; mantle plume; partial melting; Syria  相似文献   

11.
Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,Sr–Nd–Pb-isotope analysis and laser 40Ar/39Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8Ma), Si–K-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·3–0·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the Si–K-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. Izu–Bonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. Si–K-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways  相似文献   

12.
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   

13.
Major and trace element and Sr–Nd–Pb isotopic variationsin mafic volcanic rocks hve been studied in a 220 km transectacross the Kamchatka arc from the Eastern Volcanic Front, overthe Central Kamchatka Depression to the Sredinny Ridge in theback-arc. Thirteen volcanoes and lava fields, from 110 to 400km above the subducted slab, were sampled. This allows us tocharacterize spatial variations and the relative amount andcomposition of the slab fluid involved in magma genesis. TypicalKamchatka arc basalts, normalized for fractionation to 6% MgO,display a strong increase in large ion lithophile, light rareearth and high field strength elements from the arc front tothe back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constantacross the arc, which suggests a similar fluid input for Baand Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc.Rocks from the Central Kamchatka Depression range in 87Sr/86Srfrom 0·70334 to 0·70366, but have almost constantNd isotopic compositions (143Nd/144Nd 0·51307–0·51312).This correlates with the highest U/Th ratios in these rocks.Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like butdecrease slightly from the volcanic front to the back-arc. Theinitial mantle source ranged from N-MORB-like in the volcanicfront and Central Kamchatka Depression to more enriched in theback-arc. This enriched component is similar to an ocean-islandbasalt (OIB) source. Variations in (CaO)6·0–(Na2O)6·0show that degree of melting decreases from the arc front tothe Central Kamchatka Depression and remains constant from thereto the Sredinny Ridge. Calculated fluid compositions have asimilar trace element pattern across the arc, although minordifferences are implied. A model is presented that quantifiesthe various mantle components (variably depleted N-MORB-mantleand enriched OIB-mantle) and the fluid compositions added tothis mantle wedge. The amount of fluid added ranges from 0·7to 2·1%. The degree of melting changes from  相似文献   

14.
The Hawaiian–Emperor Seamount Chain (ESC), in the northernPacific Ocean, was produced during the passage of the PacificPlate over the Hawaiian hotspot. Major and trace element concentrationsand Sr–Nd–Pb isotopic compositions of shield andpost-shield lavas from nine of the Emperor Seamounts providea 43 Myr record of the chemistry of the oldest preserved Hawaiianmagmatism during the Late Mesozoic and Early Cenozoic (from85 to 42 Ma). These data demonstrate that there were large variationsin the composition of Hawaiian magmatism over this period. Tholeiiticbasalts from Meiji Seamount (85 Ma), at the northernmost endof the ESC, have low concentrations of incompatible trace elements,and unradiogenic Sr isotopic compositions, compared with youngerlavas from the volcanoes of the Hawaiian Chain (<43 Ma).Lavas from Detroit Seamount (81 Ma) have highly depleted incompatibletrace element and Sr–Nd isotopic compositions, which aresimilar to those of Pacific mid-ocean ridge basalts. Lavas fromthe younger Emperor Seamounts (62–42 Ma) have trace elementcompositions similar to those of lavas from the Hawaiian Islands,but initial 87Sr/86Sr ratios extend to lower values. From 81to 42 Ma there was a systematic increase in 87Sr/86Sr of boththoleiitic and alkalic lavas. The age of the oceanic lithosphereat the time of seamount formation decreases northwards alongthe Emperor Seamount Chain, and the oldest Emperor Seamountswere built upon young, thin lithosphere close to a former spreadingcentre. However, the inferred distance of the Hawaiian plumefrom a former spreading centre, and the isotopic compositionsof the oldest Emperor lavas appear to rule out plume–ridgeinteraction as an explanation for their depleted compositions.We suggest that the observed temporal chemical and isotopicvariations may instead be due to variations in the degree ofmelting of a heterogeneous mantle, resulting from differencesin the thickness of the oceanic lithosphere upon which the EmperorSeamounts were constructed. During the Cretaceous, when theHawaiian plume was situated beneath young, thin lithosphere,the degree of melting within the plume was greater, and incompatibletrace element depleted, refractory mantle components contributedmore to melting. KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity  相似文献   

15.
The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H2O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO2 conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New 40Ar/39Ar ages for Mts Oku and Bambouto basalts,combined with previous 40Ar/39Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;40Ar/39Ar geochronology; lithospheric and asthenospheric mantle source; hotspot  相似文献   

16.
The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)N (12–47). GroupA has slightly higher 87Sr/86Sr, similar Nd, and lower 206Pb/204Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high 238U/204Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources  相似文献   

17.
Geochronological (K–Ar or 40Ar/39Ar), major and traceelement, Sr–Nd–Pb isotopic and mineral chemicaldata are presented for newly discovered Cenozoic volcanic rocksin the western Qiangtang and central Lhasa terranes of Tibet.Alkali basalts of 65–45 Ma occur in the western Qiangtangterrane and represent primitive mantle melts as indicated byhigh mg-numbers [100 x Mg/(Mg + Fe)] (54–65), Cr (204–839ppm) and Ni (94–218 ppm) contents, and relatively lowratios of 87Sr/86Sr (0·7046–0·7061), 206Pb/204Pb(18·21–18·89), 207Pb/204Pb (15·49–15·61)and 208Pb/204Pb (38·42–38·89), and highratios of 143Nd/144Nd (0·5124–0·5127). Incontrast, younger volcanic rocks in the western Qiangtang terrane(  相似文献   

18.
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 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ 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 Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-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 Zr–Hf 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. Ti–Al–Fe-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, melt–wall-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   

19.
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 Sr–Nd–Pb 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 Nd–Sr isotopic signatures. Thesewere followed by voluminous 29–25 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 Sr–Nd isotopic signatures. Their source canbe explained by mixing EM1–Tristan da Cunha-like and depletedmantle components with subduction-related components. Post-plateau24–17 Ma alkaline flows with steep REE patterns, highincompatible element abundances, and depleted Sr–Nd 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   

20.
New high-precision Pb–Sr–Nd isotope, major and traceelement and mineral chemistry data are presented for the submarinestage of ocean island volcanism on Santiago, one of the southernislands of the Cape Verde archipelago. Pillow basalts and hyaloclastitesin the Flamengos Valley are divided into three petrographicand compositional groups; the Flamengos Formation lavas (4·6Ma) dominate the sequence, with the younger Low Si and Coastalgroups (2·8 Ma) found near the shoreline. Olivine andclinopyroxene compositions and isotopic data for minerals andtheir host melts indicate disequilibrium between some crystalsand the melt. Intra-sample disequilibrium suggests homogenisationof liquids but eruption before complete equilibration betweencrystals and melt preserves the heterogeneity. Pressures ofcrystallization for clinopyroxene (0·4–1·1GPa) indicate stalling and crystallization of the magmas overa range of depths in the lithosphere. Major element compositionsindicate melting of a carbonated eclogite source. Sr–Nd–Pbisotope data suggest the involvement of FOZO-like and EM1-likecomponents in the mantle source, which are simultaneously availableat all depths in the melting column. The Flamengos Valley lavasdisplay large compositional variations, often between stratigraphicallyadjacent flows; these frequent abrupt changes of magma compositionsuggest stalling and crystallization of discrete magma batcheson transport through the lithosphere. KEY WORDS: Cape Verde; crystal–melt disequilibrium; submarine volcanism; source heterogeneity; Pb–Sr–Nd isotopes  相似文献   

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