Lithospheric Mantle Evolution beneath the Eifel (Germany): Constraints from Sr-Nd-Pb Isotopes and Trace Element Abundances in Spinel Peridotite and Pyroxenite Xenoliths

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     

Major Element Records of Variable Plume Involvement in the North Atlantic Province Tertiary Flood Basalts

Major element variations in North Atlantic Tertiary Provinceprimitive, early erupted, alkaline-transitional-tholeiite basalts,recalculated to a restricted value of MgO, give insights intothe process of plume-related magmatism. Basalts primitive enoughto be crystallizing only olivine were recalculated to a proposedprimary magma composition of 15 wt % MgO. The recalculated datasetshows clear inter-element correlations including a strong, significant,negative correlation between Fe and Si indicating polybaricmelt segregation. Overlap between basalt compositions and experimentalmelts from a fertile, Fe-rich, low mg-number (85.5) peridotitesuggests that, relative to normal peridotite with mg-number> 89, the North Atlantic basalt source was Fe rich. Linearregression of the experimental data gives apparent pressuresof magma segregation of 17·5–37 kbar, with intra-regionvariability in the depth derivation from the melt column foreach sample, thus suggesting that lithospheric thickness ‘lid-effect’control on magma generation may have been overemphasized inrecent studies. Comparable source composition, magma segregationdepth and calculated mantle potential temperature (1440–1460°C)throughout the Province supports the previously suggested plumeimpact model, arriving below East Greenland, derived from avariably enriched and depleted lower-mantle source. Given thegood agreement between conclusions drawn from major elementdata and previously published results we suggest that restricted-MgOrecalculated datasets may be usefully applied to study otherlarge igneous provinces. KEY WORDS: basalt; Fe-rich mantle; large igneous provinces; North Atlantic Tertiary Province; restricted-MgO major element datasets     

Sr-Nd-Pb Isotopic Compositions of Peridotite Xenoliths from Spitsbergen: Numerical Modelling Indicates Sr-Nd Decoupling in the Mantle by Melt Percolation Metasomatism

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     

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

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     

Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs

An ⁴⁰Ar/³⁹Ar age of 45·1 Ma determined for lavas fromnorthern Saipan confirms that these high-silica rhyolites eruptedduring the ‘proto-arc’ stage of volcanism in theIzu–Bonin–Mariana system, which is characterizedelsewhere by eruption of boninitic lavas. Incompatible traceelement concentrations and Sr, Hf, Nd, and Pb isotope ratiosfor these rhyolites are transitional between those of c. 48Ma boninitic lavas and post-38 Ma ‘first-arc’ andesitesand dacites from Saipan and Rota that have typical subduction-relatedcompositions. These transitional compositions are modeled bycrystal fractionation of parental tholeiitic basalt combinedwith assimilation of young boninitic crust. A second stage ofRayleigh fractionation in the upper crust is required by SiO₂concentrations that exceed 77 wt % and near-zero compatibleelement concentrations. First-arc magma compositions are consistentwith fractionation of basalt and assimilation of crust similarin composition to the first-arc magmas themselves. The mantlesources of the proto-arc and first-arc lavas from Saipan andRota are similar to those of Philippine back-arc basin basaltsbased on Nd and Hf isotopic compositions. The Pb isotope compositionsof these lavas are between those of Pacific sea-floor basaltsand Jurassic and younger cherty and clay-rich sediments. Thiscontrasts with the boninitic proto-arc volcanic rocks from Guamand Deep Sea Drilling Project Sites 458 and 459 that have Pbisotope compositions similar to Pacific basin basalts and volcaniclasticsediments. The preferred explanation for the difference in thenature of proto-arc volcanism between Saipan and other fore-arclocations is that the crust ceased extending 3–4 Myr earlierbeneath Saipan. This was caused by a change from mantle upwelling,fore-arc extension, and shallow melting to an environment dominatedby more normal mantle wedge convection, stable crust, and deepermelting. KEY WORDS: rhyolite; andesite; Mariana arc; isotope ratios; trace elements     

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

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 ¹⁸O_olivine 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     

An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting

Dykes of the East Greenland Tertiary dyke swarm can be dividedinto pre- and syn-break-up tholeiitic dykes, and post-break-uptransitional dykes. Of the pre- and syn-break-up dykes, themost abundant group (Tholeiitic Series; TS) has major elementcompositions similar to the main part of the East Greenlandflood basalts. A group of high-MgO tholeiitic dykes (Picrite–AnkaramiteSeries; PAS) are much less common, and are equivalent to someof the oldest lavas of the East Greenland flood basalts. Isotopiccompositions of the TS and PAS dykes partly overlap with thosefor Iceland, but Pb isotopic compositions extend to less radiogenicvalues than those seen in either Iceland or North Atlantic mid-oceanridge basalt (MORB). The isotopically depleted source requiredto account for this isotopic variation is interpreted as subcontinentallithospheric mantle with low ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and high     

Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity

New Pb isotopic data are presented for 10 young Mesozoic toCenozoic (0–116 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 core–mantle 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     

Do We Really Need Mantle Components to Define Mantle Composition?

We discuss the concept of components in the Earth's mantle startingfrom a petrological and geochemical approach, but adopting anew method of projection of geochemical and isotopic data. Thisallows the compositional variability of magmatic associationsto be evaluated in multi-dimensional space, thus simultaneouslyaccounting for a large number of compositional variables. Wedemonstrate that ocean island basalts (OIB) and mid-ocean ridgebasalts (MORB) are derived from a marble-cake mantle, in whichdifferent degrees of partial melting of recycled lithosphere,which are heterogeneous in age and composition, contribute tothe magma genesis. This view is supported by the variabilityin the geochemical and isotopic signatures of OIB that are observedon the scale of a single ocean island as well as on that ofan ocean, mostly varying between two extreme compositions, thatare not strictly related to the commonly accepted mantle components(DMM, EMI, EMII, HIMU). Rather they are a distinctive featureof the mantle source sampled at each ocean island and are stronglydependent on the Pb isotope system. We recommend a change inperspective in studies of MORB–OIB geochemistry from onebased on physically distinct mantle components to a model basedon the existence of a marble-cake-like upper mantle. Althoughresembling the statistical upper mantle, this model impliesthat geochemical homogenization can be attained only withinthe limits of local mantle composition, so that a world-wideuniform depleted reservoir cannot be sampled by simply extendingthe volume of the region undergoing partial melting. KEY WORDS: geochemistry; isotope; mantle; OIB     

Mantle-melt Evolution (Dynamic Source) in the Origin of a Single MORB Suite: a Perspective from Magnesian Glasses of Macquarie Island

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     

Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km². It has an ⁴⁰Ar/³⁹Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO₂ (2·2wt %) and total iron (14 wt % as Fe₂O₃), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar     

Signatures of the source for the Emeishan flood basalts in the Ertan area： Pb isotope evidence

The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.     

Mantle Preconditioning by Melt Extraction during Flow: Theory and Petrogenetic Implications

Mantle preconditioning may be defined as the extraction of smallmelt fractions from mantle asthenosphere during its flow tothe site of magma generation. Equations may be written for mantlepreconditioning, assuming that the mantle comprises enriched‘plums’ in a depleted matrix. The equations takeinto account variations in mass fraction of plums, the relativerate of melting of plums and matrix, the temperature and pressureof melt extraction, the mass fraction of melt extracted, theextent of chemical exchange between plums and matrix, and theefficiency of melt extraction. Monitoring mineralogical changesand variations in partition coefficients along the inferredP–T–t path of the mantle asthenosphere allows theequations to be correctly applied to the conditions under whichmelt extraction takes place. Numerical experiments demonstratethe influence of petrogenetic variables on the shape of meltextraction trajectories and provide new criteria for distinguishingbetween melt extraction and mixing as the cause of regionalgeochemical gradients. Representative examples of arc–back-arcsystems (Scotia), continental break-up (Afar) and plume–ridgeinteraction (Azores) indicate that the compositions of the mantlesources of mid-ocean ridge basalts and island arc basalts maybe determined, at least in part, by the melt extraction historiesof their asthenospheric sources. KEY WORDS: geochemical modelling; mantle flow; isotope ratios; trace elements     

The Mesozoic to Early Cenozoic Magmatism of the Benue Trough (Nigeria); Geochemical Evidence for the Involvement of the St Helena Plume

The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of ⁴⁰ Ar-³⁹Ar ages: 147–106 Ma, 97–81 Ma and 68–49 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=7–11.1; La/Nb = 0.77–1; ⁸⁷Sr/⁸⁶Sr; =0.7042–0.7065¹⁴³Nd/¹⁴⁴Nd;_i = 0.5125–0.5127; ²⁰⁶Pb/²⁰⁴Pb_i = 17.59–18.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.6–6.8;La/Nb=0.53–0.66; ⁸⁷Sr/⁸⁶ Sr_i=0.7029–0.7037; ¹⁴³Nd/¹⁴⁴Nd_i=0.5126–0.5129;²⁰⁶Pb/²⁰⁴Pb_i = 18.54–20.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume ^*;Corresponding author.     

Petrology and Geochemistry of West Philippine Basin Basalts and Early Palau-Kyushu Arc Volcanic Clasts from ODP Leg 195, Site 1201D: Implications for the Early History of the Izu-Bonin-Mariana Arc

Site 1201D of Ocean Drilling Program Leg 195 recovered basalticand volcaniclastic units from the West Philippine Basin thatdocument the earliest history of the Izu–Bonin–Marianaconvergent margin. The stratigraphic section recovered at Site1201D includes 90 m of pillow basalts, representing the WestPhilippine Basin basement, overlain by 459 m of volcaniclasticturbidites that formed from detritus shed from the Eocene–Oligoceneproto-Izu–Bonin–Mariana island arc. Basement basaltsare normal mid-ocean ridge basalt (N-MORB), based on their abundancesof immobile trace elements, although fluid-mobile elements areenriched, similar to back-arc basin basalts (BABB). Sr, Nd,Pb and Hf isotopic compositions of the basement basalts aresimilar to those of basalts from other West Philippine Basinlocations, and show an overall Indian Ocean MORB signature,marked by high ²⁰⁸Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb and high¹⁷⁶Hf/¹⁷⁷Hf for a given ¹⁴³Nd/¹⁴⁴Nd. Trace element and isotopicdifferences between the basement and overlying arc-derived volcaniclasticsare best explained by the addition of subducted sediment orsediment melt, together with hydrous fluids from subducted oceaniccrust, into the mantle source of the arc lavas. In contrastto tectonic models suggesting that a mantle hotspot was a sourceof heat for the early Izu–Bonin–Mariana arc magmatism,the geochemical data do not support an enriched, ocean islandbasalt (OIB)-like source for either the basement basalts orthe arc volcanic section. KEY WORDS: back-arc basalts; Izu–Bonin–Marianas; Philippine Sea; subduction initiation; Ocean Drilling Program Leg 195     

Chemical Diversity of the Ueno Basalts, Central Japan: Identification of Mantle and Crustal Contributions to Arc Basalts

The Ueno Basalts of central Japan comprise a monogenetic volcaniccone complex that was active between 2·76 and 1·34Ma. Basalts were erupted at more than 14 centers scattered overa region 40 km in diameter. Alkali basalt was erupted first,followed by sub-alkaline basalt. Quasi-concentric expansionof eruption centers coinciding with uplift and with decreasingalkalinity of the lavas suggests that Ueno magmatism originatedfrom a mantle diapir as it mushroomed at the base of the lithosphere.Depleted asthenospheric mantle (alkali basalt), enriched lithosphericmantle (sub-alkaline basalt), and crustal components are identifiedas chemical end-members in the petrogenesis of the Ueno Basalts.Incompatible trace element abundances indicate that the Uenoalkali basalts are typical within-plate basalts, whereas thesub-alkaline basalts show strong affinities with normal arclavas. Sr–Nd–Pb isotopic compositions indicate thatthe mantle source of the alkali basalts was more depleted thanthat of the sub-alkaline basalts. About 7% melting of asthenosphericmantle in the garnet-lherzolite stability field produced theprimitive alkali basalts and 12% melting of spinel lherzolitewithin the subcontinental lithosphere produced the primitivesub-alkaline basalts. Isotopic compositions and fluid mobile/immobileelement ratios broadly covary with SiO₂ contents in the sub-alkalinesuite, and increasing silica content is associated with strongerEMII (Enriched Mantle II) isotope affinities and fluid mobileelement abundances. A progressive AFC (assimilation–fractionalcrystallization) model assuming assimilation of a low-K silicicmelt reproduces the chemical variations observed in the sub-alkalinesuite. Melting of a flattening mantle diapir at the base ofthe lithosphere is the dominant cause of Ueno magmatism, accompaniedby the assimilation of older arc crust. KEY WORDS: arc basalt; crustal assimilation; mantle heterogeneity; Ueno Basalts     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Rapid Change of Lava Composition from 1998 to 2002 at Piton de la Fournaise (Reunion) Inferred from Pb Isotopes and Trace Elements: Evidence for Variable Crustal Contamination

After an unusually long quiet period of nearly 6 years, in 1998the Piton de la Fournaise volcano started a new cycle of intensevolcanic activity. We report geochemical data on the first nineevents (53 samples), from the long-lived initial eruption (sixand a half months) of 1998 to the high-flux picritic eruptionof January 2002. Pb isotopes and trace elements display systematic,coupled variations, which are mostly confined to the beginningand the end of the period. Two well-defined binary mixing trendsare shown by Pb–Pb and Pb–trace element relationships.These trends indicate a change of end-member components betweenMarch and June 2001 that coincides with the transition fromsteady-state basalts to picrites. A three-component mixing modelinvolving a homogeneous plume and two contaminants successfullyexplains the data. The Pb–Pb relationship requires thattwo mixing processes occur successively: plume-derived magmainteracts first with altered oceanic crust, and the resultinghybrid product then interacts at shallower level with the oldlavas constituting the base of the volcanic edifice. Assimilationof edifice material decreased continuously from 1998 to 2002,whereas assimilation of oceanic crust drastically increasedduring the late-stage picritic eruption. These results suggestthat picrites may have resided for an unusually long time atan oceanic crustal level before ascending rapidly through thevolcanic edifice with little interaction with channel walls. KEY WORDS: assimilation; lead isotopes; picrites; Piton de la Fournaise; trace elements     

Carbonatite Magmatism and Plume Activity: Implications from the Nd, Pb and Sr Isotope Systematics of Oldoinyo Lengai

New Nd (0.51261–0.51268), Pb (²⁰⁶Pb/²⁰⁴Pb: 19.24–19.26),and Sr (0.70437–0.70446) isotopic compositions from tennatrocarbonatite lavas, collected in June 1993 from OldoinyoLengai, the only known active carbonatite volcano, are relativelyuniform, and are similar to data from the 1960 and 1988 flows.Three of the samples contain silicate spheroids, one of whichhas Nd and Sr isotopic ratios similar to host natrocarbonatite,consistent with an origin by liquid immiscibility or the mixingof melts with similar isotopic compositions. Pb isotope datafor two samples of trona are inconsistent with its involvementin the genesis of natrocarbonatite. New Pb isotope data fromsilicate volcanic and plutonic blocks (ijolite, nephelinite,phonolite, syenite) from Oldoinyo Lengai are highly variable(²⁰⁶Pb/²⁰⁴Pb, 17.75–19.34; ²⁰⁷Pb/²⁰⁴Pb, 15.41–15.67;²⁰⁸Pb/²⁰⁴Pb, 37.79–39.67), and define near-linear arraysin Pb-Pb diagrams. The isotopic data for the silicate rocksfrom Oldoinyo Lengai are best explained by invoking discretepartial melting events which generate undersaturated alkalinesilicate magmas with distinct isotopic ratios. Pb isotope ratiosfrom most ijolites and phonolites are predominantly lower andmore variable than from the natrocarbonatites, and are attributedto interaction between silicate melts involving HIMU and EMIsource components and an additional component, such as lower-crustalgranulites, DMM or PREMA (prevalent mantle). Variations in Nd,Pb and Sr isotope ratios from Oldoinyo Lengai, among the largestyet documented from a single volcano, are attributed to mantlesource heterogeneity involving mainly the mixing of HIMU andEMI mantle components. Based on the new isotopic data from OldoinyoLengai and data from other East African carbonatites, and mantlexenoliths, we propose a two-stage model in an attempt to explainthe isotope variations shown by carbonatites in this area. Themodel involves (I) the release of metasomatizing agents withHIMU-like signatures from upwelling mantle (‘plume’)source, which in turn metasomatize the sub-continental (old,isotopically enriched, EMI-like) lithosphere, and (2) variabledegrees and discrete partial melting of the resulting heterogeneous,metasomatized lithosphere. KEY WORDS: carbonatite; isotopes; Oldoinyo Lengai; mantle plumes ^*Telephone: (613) 788–2660, ext. 4419. Fax: (613) 788–4490. e-mail: kbell{at}ccs.carleton.ca     

Petrogenesis and Geodynamic Implications of Late Cenozoic Basalts in North Queensland, Australia: Trace-element and Sr-Nd-Pb Isotope Evidence

Radiogenic isotopic (Sr–Nd–Pb) and trace-elementcompositions of late Cenozoic basalts from two discrete geographicalregions in North Queensland, Australia, can be used to identifycontributions from geochemically distinctive mantle source components.The North Queensland basalts have positive