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1.
《International Geology Review》2012,54(12):1456-1474
We present new major element, trace element, and Sr–Nd–Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e.g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr–Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher 87Sr/86Sr for a given 143Nd/144Nd ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.  相似文献   

2.
The Cameroon line comprises a 1600-km long Y-shaped chain of< 30 m.y. old volcanoes and <70 m.y. old plutons extendinginto mainland Africa from the Atlantic island of Pagalu. Thedistribution of basaltic volcanic centres is ideal for comparingsub-continental and sub-oceanic sources for basalts and forstudying the influence of the lithosphere on magma generation.We report Nd, Sr, Pb and O isotopic data for more than thirty(principally basaltic) samples from all the main volcanic centrestogether with data for two granulite facies xenoliths. Thosebasalts which display no obvious evidence of crustal contaminationyield initial 87Sr/86Sr ratios ranging from 0.7029 to 0.7035,Nd between +2 and +7 and 206Pb/204Pb between 19?0 and 20?6.The Nd and Sr isotopic compositions define a field on the lefthand side of the ‘mantle array’ (that is with relativelyunradiogenic Sr) and include some data which show overlap withcompositions observed for St. Helena. 208Pb/204Pb ratios extendto 40?4—amongst the more radiogenic observed for alkalibasalts. The Nd and Sr isotopic data are similar in oceanicand continental sectors indicating that the magmas are derivedfrom generally similar mantle sources. Despite this overallsimple picture, the source of the Cameroon line volcanics hasin fact been variable in both time and space. Pb is less radiogenicand Sr is more radiogenic in transitional to hypersthene-normativecompositions. There is a progression to more radiogenic leadisotopic compositions with time for the Cameroon line as a wholethat is most strikingly displayed in the 30 m.y. eruptive historyof Principe. These space-time data are difficult to reconcilewith conventional plume models or with some dispersed ‘plumpudding’ models. The heterogeneities require isolationtimes considerably longer than the age of the south Atlanticsea floor (120 Ma). The eruptive lavas with the most radiogenicPb observed (accompanied by unradiogenic Nd) precisely straddlethe continental edge (i.e. occur in both oceanic and continentalsectors) with no dependency on Nd and Pb concentrations. A modelis proposed which links these observations with the destructionof lithosphere, and the impregnation of the uppermost mantleby the St. Helena hot spot during the formation of the SouthAtlantic ocean. This mantle was subsequently melted to formthe Cameroon line which appears to be derived from a risinghot zone initiated by the early plume activity. The magmaticproducts reflect the mantle mixing that took place during continentalbreakup, the consequent cooling and thickening of the lithosphereand the continued interaction between rising plume componentsand this lithosphere. The depth from which magmas are currentlybeing tapped at the continent/ocean boundary is estimated atless that 150 km.  相似文献   

3.
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/204Pbi = 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 Sri=0.7029–0.7037; 143Nd/144Ndi=0.5126–0.5129;206Pb/204Pbi = 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.  相似文献   

4.
Eclogite xenoliths from the Colorado Plateau, interpreted asfragments of the subducted Farallon plate, are used to constrainthe trace element and Sr–Nd–Pb isotopic compositionsof oceanic crust subducted into the upper mantle. The xenolithsconsist of almandine-rich garnet, Na-clinopyroxene, lawsoniteand zoisite with minor amounts of phengite, rutile, pyrite andzircon. They have essentially basaltic bulk-rock major elementcompositions; their Na2O contents are significantly elevated,but K2O contents are similar to those of unaltered mid-oceanridge basalt (MORB). These alkali element characteristics areexplained by spilitization or albitization processes on thesea floor and during subduction-zone metasomatism in the fore-arcregion. The whole-rock trace element abundances of the xenolithsare variable relative to sea-floor-altered MORB, except forthe restricted Zr/Hf ratios (36·9–37·6).Whole-rock mass balances for two Colorado Plateau eclogite xenolithsare examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rareearth elements, Pb, Th and U. Mass balance considerations andmineralogical observations indicate that the whole-rock chemistriesof the xenoliths were modified by near-surface processes afteremplacement and limited interaction with their host rock, aserpentinized ultramafic microbreccia. To avoid these secondaryeffects, the Sr, Nd and Pb isotopic compositions of mineralsseparated from the xenoliths were measured, yielding 0·70453–0·70590for 87Sr/86Sr, –3·1 to 0·5 for Nd and 18·928–19·063for 206Pb/204Pb. These isotopic compositions are distinctlymore radiogenic for Sr and Pb and less radiogenic for Nd thanthose of altered MORB. Our results suggest that the MORB-likeprotolith of the xenoliths was metasomatized by a fluid equilibratedwith sediment in the fore-arc region of a subduction zone andthat this metasomatic fluid produced continental crust-likeisotopic compositions of the xenoliths. KEY WORDS: Colorado Plateau; eclogite xenolith; geochemistry; subducted oceanic crust  相似文献   

5.
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   

6.
Augustine Volcano, a Quaternary volcanic centre of the easternAleutian Arc, produces predominantly andesites and dacites oflow- to medium-K calc-alkaline composition. Mineralogical andmajor element characteristics of representative lavas suggestthat magmatic evolution has been influenced by both crystalfractionation and magma-mixing processes. However, incompatibletrace element variations (e.g. K/Rb) indicate that these evolvedlavas have been contaminated by the mafic arc crust of the underlyingTalkeetna accreted terrane. The limited range of isotope compositionsalso supports the assimilation of non-radiogenic mafic crust(e.g. 87Sr/86Sr = 0.7032–0.7034; 143Nd/144 Nd = 0.51301–0.5130).In addition, Pb-isotope compositions parallel the North Pacificmean oceanic trend (206Pb/204 Pb = 18.3–18.8; 207Pb/204Pb= 15.5–15.6; 208Pb/204Pb = 38.2–38.3) and do notrequire a subducted sediment component in the source. Relativelyhigh (Ba/La) N (0.79–18.10) and B/Be (14.5) ratios do,however, suggest a metasomatic fluid component derived fromthe dehydration of the subducting plate. The thickened continental crust (35 km) of the eastern AleutianArc prevents the ascent of basaltic melts, which fractionateand assimilate at various depths to produce andesitic magmas.These andesites evolve towards more silicic compositions byfractional crystallization. The absence of evidence for a largehigh-level crustal magma chamber implies that the magmatic systembeneath the volcano is young and at an immature stage of evolution. KEY WORDS: Augustine Volcano; Aleutians; assimilation; melasomatism; geochemistry *Corresponding author. Present address: Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA  相似文献   

7.
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   

8.
Petrological and geochemical studies performed with invoking data on the compositions of clinopyroxenes have clarified the conditions of formation of Vendian-Cambrian basaltic complexes in the Dzhida zone of the Paleoasian Ocean (northern Mongolia and southwestern Transbaikalia). The research was based on a comparative analysis with reference igneous basaltic associations. Of special importance are our microprobe data on trace and rare-earth elements in clinopyroxenes from igneous rocks of different present-day geodynamic settings, namely, N-MORB (Mid-Atlantic Ridge, Central Atlantic), OIB (Bouvet Island, South Atlantic), WPB (within-plate tholeiitic plateau basalts of the Siberian Platform), and boninites of ensimatic arcs (Izu-Bonin island arc, Pacific). The studies have shown that the paleo-oceanic structures in the district of the Urgol guyot formed during geodynamic processes under the impact of mantle plumes on oceanic spreading crust, which resulted in oceanic basaltic plateaus and within-plate oceanic islands. All these structures were later superposed by typical island-arc structure-lithologic associations. Formation of basalt complexes in the Dzhidot guyot district proceeded with a stronger effect of enriched plume melts of within-plate oceanic islands as compared with the Urgol guyot. This is evidenced from petrochemical and geochemical data showing the development of OIB-type magmatic systems on the oceanic basement. Data on clinopyroxenes confirm the participation of mantle plume in this process, which led to the evolution of magmas from typical oceanic basalts (MORB) to plateau basalts and OIB.  相似文献   

9.
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   

10.
The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

11.
Lavas from Heard Island, located on the Kerguelen Plateau inthe southern Indian Ocean, exhibit the largest range (e.g.,87Sr/86Sr=0.7047–0.7079) of isotopic compositions yetobserved on a single oceanic island. Isotopic compositions arewell correlated and are accompanied by systematic changes inincompatible trace element ratios, particularly those involvingNb. These variations are interpreted as resulting from mixingbetween two components. One is characterized by high 87Sr/86Sr,low 206Pb/204Pb and 143Nd/144Nd ratios, and negative Nb andEu anomalies, and is derived ultimately from the upper continentalcrust. The other has lower 87Sr/86Sr, and higher 206Pb/204Pband 143Nd/144Nd ratios, and lacks the depletions in Nb and Eu.Two possible compositions are considered for the low-87Sr/86Srcomponent of the source. The first is at the low-87Sr/86Sr endof the Heard Island data array, represented most closely bylavas from the Laurens Peninsula. However, trace element variationssuggest that these lavas might not be representive of the Heardplume. The second is close to the low-87Sr/86Sr end of the isotopicarray for lavas from the main volcano. In this case a lithosphericmantle origin is suggested for the Laurens Peninsula lavas.The relationships between isotopic data, major element compositions,and incompatible trace element ratios indicate that the continent-derivedmaterial is probably present in the mantle source, where itmakes a maximum contribution of <4 wt.% for all but one HeardIsland sample. However, if the Kerguelen Plateau is a submergedcontinental block, shallow-level contamination cannot be ruledout. The binary mixing model developed to explain the Heard Islandgeochemical variations is extended to include other Indian Oceanoceanic island and mid-ocean ridge basalts (OIB and MORB). Weshow that isotopic compositions of Indian Ocean OIB are consistentwith sampling of a regional reservoir in which the same twocomponents exist in variable proportions (generally 1–5wt.% of the continent-derived component). The distinctive isotopiccompositions of Indian Ocean MORB are consistent with mixingof a similar component into an Atlantic-or Pacific-like MORBmantle source. The relatively unradiogenic 206Pb/204Pb isotopiccompositions of these ‘enriched’ Indian Ocean mantlecomponents are unlike any present-day marine sediments and indicatethat their source has had 238U/204Pb ratios (µ) much lowerthan typical upper continental crust for > 1 Ga. These agespre-date the formation of Gondwana (600-130 Ma) and thereforedo not support sediment subduction beneath Gondwana as the causeof enrichment in the sub-Indian Ocean mantle. We propose thatthe enrichment of Indian Ocean OIB sources was due to subductionof upper-crustal material beneath a Proterozoic precursor ofGondwana at 1–2 Ga. The enrichment of the Indian OceanMORB sources could have had a similar origin, or could havebeen derived from sub-continental lithospheric mantle returnedto the asthenospheric mantle, perhaps during the break-up ofGondwana (200–130 Ma).  相似文献   

12.
Calc-alkaline olivine andesite and two-pyroxene dacite of theTaos Plateau volcanic field evolved in an open magmatic system.mg-numbers of spatially and temporally associated ServilletaBasalt (54–61) and ohvine andesite (49–59) are comparableand preclude fractional crystallization of ferromagnesian mineralsas the major differentiation process. If Servilleta olivinetholeiite is assumed to be the parental magma type, enrichmentsof highly incompatible trace elements (up to 17 ?) oVer concentrationsin the basalts require that andesitic and dacitic magmas containa substantial proportion of assimilated crust. Isotopic compositionsof andesite and dacite, which have slightly higher 87Sr/86Srratios than the basalts but lower 143Nd/144Nd, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, are consistent with contaminationof parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pbcontinental crust. Concentrations of highly incompatible traceelements in andesite and dacite lavas are decoupled from majorelement compositions; the highest concentrat ions of these elementsoccur in andesitic, rather than dacitic compositions, and andesitelavas are more variable in trace element contents. Assimilationof heterogeneous crust concurrent with fractional crystallizationof varying mineral assemblages could cause this decoupled behavior.High mg-numbers in andesite and dacite, skeletal olivine phenocrysts,and reversely zoned pyroxene phenocrysts are manifestationsof mafic replenishment and magma mixing in the Taos Plateaumagmatic system. Taos Plateau volcanoes are monolithologic and are distributedin a semi-concentric zoned pattern that is a reflection of thecomplex subvolcanic magmatic system. A central focus of basaltshields developed above the main basaltic conduit system; thesemagmas contain 10–35% admixed andesitic and dacitic magma.Basalt shields are surrounded by a partial ring of olivine andesiteshield volcanoes, where replenishment of basaltic magma providedthe heat necessary for prolonged assimilation of crust, resultingin intermediate-composition lavas. Dacite shields are locatedaround the periphery of the more mafic volcanoes and reflecta decrease in mafic input on the fringes of the magmatic system.  相似文献   

13.
New thermal ionization mass spectrometry U-series disequilibriumdata are presented for 24 basaltic to dacitic glasses from activespreading centres in the back-arc Lau Basin (SW Pacific), togetherwith additional inductively coupled plasma mass spectrometrytrace element analyses and Sr–Nd–Pb isotope data.Valu Fa Ridge samples, adjacent to the arc front, have highU/Th and (230Th/238U) <1, implying a recent (<<350ka) addition of a U-rich slab-derived fluid. The Valu Fa datacan be combined with existing 230Th–238U data for theCentral Tonga arc to infer a fluid addition event at  相似文献   

14.
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   

15.
We report Sr-, Nd- and Pb-isotopic compositions for the lavasof Mauritius, the second youngest volcanic island in the Réunionhotspot. The lavas of the Older Series (7·8–5·5Ma) have identical isotopic compositions (87Sr/86Sr = 0·70411to 0·70422,143Nd/144Nd = 0·512865 to 0·512854,and 206Pb/204Pb = 19·016 to 19·041) to those ofRéunion, where the center of volcanic activity is currentlylocated. The lavas of the Intermediate Series (3·5–1·9Ma) and Younger Series (0·70–0·17 Ma) areshifted to lower Sr-isotopic compositions (0·70364–0·70394,with 143Nd/144Nd = 0·512813 to 0·512948 and 206Pb/204Pb= 18·794 to 18·984). The Intermediate Series lavashave similar trace-element characteristics (e.g. Zr–Nb,Ba–Y) to those of Rodrigues, in both cases requiring theinvolvement of an enriched mantle-like component in the mantlesource. During the volcanic history of Mauritius, the magmaslost the principal isotopic characteristics of the Réunionhotspot with time, and became gradually imprinted with the isotopicsignature of a shallower mantle source that produced the CentralIndian Ridge basalts. KEY WORDS: hotspot; isotopes; Mauritius; Réunion; trace element  相似文献   

16.
Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their '18O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704-0.708) and lower 143Nd/144Nd (0.5128-0.5124), with higher '18O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in '18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.  相似文献   

17.
Whole-rock geochemical data on basaltic to rhyolitic samplesfrom 12 volcanic centers are used to constrain the role of continentalcrust in the genesis of magmas formed beneath the anomalouslywide subduction-related volcanic arc in Ecuador. Relativelyhomogeneous, mantle-like, isotopic compositions across the arcimply that the parental magmas in Ecuador were produced largelywithin the mantle wedge above the subduction zone and not byextensive melting of crustal rocks similar to those upon whichthe volcanoes were built. Cross-arc changes in 143Nd/144Nd and7/4Pb are interpreted to result from assimilation of geochemicallymature continental crust, especially in the main arc area, 330–360km from the trench. Mixing calculations limit the quantity ofassimilated crust to less than 10%. Most andesites and dacitesin Ecuador have adakite-like trace element characteristics (e.g.Y <18 ppm, Yb <2 ppm, La/Yb >20, Sr/Y >40). Availablewhole-rock data do not provide a clear basis for distinguishingbetween slab-melting and deep crustal fractionation models forthe genesis of Ecuador adakites; published data highlightinggeochemical evolution within individual volcanoes, and in magmaticrocks produced throughout Ecuador since the Eocene, appear tosupport the deep fractionation model for the genesis of mostevolved Ecuadoran lavas. A subset of andesites, which displaya combination of high Sr (>900 ppm), Nd >4·1 and7/4Pb <6·0, appear to be the best candidates amongEcuador lavas for slab-melts associated with the subductionof the relatively young, over-thickened, oceanic crust of theCarnegie Ridge. KEY WORDS: andesite; Ecuador; trace elements; isotopes; adakite  相似文献   

18.
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   

19.
Detailed studies by submersible were carried out in the axialzone of the Red Sea Rift near 18?N during the Soviet Red Seaexpedition of the Oceanological Institute of the Academy ofSciences (December 1979–March 1980). The initial bathymetric,magnetic and seismic surveys established the general organizationof the symmetric tectonic steps (1–3) descending towardsthe axial rift. The 4–5 km wide inner floor of the riftwas explored during 21 dives. It is occupied by 100–300m high, young pillowed volcanoes, isolated or grouped to formelongated hills, frequently cut by open fissures except in thezone of most recent extrusion. The 42 samples collected are typical plagioclase ? olivine ?clinopyroxene ? spinel, more or less porphyritic mid-ocean ridgebasalts whose compositions were mainly controlled by polybaricfractionation of plagioclase, olivine and minor clinopyroxene.They have been separated into porphyritic and sub-aphyric groupsusing modes and mineralogical criteria. Mineral-liquid equilibria,crystal zonation, and modal proportions indicate some magmamixing but probably only of closely related magma batches withineach described group, as can occur inside a single magma chamber.Crystal accumulation is believed to have played a significantrole in only a few porphyritic samples. Three sub-groups (from less to more evolved; (a) FeO*/MgO<1?22;(b) 1?16<FeO*/MgO < 1?48; and (c)FeO*/MgO>1?49) weredistinguished on the basis of glass and whole-rock major elementchemistry. Glass compositions follow the multisaturated cotectic-likecurve for MORB-type basalts and show a general evolution verycomparable to what is observed on the Mid-Atlantic Ridge near36?N, but arc less diverse than in the FAMOUS area. 87Sr/86Sr,143Nd/144Nd, and 206Pb/204Pb data for 4 samples show strongsimilarities to those from the Mid-Atlantic and East PacificRidges, and indicate no continental contamination despite thefact that they have been produced during recent continentalbreak-up and ocean opening. 206Pb/204Pb values, Th/Ta vs. Th/Tbcorrelations, and rare earth element patterns allow recognitionof three different groups of samples, indicating that the RedSea Rift near 18?N is fed by a heterogeneous mantle source.The chondrite-normalized LREE.  相似文献   

20.
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  相似文献   

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