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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 6 (1873), S. 345-352 
    ISSN: 0365-9496
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The two parallel chains of Hawaiian volcanoes (‘Loa’ and ‘Kea’) are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples ...
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Kostomuksha greenstone belt consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, separated by a major shear zone. The former contains submarine 2.8 Gyr old komatiite-basalt lavas and volcaniclastic lithologies with trace element and isotopic compositions resembling those of recent oceanic flood basalts [ɛNd(T) =+ 2,8, μ.1= 8.73 (Nb/Th)N= 1.5–2.1 (Nb/La)N= 1.0–1.5]. We suggest that the mafic terrane is a remnant of the upper crustal part of an Archaean oceanic plateau derived from partial melting of a mantle plume head. When the plateau reached the continental margin, it collided with the sedimentary terrane but was too buoyant to subduct. As a result, the volcanic section of the plateau was imbricated and obducted thus becoming a new piece of continental crust. The deeper zones were subducted and disappeared from the geological record.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1437-3262
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Abstract Crust formation ages inferred on the basis of Nd model age, U-Pb zircon and Pb-Pb data for granulites from central Sri Lanka, and the late Archean Lewisian (Scotland) and Nuk (south-western Greenland) high-grade gneisses are comparable, and in the range 2.2–3.0 Ga and 2.8–3.0 Ga respectively. The time interval between crust formation and high-grade metamorphism (with its attendant depletions of incompatible and heat-producing elements) is in the order of ∼ 2 Ga for the ∼ 0.6 Ga Highland granulites of Sri Lanka, but are less than ∼ 0.1–0.2 Ga for the Lewisian and Nuk gneisses. Severe depletions of U and Th produced the distinctive unradiogenic present-day Pb ratios of the Lewisian and Nuk gneisses. The Sri Lanka granulites, in contrast, preserve a record of an extended residence in a high U/Pb, high Th/Pb environment and display radiogenic »upper crustal« — like present-day isotope ratios. Relative to primitive mantle compositions, the Pb isotopes identify the Lewisian and Nuk terrains as crustal reservoirs depleted in Th and U (relative to Pb), whereas the Highland granulites are enriched. The global Pb isotope mass balance requires the existence of low U/Pb, low Th/Pb and high Th/U reservoir(s) for successful solutions, and the lower crust is a likely candidate for this reservoir. High-grade terrains like Sri Lanka show depletions of measured U and Th abundances relative to Pb, but do not show Pb isotope signatures of »model bulk lower crust«. Archean high-grade gneisses display the appropriate combination of retarded Th/Pb and U/Pb although their absolute compositions are much less radiogenic than those of »model bulk lower crust« predicted from simple mass balance calculations.
    Abstract: Résumé Les âges modèles de formation de la croûte, estimés à partir des méthodes Nd, U-Pb (zircon) et Pb-Pb, pour les granulites du Sri Lanka central d'une part et les gneiss de degré élevé lewisiens (Archéen sup.) d'Ecosse et de Nuk (sud-ouest du Groenland) d'autre part, sont comparables et se situent respectivement dans les intervalles de 2,2–3,0 Ga et 2,8–3,0 Ga. L'intervalle de temps qui sépare la formation de la croûte et le pic du métamorphisme (accompagné de pertes d'éléments incompatibles et producteurs de chaleur) est de l'ordre de ± 2 Ga pour les granulites du Sri Lanka, âgées de ± 0,6 Ga, mais n'excède pas 0,1 à 0,2 Ga pour le Lewisien et les gneiss de Nuk. Des pertes sévères en U et en Th sont responsables des rapports Pb actuels peu radiogéniques dans le Lewisien et les gneiss de Nuk. Par contre, les granulites du Sri Lanka se sont maintenues dans des conditions de rapports U/Pb et Th/Pb élevés et fournissent des rapports isotopiques analogues à ceux de la croûte supérieure actuelle. Par rapport aux compositions mantéliques primitives, les isotopes du Pb identifient le Lewisien et les formations de Nuk comme des réservoirs crustaux appauvris en Th et en U (par rapport au Pb), tandis que les granulites du Sri Lanka ont été enrichies. Le bilan de masse des isotopes du Pb à l'échelle globale requiert l'existence d'un ou de plusieurs réservoirs à faible U/Pb, faible Th/Pb et haut Th/U; la croûte inférieure pourrait constituer ce réservoir. Des formations très métamorphiques comme celles du Sri Lanka présentent un appauvrissement des teneurs en U et en Th par rapport au Pb, mais ne montrent pas les signatures isotopiques du Pb correspondant à un modèle de croûte inférieure globale. Les gneiss archéens de degré élevé possèdent la combinaison adéquate des rapports Th/Pb et U/Pb déprimés, bien que leurs compositions absolues soient moins radiogéniques que celle de la »croûte inférieure type« prévue par les bilans de masse.
    Notes: Zusammenfassung Nd-Modellalter, U-Pb-Zirkondaten sowie Pb-Pb-Alter weisen auf vergleichbare Krustenbildungsalter von 2.2–3.0 Ga für den zentralen Granulitgürtel Sri Lankas bzw. 2.8–3.0 Ga für die spätarchaischen Granulitgebiete der Lewisian-(Schottland) und Nuk-Komplexe (südwestliches Grönland) hin. Die extrem lange Zeitspanne von ca. 2 Ga zwischen Krustenbildung und hochgradiger Metamorphose und die daraus resultierende Verarmung an hochinkompatiblen Elementen unterscheidet jedoch die ca. 0.6 Ga alten Granulite der Highland- und Südwest-Gruppe Sri Lankas von den Lewisian- und Nuk-Gneisen. Hier erfolgte die Granulitmetamorphose verhältnismäßig bald nach der Krustenbildung (≤ 0.1–0.2 Ga), und die frühe signifikante Verarmung an U und Th ist verantwortlich für die heutige, wenig radiogene Pb-Isotopie in diesen Gesteinen. Im Gegensatz dazu zeigen Konzentrationsmessungen an den Granuliten Sri Lankas zwar eine Verarmung von U und Th relativ zu Pb, was aber aus der hochradiogenen Pb-Isotopie nicht ersichtlich ist. Der Grund hierfür ist, daß dieser Krustenkomplex eine sehr lange Zeit zur kontinentalen Oberkruste gehörte, mit entsprechend hohen U/Pb- und Th/Pb-Verhältnissen und dem daraus entstandenen Zuwachs an radiogenem Blei. Die terrestrische Massenbilanz des Bleis erfordert, daß die zeitintegrierten erhöhten U/Pb-, Th/Pb- und erniedrigten Th/U-Werte sowohl der Oberkruste als auch fast aller Mantel-derivate durch ein Reservoir, wie z.B. die Unter-kruste, mit langfristig erniedrigtem U/Pb und Th/ Pb und erhöhtem Th/U kompensiert werden muß. Krustenkomplexe wie Sri Lanka, die erst in jüngster Zeit in die Unterkruste versenkt wurden, kommen dafür nicht in Frage. Archaische Granulitgebiete, jedoch, zeigen das entsprechende Th/Pb und U/Pb, wenn auch ihre absolute Zusammensetzung deutlich weniger radiogen ist als es die Modellunterkruste verlangt.
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  • 5
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (〈10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (87Sr/86Sr=0.702203–0.702285; 143Nd/144Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Onega plateau constitutes part of a vast continental flood basalt province in the SE Baltic Shield. It consists of Jatulian-Ludikovian submarine volcanic, volcaniclastic and sedimentary sequences attaining in places 4.5 km in thickness. The parental magmas of the lavas contained ∼10% MgO and were derived from melts generated in the garnet stability field at depths 80–100 km. The Sm-Nd mineral and Pb-Pb whole-rock isochron ages of 1975 ± 24 and 1980 ± 57 Ma for the upper part of the plateau and a SHRIMP U-Pb zircon age of 1976 ± 9 Ma for its lower part imply the formation of the entire sequence within a short time span. These ages coincide with those of picrites in the Pechenga-Imandra belt (the Kola Peninsula) and komatiites and basalts in the Karasjok-Kittilä belt (Norway and Finnmark). Together with lithostratigraphic, chemical and isotope evidence, these ages suggest the derivation of the three provinces from a single large (∼2000 km in diameter) mantle plume. These plume-generated magmas covered ∼600,000 km2 of the Baltic Shield and represent a major contribution of juvenile material to the existing continental crust at 2.0 Ga. The uppermost Onega plateau lavas have high (Nb/Th)N = 1.4–2.4, (Nb/La)N= 1.1–1.3, positive ɛNd(T) of +3.2 and unradiogenic Pb-isotope composition (μ1 = 8.57), comparable with those of modern oceanic plume-derived magmas (oceanic flood basalt and ocean island basalt). These parameters are regarded as source characteristics. The lower sequences have (Nb/Th)N= 0.58–1.2, (Nb/La)N= 0.52–0.88 and ɛNd(T) =−2.6. They have experienced mixing with 10–30% of continental crust and resemble contaminated lavas from other continental flood basalt provinces. The estimated Nb/U ratios of 53 ± 4 in the uncontaminated rocks are similar to those found in the modern mantle (∼47) suggesting that by 2.0 Ga a volume of continental crust similar to the present-day value already existed.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Contributions to mineralogy and petrology 97 (1987), S. 31-42 
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ɛ 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by “phlogopite metasomatism”. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ɛ 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic “crustal residence ages” of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Contributions to mineralogy and petrology 98 (1988), S. 408-416 
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had 87Sr/86Sr=0.7039, 143Nd/144Nd=0.512678 (ɛ Nd=+3.8) and δ 18O=6.1‰, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in δ 18O, to about 7.8‰, is the result of fractional crystallization. Mafic rocks show a large range in initial 143Nd/144Nd ratio, from about 0.51267 to 0.51236 (ɛ Nd= +3.7 to −2.5), which is correlated with elevated 87Sr/86Sr ratios and δ 18O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement. The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.
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  • 9
    ISSN: 1432-1130
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The photoplate detection system of a spark source mass spectrometer has been recently replaced by a detector array consisting of 20 separate small channeltrons for simultaneous ion counting of up to 20 trace elements. The new multi-ion counting – spark source mass spectrometry (MIC-SSMS) technique combines the advantages of conventional SSMS with modern on-line detection of elements. It has important analytical features, such as simple and fast solid-state sample preparation, high precision (about 1–2%) and accuracy (4%) using multielement isotope dilution, high sensitivity which leads to short measuring times (10–50 min) and low detection limits (about 0.001–0.01 μg/g).
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 306 (1983), S. 431-436 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Analyses on fresh glass samples of mid-ocean ridge basalt yield a uniform ratio K/U = 12,700 ± 200. In contrast, Th/U increases systematically with Th concentration. From these results we calculate an upper limit (1.5 pW kg−1) and a best estimate (0.6 pW ...
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