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  • 1
    Monograph available for loan
    Monograph available for loan
    Nantwich : Shiva Publ.
    Call number: M 93.0167
    Type of Medium: Monograph available for loan
    Pages: VIII, 272 S.
    ISBN: 0906812348
    Language: English
    Location: Upper compact magazine
    Branch Library: GFZ Library
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Lavas from Karisimbi, the largest volcano in the Virunga province in the Western Branch of the African rift on the Zaire-Rwandan border, constitute a suite of mafic potassic basanites and more evolved potassic derivatives. All of the lavas are potassic with K2O/Na2O≥1, and enriched in incompatible elements, with chondrite normalised (La/Yb)n〉18 and Nb/Zr〉0.25. The 87Sr/86Sr and 143Nd/144Nd ratios reflect these enriched compositions, varying from 0.7052 and 0.51258 respectively in the K-basanites to 0.7132 and 0.51226 in the most evolved K-trachyte, although at MgO abundances 〉4% there is no systematic variation of isotope ratios with fractionation. At 〉4% MgO, lava compositions were controlled by assimilation and fractional crystallization in a sub-volcanic magma chamber. Trace-element and isotope variations in the more mafic lavas appear to reflect mixing between a “primitive” K-basanite (PKB) magma and a Sr-rich end-member, similar to melilite nephelinites from the neighbouring volcano, Nyiragongo. Both endmembers are mantle-derived and isotopically distinct, with the PKB being characterised by 87Sr/86Sr up to 0.707 and 143Nd/144Nd as low as 0.51236. Alternatively, isotope variations may be the time-integrated response to trace-element fractionations in a variably enriched mantle source. The Pb isotope variations within Karisimbi are complex. In the more evolved lavas all three ratios increase coherently with fractionation, whereas in the mafic varieties 206Pb/204Pb remains roughly constant at ∼19.2 while 207Pb/204Pb and 208Pb/204Pb vary from 15.67 to 15.78 and 39.49 to 40.80 respectively, defining sub-vertical trends, consistent with PKB-nephelinite magma mixing. The Nd and Sr isotopes indicate trace-element fractionation in the PKB source at ∼1 Ga, similar to ages derived from the overlying crust and suggesting a lithospheric origin. Elevated 208Pb/204Pb and 208Pb*/206Pb* values of the PKB are also consistent with Th/U fractionation at a similar time. However, this 1Ga age contrasts with that derived from the elevated 207Pb/204Pb ratios which indicate U/Pb fractionation during the Archaean. Crustal contamination can be excluded as the major control of Pb isotope variation in the PKB because their high Ce/Pb ratios (∼27) are similar to those typical of oceanic basalts. Parent/daughter trace-element fractionation and the high Ti, Nb and Ta abundances of the PKB lavas are all consistent with enrichment of a lithospheric source region by small-degree silicate melts at ∼1Ga. Comparison between measured and time-integrated trace-element ratios suggests that the degree of melting associated with recent magmatism was ≥5%. These data show that significant Th/U and Rb/Sr fractionation can be produced by intra-mantle melting processes and that high 208Pb/204Pb and 208Pb*/206Pb* values can evolve within the upper mantle and do not necessarily require the recycling of crustal material. Comparable isotope features in continental flood basalts and DUPAL ocean island basalts may be explained in a similar way.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in 87Sr/86Sr (0.70327–0.70610), 143Nd/144Nd (0.51302–0.51229) and 208Pb*/206Pb* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high Δ8/4, and to a lesser extent high Δ7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS ‘subduction component’ interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (〈5%) of subducted S China Sea terrigenous sediment. The 87Sr/86Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with 143Nd/144Nd and 208Pb*/206Pb*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.
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  • 4
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Late Permian to Early Triassic Siberian Traps have been sampled by drill core (core SG-9) and from surface exposure (section 1F) in the Noril'sk region of the Siberian Platform, Russia. Combined major, trace element, and Nd-, Sr-, and Pb-isotope data on selected samples through the Siberia Trap, offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near Noril'sk. A Lower Sequence of sub-alkalic basalts, tholeiites, and picritic basalts (upwards these are the Ivakinsky, Syverminsky, and Gudchichinsky formations) are overlain by an Upper Sequence of picritic basalts and tholeiites interbedded with tuffs (upwards, these are the Khakanchansky, Tuklonsky, Nadezhdinsky, Morongovsky, Mokulaevsky and Kharayelakhsky formations).The Gudchichinsky and Tuklonsky formations contain both picritic and tholeiitic lavas. The Tuklonsky formation tholeiites and picrites have moderate Gd/Yb (1.6–1.8), low TiO2 (0.45–0.95 wt%), a significant negative Ta and Nb anomaly (Nb/La =0.42–0.57) and unradiogenic Nd (ɛ Nd CHUR = to -4.6). In contrast, both the Gudchichinsky formation tholeiites and picrites have high Gd/Yb (2.3–3.1), and TiO2 (1.2–2.3 wt%), no significant Nb or Ta anomaly (Nb/La =0.8–1.1), and radiogenic Nd (ɛ Nd CHUR = to 7.3). The low-Ti and Nb/La, high La/Sm, and unradiogenic Nd-isotope signatures of the picritic Tuklonsky formation lavas and the tholeiitic lavas of the Upper Sequence are characteristic of magmas strongly influenced by material from the continental lithosphere, whereas the high-Ti and Nb/La, low La/Sm and radiogenic Nd-isotope signatures of the Lower Sequence are more comparable to deeper asthenospheric mantle-plume generated lavas similar to oceanic island basalts. The lavas overlying the Tuklonsky formation have mg-numbers of 0.63 to 0.68, and are more evolved than the Tuklonsky (Mg-number 〈 0.62) and have more radiogenic ɛ Nd CHUR (Tuklonsky:-0.03 to-4.66; Mokulaevsky: + 0.60 to + 1.61), but have many of the incompatible trace element features of the Tuklonsky sky type magma. These lavas show a progressive upwards decline in SiO2 (55–49 wt%), La/Sm (4.6–2.0), and ɛ UR Sr ( + 67 to + 13) which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma. This paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust.
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  • 5
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Strontium isotope data are reported for primitive lavas (leucitites, tephritic leucitites, K-rich basalts, and related types) from the Roccamonfina volcano. A strong positive correlation is found between 87Sr/86Sr and the abundances of K, Rb, Sr, Ba and Zr. It is argued that the present contents of these elements in the lavas are not far removed from their concentrations in their parental primary magmas. Models involving disequilibrium and equilibrium melting of respectively homogeneous and heterogeneous source rocks are discussed. It is concluded that a heterogeneous source highly enriched in incompatible elements at some stage in the past (? 300 m.y.) is indicated.
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  • 6
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.
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  • 7
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking compositional gap between 50 and 65% SiO2 suggests that the high SiO2 rocks evolved by 10–15% partial melting followed by variable amounts of fractional crystallisation. The source material could have been basalt within the Deccan Trap, or related gabbroic rocks in deep crustal sill complexes. The rhyolites yield an Rb-Sr whole rock age of 61.5±1.9 Ma, with a slightly high initial 87Sr/86Sr=0.7085±18. It is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks.
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  • 8
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract 143Nd/144Nd, 87Sr/86Sr, and REE analyses are presented on a wide variety of Pliocene-Recent volcanic rocks from central Italy. 143Nd/144Nd varies from 0.51214–0.51289 and 87Sr/86Sr from 0.7255-0.7036; while the rare earth elements are characterised by light RE enrichment and a significant negative Eu anomaly. These Italian volcanics are tentatively subdivided into three zones: (1) N. Tuscany where the magmas are believed to reflect crustal anatexis. (2) A central zone in which hybrid (crust/ mantle) rocks have been recognised. (3) A southern zone, south of Rome, where mantle-derived magmas are identified which have been largely unaffected by interaction with continental crust. At Roccamonfina, in zone 3, Rb/Sr and Sm/Nd pseudo isochrons are observed but since the calculated ages are 0.5 and 2.0 b.y. respectively it is argued that a simple isochron model is not applicable and that the data are most easily explained by a recent mixing event within the upper mantle. It is envisaged that this occurred during metasomatism of the upper mantle source region by a fluid that had high 87Sr/86Sr and low 143Nd/144Nd and was enriched in K, Rb, and LREE's but relatively depleted in Sr2+ and Eu2+.
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  • 9
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A numerical inversion program has been developed to investigate isotope and trace element variations in suites of related rocks in terms of assimilation and fractional crystallisation processes. Major element compositions constrain the degree of fractionation, and the program may be used to calculate the isotope and trace element composition of either the crustal contaminant, or of the parental magmas once the contaminant is known. The ratio of the rate of assimilation to the rate of fractional crystallisation, and the bulk partition coefficients, can be varied with changes in magma composition. The approach is illustrated with a suite of calc-alkaline rocks from the Cerro Galan centre in NW Argentina, and a suite of continental flood basalts from the southern Paraná in Brazil. Both exhibit striking increases in 87Sr/86Sr with increasing SiO2 consistent with progressive contamination during differentiation within the continental crust. In detail, both suites contain a subset of samples which have undergone relatively little contamination, and a subset characterised by relatively high SiO2 and 87Sr/86Sr reflecting larger crustal contributions. Individual samples within the more contaminated subset can be modelled in terms of assimilation with fractional crystallisation (AFC) processes, although no clear progressively contaminated liquid lines of descent are preserved. At both Cerro Galan and the southern Paraná, the associated dacites and rhyodacites appear to have similar compositions to the crustal contaminants. Thus the inversion program has been used to investigate the amount of contamination, the relationship between the amount of contamination and the amount of crystals removed by fractional crystallisation, and the isotope and trace element contents of the parental magmas prior to AFC. The latter are estimated to have had 87Sr/86Sr=0.7055 and 143Nd/144Nd=0.51256 at Cerro Galan, and 0.7090 and 0.51226 respectively in the southern Paraná.
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  • 10
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The variations in trace element abundances of a suite of alkali-olivine basalts from the Big Pine volcanic field, California, have been ‘inverted’ following the method of Hofmann and co-workers to obtain source concentration and distribution coefficient data. The high Mg-numbers and ne-normative mineralogy of these lavas allow a simple correction to be made for fractional crystallisation, and together with a limited range in 87Sr/86Sr (0.7056–0.7064), suggest derivation from a relatively homogeneous source region. Negative correlations between SiO2 and P2O5, and SiO2 and Rb in the calculated primary magmas imply that both major and trace elements vary in a coherent fashion as a function of the degree of partial melting. The Big Pine lavas are characterised by high ratios of large-ion lithophile to high-field strength elements (Ba/Nb〉60), and the inverse procedure demonstrates that this reflects source concentrations, as opposed to a mineralogical control. The calculated mantle source is further characterised by generally high abundances of Sr, Ba, K, and Th relative to Nb and Ta which imply that incompatible element enrichment of the source occurred above a subduction zone. A model Sm/Nd age of 1.8 Ga for this enrichment coincides with the regional crustal formation age. Such features imply that both the major and trace element components of the Big Pine lavas are derived from within lithospheric mantle, perhaps mobilised by the high geothermal gradients which characterise the extensional environment of the Basin and Range Province. A comparison with other Cenozoic mafic lavas throughout the western United States suggests that a substantial proportion of the mantle lithosphere in this area has similar chemical characteristics to the source of the Big Pine lavas. If this is the case, then it implies that convergent margins represent an important tectonic environment for the formation of lithospheric mantle.
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