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  • Articles  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Contributions to mineralogy and petrology 92 (1986), S. 104-112 
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The porphyritic quartz diorites of the Caledonian Brabant Massif have been totally altered. Ca, Rb, Sr, Zr, Ce, Y measurements and Sr-Nd isotopic analyses were performed on the Quenast plug and the Lessines sill, in an attempt to study the relative mobility of Sr and evaluate the extent, direction and magnitude of the 87Sr/86Sr alterations. Sr electron microprobe analyses of epidote were also carried out to assess its role in the Sr distribution. The initial 87Sr/86Sr ratio is shown to have had an unsteady behaviour during the studied water/rock interactions since it has been sometimes enhanced, sometimes depressed and occasionally not modified. The possibility and magnitude of the 87Sr contamination turn out to be strictly related to the degree of Sr accommodation in the secondary minerals. Epidote in particular has proved to be the main trap for the hydrothermal Sr and this mineral is thus regarded as the major controlling factor of 87Sr hydrothermal contamination. The epidote-poor rocks (albite+chlorite-rich rocks) seem to have been unaffected by any Sr interchange with the aqueous solutions. Therefore, as alteration quickly follows the crystallization of the magma, their initial 87Sr/ 86Sr ratio, which is deduced from an isochron, might be a primary petrogenetic feature enabling interpretation of the genesis of their parental magmas. On the other hand, in the epidote-rich rocks, this ratio has been readily altered; it could thus generally be used only to trace the origin of the hydrothermal solutions. As a consequence, these rocks should not be selected for dating an alteration event by the Rb-Sr method.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2018-10-29
    Description: There is increasing evidence that abrupt vegetation shifts and large-scale erosive phases occurred in Central Africa during the third millennium before present. Debate exists as to whether these events were caused by climate change and/or intensifying human activities related to the Bantu expansion. In this study, we report on a multi-proxy investigation of a sediment core (KZR-23) recovered from the Congo submarine canyon. Our aim was to reconstruct climate, erosion and vegetation patterns in the Congo Basin for the last 10,000yrs, with a particular emphasis on the late Holocene period. Samples of modern riverine suspended particulates were also analyzedto characterize sediment source geochemical signatures from across the Congo watershed. We find that a sudden increase of bulk sediment aluminium-to-potassium (Al/K) ratios and initial radiocarbon ages of bulk organic matter occurred after 2,200yrs ago, coincident with a pollen-inferred vegetation change suggesting forest retreat and development of savannas. Although hydrogen isotope compositions of plant waxes (δDwax) do not reveal a substantial hydroclimate shift during this period, neodymium isotopes and rare earth elements in detrital fractions indicate provenance changes for the sediment exported from the Congo Basin at that time, hence suggesting a reorganization of spatial rainfall patterns across Central Africa during this event. Taken together, these findings provide evidence for changing landscapes in Central Africa from about 2,200yrs ago, associated with synchronous events of vegetation changes and enhanced erosion of pre-aged and highly weathered soils. These events coincided remarkably well with the arrival of Iron Age communities into the rainforest, as inferred from comparison to regional archaeological syntheses. While the human impact on the environment remains difficult to quantify at the scale of the vast Congo Basin, we tentatively propose that strengthening of El Niño-Southern Oscillation (ENSO) variability at that time played a key role in triggering the observed environmental changes, and possibly acted as a driver for the eastward migration of Bantu-speaking peoples across Central Africa.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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