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Cohen, R.S., O'Nions, R.K. and Dawson, J.B. (1984). Isotope geochemistry of xenoliths from East Africa: implications for development of mantle reservoirs and their interaction. Earth and Planetary Science Letters 68(2): 209-220. |
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Pb, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenic Pb ( (super 206) Pb/ (super 204) Pb = 15.55) and Nd ( (super 143) Nd/ (super 144) Nd = 0.51127) isotope compositions recorded so far for an ultramafic xenolith, and (super 87) Sr/ (super 86) Sr = 0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77< (super 206) Pb/ (super 204) Pb<17.50 and some show evidence for depletion of U relative to Pb up to 2.0 Ga ago. A phlogopite-amphibole vein from the Pello Hill sample has Sr, Nd and Pb isotope compositions similar to those of mid-ocean ridge basalts, indicating both a young emplacement age for the vein material and a source which had an isotopic signature characteristic of depleted mantle. The Sr, Nd and Pb isotope systematics of ultramafic xenoliths do not conform with those of MORB, particularly in terms of their Pb-Sr, and Nd-Pb relationships. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved. |
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Keywords
Africa; alkaline earth metals; continental crust; crust; East, Africa; evolution; geochemistry; igneous rocks; inclusions;, isotopes; lead; lherzolite; lithosphere; mantle; metals; Nd-144, /Nd-143; neodymium; Pello Hill; peridotites; petrology; plutonic, rocks; rare earths; ratios; Sr-87/Sr-86; stable isotopes;, strontium; Tanzania; ultramafics; xenoliths, 05A Igneous and metamorphic petrology; 02D Isotope geochemistry |
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Publisher
Elsevier Science
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1000 AE Amsterdam
The Netherlands
(+31) 20 485 3757
(+31) 20 485 3432
nlinfo-f@elsevier.com |
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