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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The 206Pb/204Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ 87Sr/86Sr of 0.0002 and ΔɛNd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the 206Pb/204Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination.
    Type of Medium: Electronic Resource
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