Indirect crustal contamination; evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

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doi: 10.1007/s004100050222
Authors:Paslick, C. R.; Halliday, A. N.; Lange, R. A.; James, D.; Dawson, J. B.
Author Affiliations:Primary:
Rock Valley College, Rockford, IL, United States
Other:
University of Michigan, United States
University of Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 125(4), p.277-292. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1996
Note:In English. 37 refs.; illus., incl. 10 tables, geol. sketch map
Summary:Alkali basalts and nephelinites from the volcanic province of this area show evidence of 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 analysed for Sr, Nb and Pb isotopic compositions. The 206Pb/204Pb ratios vary 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 Δ87Sr/86Sr of 0.0002 and Δ εV>Nd 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 N Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variation. The xenocrystic phases present appear rather to have been derived from earlier alkali basalt rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that later 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 changed by <<<[dollar]I> 0.25% from this indirect crustal contamination. [R.E.S.]
Subjects:Alkali basalts; Basalts; Cenozoic; Chain silicates; Chemical composition; Continental crust; Crust; Igneous rocks; Isotopes; Lead; Magma contamination; Major elements; Mantle; Metals; Nd-144/Nd-143; Neodymium; Neogene; Nephelinite; Nesosilicates; Olivine; Olivine group; Orthosilicates; Oxides; Pb-206/Pb-204; Peralkalic composition; Perovskite; Petrology; Provenance; Pyroclastics; Pyroxene group; Radioactive isotopes; Rare earths; Silicates; Stable isotopes; Tertiary; Trace elements; Tuff; Volcanic rocks; Whole rock; Africa; East Africa; East African Rift; Tanzania; Northern Tanzania
Abstract Numbers:97M/3951
Record ID:1997042382
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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