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zeige Details Walter, Robert C., Hart, William K., ... Petrogenesis of a basalt-rhyolite tephra from the west-central Afar, Ethiopia
in: Contributions to mineralogy and petrology , ISSN 1432-0967, Vol. 95 (4. 1987), p. 462-480
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1987
Abstract The Cindery Tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption. It contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage. Brown glasses are ferrobasaltic in composition and are similar to the voluminous Pliocene tholeiites of the surrounding west-central Afar volcanic field; both are enriched in the light rare earth and incompatible elements and possess higher 87Sr/86Sr and lower 143Nd/144Nd than MORB. Rhyolitic glasses are subalkaline and, compared to the basaltic glasses, are strongly depleted in the compatible elements and enriched in the incompatible elements. Both glass types have similar incompatible element and isotopic ratios, and with the rhyolite glass showing a 2-fold parallel enrichment in rare earth element abundances over the basaltic glass. These observations suggest that the two glasses are genetically related. Rare glasses with intermediate compositions occur as phenocryst melt inclusions, as mantles on phenocrysts and as free pumice clasts. Their major element contents do not point to an origin by simple hybrid mixing of the basaltic and rhyolitic melts. Rather, major element mixing calculations indicate formation of the intermediate and rhyolite melts by fractionation of the observed phenocryst assemblage, using a starting composition of the observed basaltic glass. Model calculations from trace element data, though lacking from the intermediate glasses, support fractional crystallization. The bimodal mineral assemblage argues against an immiscible liquid origin for the contrasting glass compositions.
Copyright: Copyright 1987 Springer-Verlag
beteiligte Personen: Walter, Robert C. , Hart, William K. , Westgate, John A.
Format: Elektronisch
Erschienen: 1987.
Serie: Springer Online Journal Archives 1860-2000 [Dig. Serial]
Schlagwörter:
URL: http://dx.doi.org/10.1007/BF00402206

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