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Mass transfer during sub-seafloor alteration of the upper Troodos crust (Cyprus)
Authors:Ulrich Bednarz  Hans -Ulrich Schmincke
Institution:(1) Institut für Mineralogie, Ruhr-Universität Bochum, Postfach 102148, D-4630 Bochum, Germany
Abstract:Five major alteration zones in the Extrusive Series and the Sheeted Dike Complex of the Troodos Ophiolite are each characterized by (a) distinct elemental changes compared to the original composition and (b) secondary mineralogy. The upper ca. 300 m of the extrusive crust, the highly oxidatedcold seawater alteration zone (CSA), is strongly enriched in K2O and depleted in Na2O. It is followed downwards by alow temperature alteration zone (<170° C) which is most widespread in the Troodos extrusives and where Na2O and K2O are enriched, the latter less strongly than in the CSA zone. Three types ofhigh temperature alteration zones (<440° C; HTA I–III) are found in the Sheeted Dike Complex. All are marked by thorough leaching of K2O, while the behavior of Na2O (e.g. unchanged in type III) and CaO (depleted in type I, enriched in types II, III) is variable. Mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section. The Troodos extrusive crust and upper Sheeted Dike Complex are a major sink for MgO, K2O, and Na2O, and a source for CaO; the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust.
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