Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley |
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Authors: | SCAILLET BRUNO; MACDONALD RAY |
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Institution: | 1 INSTITUT DES SCIENCES DE LA TERRE D'ORLÉANS, CNRS, 1A RUE DE LA FÉROLLERIE, 45071, ORLÉANS CEDEX 02, FRANCE
2 ENVIRONMENT CENTRE, LANCASTER UNIVERSITY, LANCASTER LA1 4YQ, UK |
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Abstract: | Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO2 conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalitemagnetitequartz buffer)and at high fF2 inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite molar (Na2O+ K2O)/Al2O3 (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A |
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