Alkali feldspar liquid equilibrium relationships in peralkaline oversaturated systems and volcanic rocks |
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| Authors: | Jacques Roux Jacques Varet |
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| Institution: | (1) Centre de Synthése et Chimie des Minéraux, 45045 Orléans-Cedex;(2) Laboratoire de Pétrographie-Volcanologie, Université Paris-Sud, 91405 Orsay |
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| Abstract: | A critical review of both experimental data and natural rock sequences allows a comparison of liquid lines of descent in peralkaline oversaturated systems. Available experimental data have been used to draw the liquidus surfaces and the fractionation curves in various planes including the alkali feldspar join. The thermal valley becomes steeper, the fractionation curves being channeled faster towards the thermal valley with increasing alkali-silica ratio. The slope of the thermal valley varies so that the higher the peralkalinity, the more sodic the most fractionated liquids. Rock series from Easter Island, Gran Canaria, Pantellaria Island, Boina (Afar), Fantale (Ethiopia) and Menengai (Kenya) in which there is good evidence for alkali feldspar fractionation, show that such variations do occur in nature. Factors other than the peralkalinity of the series, such as additional components and changes in the confining pressure are shown to affect the orientation and location of the thermal valley. |
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