Seven rock samples were collected from the cross-section of a 2.8-m-thick andesitic dyke on Alicudi (Aeolian Islands, Sicily). Thin section microprobe analysis of phenocrysts (pyroxenes, olivine and plagioclase) and whole-rock X-ray fluorescence show that samples at the chilled margins are sharply distinct, with the highest Mg, Cr and Ni contents. Seven single crystals of clinopyroxene were analysed by microprobe techniques, while their structures were refined using the X-ray diffraction method; different Mg-Fe2+ ordering in the M2 and M1 sites of clinopyroxene were observed across the dyke section. The equilibration temperature, evaluated by applying the geothermometer proposed by Molin & Zanazzi (1991), gives the highest values at the dyke margins (850-degrees-C) and the lowest values close to the middle (630-degrees-C). By applying the theoretical model of Jaeger (1968) for the cooling of volcanic bodies, as well as the Kudo & Weill (1970) plagioclase-liquid geothermometer (modified by Mathez, 1973), it was possible to calculate the cooling rates across the dyke section: from 8 and 34-degrees-C/h near the boundaries to 1-degrees-C/h in the middle.
COOLING HISTORY OF A DYKE ON ALICUDI (AEOLIAN ISLANDS) FROM INTRACRYSTALLINE FE-2+-MG EXCHANGE-REACTION IN CLINOPYROXENE
MOLIN, GIANMARIO;
1993
Abstract
Seven rock samples were collected from the cross-section of a 2.8-m-thick andesitic dyke on Alicudi (Aeolian Islands, Sicily). Thin section microprobe analysis of phenocrysts (pyroxenes, olivine and plagioclase) and whole-rock X-ray fluorescence show that samples at the chilled margins are sharply distinct, with the highest Mg, Cr and Ni contents. Seven single crystals of clinopyroxene were analysed by microprobe techniques, while their structures were refined using the X-ray diffraction method; different Mg-Fe2+ ordering in the M2 and M1 sites of clinopyroxene were observed across the dyke section. The equilibration temperature, evaluated by applying the geothermometer proposed by Molin & Zanazzi (1991), gives the highest values at the dyke margins (850-degrees-C) and the lowest values close to the middle (630-degrees-C). By applying the theoretical model of Jaeger (1968) for the cooling of volcanic bodies, as well as the Kudo & Weill (1970) plagioclase-liquid geothermometer (modified by Mathez, 1973), it was possible to calculate the cooling rates across the dyke section: from 8 and 34-degrees-C/h near the boundaries to 1-degrees-C/h in the middle.Pubblicazioni consigliate
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