COOLING HISTORY OF A DYKE ON ALICUDI (AEOLIAN ISLANDS) FROM INTRACRYSTALLINE FE-2-MG EXCHANGE-REACTION IN CLINOPYROXENE()

Seven rock samples were collected from the cross-section of a 2.8-m-th ick 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 m argins are sharply distinct, with the highest Mg, Cr and Ni contents. Seven single crystals of clinopyroxene were analysed by microprobe tec hniques, while their structures were refined using the X-ray diffracti on method; different Mg-Fe2+ ordering in the M2 and M1 sites of clinop yroxene were observed across the dyke section. The equilibration tempe rature, evaluated by applying the geothermometer proposed by Molin & Z anazzi (1991), gives the highest values at the dyke margins (850-degre es-C) and the lowest values close to the middle (630-degrees-C). By ap plying the theoretical model of Jaeger (1968) for the cooling of volca nic bodies, as well as the Kudo & Weill (1970) plagioclase-liquid geot hermometer (modified by Mathez, 1973), it was possible to calculate th e cooling rates across the dyke section: from 8 and 34-degrees-C/h nea r the boundaries to 1-degrees-C/h in the middle.