STRUCTURAL AND CHEMICAL VARIATIONS IN CLINOPYROXENES FROM THE ISLAND OF ALICUDI (AEOLIAN ARC) AND THEIR IMPLICATIONS FOR CONDITIONS OF CRYSTALLIZATION

The crystal chemistry of clinopyroxenes from calc-alkaline volcanic ro cks from the island of Alicudi (Aeolian Are) was studied by single-cry stal X-ray diffraction and microprobe techniques, in order to explore the relationships between clinopyroxene structural and chemical parame ters and the physicochemical conditions of their crystallization. Crys tal chemical data indicate considerable differences between clinopyrox enes from early eruptive stages and those from the latest volcanic act ivity. Early-stage clinopyroxenes are characterized by smaller cell vo lumes and a more closely packed structure than late clinopyroxenes, su ggesting a higher pressure of crystallization for the early clinopyrox enes. The crystal chemical pattern of clinopyroxene follows an overall trend of rock composition, which is basaltic and andesitic-basaltic f or the earliest products and becomes slightly more acidic at the end o f volcanic activity. This geochemical and petrologic evolution is also found in other Aeolian islands and may be explained by assuming the i ncreasing role of crystal fractionation with respect to mixing with pr imary mafic melts. The variations in crystal chemical parameters of cl inopyroxenes at Alicudi, which indicate decreasing pressure of crystal lization with time, lend support to the hypothesis of polybaric evolut ion of the Alicudi magmas.