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 rocks from the island of Alicudi (Aeolian Are) was studied by single-crystal X-ray diffraction and microprobe techniques, in order to explore the relationships between clinopyroxene structural and chemical parameters and the physicochemical conditions of their crystallization. Crystal chemical data indicate considerable differences between clinopyroxenes from early eruptive stages and those from the latest volcanic activity. Early-stage clinopyroxenes are characterized by smaller cell volumes and a more closely packed structure than late clinopyroxenes, suggesting a higher pressure of crystallization for the early clinopyroxenes. The crystal chemical pattern of clinopyroxene follows an overall trend of rock composition, which is basaltic and andesitic-basaltic for the earliest products and becomes slightly more acidic at the end of volcanic activity. This geochemical and petrologic evolution is also found in other Aeolian islands and may be explained by assuming the increasing role of crystal fractionation with respect to mixing with primary mafic melts. The variations in crystal chemical parameters of clinopyroxenes at Alicudi, which indicate decreasing pressure of crystallization with time, lend support to the hypothesis of polybaric evolution of the Alicudi magmas.