Abstract Crystal chemistry and structural data for clin- opyroxene from the Aeolian islands (Southern Tyrrhe- nian Sea, Italy) were determined with the aim of obtain- ing geobarometric information and exploring implica- tions for the structure of volcanic plumbing systems. Cell and M1 site volumes for clinopyroxenes, which are known to decrease with increasing pressure of crystalli- zation, revealed variable values, both within some single islands and along the entire arc, indicating polybaric conditions of crystallization. The lowest cell and M1 volumes were found at Filicudi, plotting close to values of clinopyroxenes from high-pressure ultramafic xeno- liths entrained in alkali basalts. Indications of high-pres- sure crystallization were also found at Salina and, to a lesser extent, at Alicudi, all situated in the western sector of the Aeolian Arc. The central and eastern islands of Li- pari, Vulcano, Panarea and Stromboli generally show higher values of cell parameters, suggesting crystalliza- tion in shallow magma chambers. These islands are char- acterized by the occurrence of large calderas, which are apparently lacking at Salina and Filicudi. Time-related variations were observed for cell and M1 volumes of clinopyroxene for some islands. At Salina, the early- erupted products display low values of cell parameters with respect to later activity, thus indicating a decrease in crystallization pressure with time. A similar, although less striking, pattern is observed at Alicudi and Lipari. An overall increase in cell parameters with time was ob- served at the scale of the entire arc. The observed varia- tions in clinopyroxene structural parameters highlight the significance of pyroxene crystal chemistry for petro- genetic and volcanological interpretation. Correlation with time and the structural characteristics of volcanoes suggest significant regional and temporal modifications in the plumbing systems of Aeolian volcanoes. Clin- opyroxenes from Filicudi and the older Salina crystal- lized at high pressure in deep magma chambers, in the lower crust or at the mantle–crust boundary. The lower crystallization pressure in the younger Salina is interpret- ed as evidence of upward migration of magma chambers with time. Similar evolution can be envisaged for Ali- cudi. Instead, the entire evolutionary history of the cen- tral and eastern islands was dominated by low-pressure crystallization, with formation of calderas and generation of abundant acid products that are scarce or absent in the western islands. Evolution of the plumbing system of single volcanoes and of the Aeolian arc in general is probably related to modification of stress regimes and/or thinning of the arc basement, due to the effect of upris- ing mantle material above the Ionian subduction zone. Keywords Crystal chemistry · Clinopyroxene · Geobarometry · Magma chambers · Arc volcanism · Aeolian Arc Introduction The Aeolian Archipelago, Southern Tyrrhenian Sea (Fig. 1) consists of seven volcanic islands (Alicudi, Fili- cudi, Salina, Lipari, Vulcano, Panarea, Stromboli), of which two volcanoes, Stromboli and Vulcano, are still active. The Aeolian rocks have variable magmatic affini- ties, from calc-alkaline (CA), high-K calc-alkaline (HKCA) and shoshonitic (SHO) to potassic (KS). A gen- eral evolution from basic to more evolved products with time in some islands has been recognized, and rhyolites are the typical final products at Lipari, Vulcano, Panarea Editorial responsibility: W. Hildreth S. Nazzareni ( ✉ ) · A. Peccerillo · P. F. Zanazzi Dipartimento Scienze della Terra, Università di Perugia, Piazza Università 1, 06100 Perugia, Italy e-mail: crystal1@unipg.it Tel.: +39-75-5852612, Fax: +39-75-5852603 S. Nazzareni · G. Molin Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, 35100 Padova, Italy G. Molin C.N.R. Centro di Studio per la Geodinamica Alpina (C.N.R), c/o Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, 35100 Padova, Italy Bull Volcanol (2001) 63:73–82 DOI 10.1007/s004450100125 RESEARCH ARTICLE Sabrina Nazzareni · Gianmario Molin Angelo Peccerillo · Pier Francesco Zanazzi Volcanological implications of crystal-chemical variations in clinopyroxenes from the Aeolian Arc, Southern Tyrrhenian Sea (Italy) Received: 3 April 2000 / Accepted: 18 December 2000 / Published online: 3 March 2001 © Springer-Verlag 2001