CRYSTAL-CHEMISTRY OF CLINOPYROXENES FROM FILICUDI AND SALINA (AEOLIAN ISLANDS, ITALY) - GEOTHERMOMETRY AND BAROMETRY

Single clinopyroxene crystals from calc-alkaline volcanic rocks of Filicudi and Salina (Aeolian Islands, Italy) have been studied by X-ray diffraction and microprobe techniques to examine the relations between clinopyroxene crystal chemistry and the petrological features of the host rocks. The clinopyroxenes of Filicudi and Salina are chemically quite similar, but two different groups can be distinguished, diopsides and augites, often coexisting in the same rock. The Filicudi clinopyroxenes are generally characterized by a smaller cell volume and a more closely-packed structure; comparison with crystals from different environments suggests a higher pressure of crystallization for the Filicudi rocks. A relationship between SiO2 content of magma and clinopyroxene intracrystalline equilibration temperature is discussed in terms of different cooling history due to the more pronounced explosive character of the more silicic magmas, leading to more rapid quenching of eruption products. For a given rock-type, clinopyroxene equilibration temperature is also a function of sample position in the volcanic sequence, to which different cooling rates can be assigned.