Abstract
In the past 5 ka volcanism has been very intense at Ischia, a densely populated and still active volcanic island in the Gulf of Napoli. Mineralogical, geochemical and isotopic investigations have been carried out on the pyroclastic deposits and lava flows of the historical eruptions of Fiaiano (530–960 A. D.) and Arso (1302 A. D.). These products are characterized by strong petro-chemical disequilibria. Mineralogical disequilibria are shown by the occurrence of resorbed and reversely zoned phenocrysts, and by the coexistence of phenocrysts and microlites with variable composition. Chemical inhomogeneity characterizes the glass of the juvenile fragments. Isotopic disequilibria have been detected among all separated mineral phases and whole-host rock. The detected disequilibria are interpreted as the result of a physical mixing process between variable fractionated and isotopically distinct batches of magmas. Petrochemical data suggest that mingling occurred between a highly crystallized alkali-trachytic and an isotopically distinct latitic to trachytic magmas, for both the eruptions. The pre-eruptive scenario proposed is the uprising of a latitic-to-trachytic magma (characterized by a Sr-isotope ratio of about 0.7068) from a deep reservoir and successive mingling with batches of more viscous, highly crystallized alkali-trachytic magma (with a Sr-isotope composition of about 0.7061) at shallow depth.
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