Magma dynamics beneath Kameni volcano, Thera, Greece, as revealed by crystal size and shape measurements

Size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from Kameni volcano, Greece, can be modelled by mixing two populations of crystals, each with overlapping linear crystal size distributions (CSD)-termed microlites and megacrysts. The magmas bearing the microlites and megacrysts started to crystallise 6-13 and 24-96 years, respectively, before each eruption. The dates of initiation of crystallisation of the megacrysts indicate that they are left-overs of earlier injections of new magma into a shallow chamber: some magma remains after each eruption and continues to crystallise. New magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma. Eruption followed 6-13 years after mixing. Such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied, just topped up from time to time.