The Minoan and post-Minoan eruptions, Santorini (Greece), in the light of melt inclusions: chlorine and sulphur behaviour

Mineralogical and melt inclusion studies were carried out on plinian tephra (Pumice fall unit, Phira) ejected during the first phase of the Minoan eru ption, as well as on the most basic pyroclastic ejecta emitted during the r ecent activity of Nea Kameni. Heating stage experiments were performed on c rystallised melt inclusions. The temperature of melting (T-m) varies from 1 095 +/- 10 degrees C (Fo(80-77), high Al-basaltic melts) to 995 +/- 10 degr ees C (Fo(76-73), andesitic melts). A subsequent quenching and electron mic roprobe analysis of glass was performed in order to determine the compositi on of entrapped magmas. After heating experiments, the melt inclusions are parent liquids of a high Al-basaltic composition; these parent magmas have not been found as whole rocks in post-Minoan deposits. Melt inclusions, who le rocks and matrix glasses define a compositional trend from high Al-basal t to rhyolite, which attests to either mixing/mingling of cogenetic magmas or fractional crystallisation as processes controlling the chemical evoluti on of these magmas, and corresponds to a fraction of crystallised parent ma gmas of about 70%. Rhyolitic melts exhibit the highest chlorine contents (3 000-3200 mu g/g Cl) and the lowest sulphur contents (about 100 mu g/g S). B asaltic and andesitic melts show the lowest chlorine contents (900-1000 mu g/g Cl) and the highest sulphur contents (1000-600 mu g/g S). This shows a difference in behaviour between chlorine and sulphur, which is probably lin ked to the difference in solubility. This suggests also that the present-da y sulphur emission at Santorini does not originate only from differentiated magmas but also from basaltic and andesitic ones. On this assumption, the minimal chlorine and sulphur release to the atmosphere is estimated to be a bout 2 x 10(10) and 2.5 x 10(11) kg for the Minoan eruption. This shows tha t volcanic hazards related to volatile degassing, especially to SO2 emissio ns, could represent a serious risk at Santorini. (C) 2000 Elsevier Science B.V. All rights reserved.