Crystallization conditions in the Upper Pollara magma chamber, Salina island, Southern Tyrrhenian Sea

Pyroclastites erupted from the Upper Pollara magma chamber (13 ka, Salina Island,Aeolian Archipelago) resulted from mingling and mixing of rhyolitic and andesiticmagmas. An experimental study has been conducted on the rhyolitic end-member toconstrain the pre-eruptive conditions of the magma. In order to check for the role ofmixing on the equilibrium phase assemblage, three different starting compositions,corresponding to three different mixing degrees, have been used. The crystallizationexperiments were conducted at two different oxygen fugacities and at variable temperatureand fluid contents. The results indicate that the natural mineralogical assemblagecan only be reproduced from a composition showing a certain degree of mixing.Assuming a pressure of 200MPa (generally accepted for the Aeolian Islands), the preeruptivetemperature of the magmas is estimated between 755 and 800 °C and the watercontent of the melt was higher than 4–4.5 wt.%.The Upper Pollara magma crystallized at relatively high fO2 (deltalog fO2=Ni–NiO+1 log unit), compared to rhyolitic magmas from Lipari and Vulcano. As thisdifference has not been observed for the most primitive magmas the difference in fO2could be related to different degassing processes operating in Salina and Lipari –Vulcano magmas.