Crystallization conditions in the Upper Pollara magma chamber, Salina Island, Southern Tyrrhenian Sea |
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| Authors: | P Donato H Behrens R De Rosa F Holtz F Parat |
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| Institution: | Dipartimento Scienze della Terra, Università della Calabria, Arcavacata di Rende (CS), Italy
Institut für Mineralogie, Universit?t Hannover, Hannover, Germany
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| Abstract: | Summary Pyroclastites erupted from the Upper Pollara magma chamber (13 ka, Salina Island, Aeolian Archipelago) resulted from mingling
and mixing of rhyolitic and andesitic magmas. An experimental study has been conducted on the rhyolitic end-member to constrain
the pre-eruptive conditions of the magma. In order to check for the role of mixing on the equilibrium phase assemblage, three
different starting compositions, corresponding to three different mixing degrees, have been used. The crystallization experiments
were conducted at two different oxygen fugacities and at variable temperature and fluid contents. The results indicate that
the natural mineralogical assemblage can only be reproduced from a composition showing a certain degree of mixing. Assuming
a pressure of 200 MPa (generally accepted for the Aeolian Islands), the pre-eruptive temperature of the magmas is estimated
between 755 and 800 °C and the water content of the melt was higher than 4–4.5 wt.%.
The Upper Pollara magma crystallized at relatively high fO2 (ΔlogfO2 = Ni–NiO + 1 log unit), compared to rhyolitic magmas from Lipari and Vulcano. As this difference has not been observed for
the most primitive magmas the difference in fO2 could be related to different degassing processes operating in Salina and Lipari – Vulcano magmas. |
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