Rheology of the evolved alkaline magmas of Vesuvius and the Phlegraean Fields: The anomalous effects of chlorine and fluorine

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The effect of Cl and F together, on the rheology and structure of evolved hydrous alkaline melts will be determined via micropenetration viscosity techniques and NMR and Raman spectroscopy. Preliminary studies have shown that chlorine tends to increase the viscosity of dry peralkaline melts and to decrease the viscosity of peraluminous compositions. Chlorine and fluorine together in these melts produces the opposite effect; that is, a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. Fluorine alone always decreases silicate melt viscosity. NMR measurements have shown that high field strength cations compete with Al3+ to form bonds with F; while chlorine avoids Al3+ and bonds with both the alkali and alkaline earth cations present in the melt. As explosive volcanic eruptions can be caused by the growth of volatile-filled bubbles in an increasingly high viscosity volatile-poor melt, it is of major importance to understand the interaction of the common volatiles F and Cl and H2O in the melt structure and their effect on magma viscosity. The melt compositions used here are based on the magmas of Vesuvius and the Phlegraean Fields, which have chlorine contents up to 2 mol%, and fluorine contents up to 2 mol%; as well as water content up to 4 wt %.