On the behaviour of fluorine in volcanic processes

Volatile compunds play a very important role in both the evolution of magmas and the eruptive processes. Despite great interest in the knowledge of volatile distribution in magma chambers prior to eruptions, direct evidence of this kind is very difficult to obtain because a major quantity of gaseous species is released to the atmosphere during volcanic phenomena.Good estimates of volatile contents in magmas have been obtained by their distribution in coexisting mineral phases and microprobe analysis of glass inclusions; however, a sufficient set of data is not yet available to provide direct evidence of volatile concentrations in magma chambers before eruptive processes.Owing to their volatility, water, hydrogen, carbon dioxide, sulphur and chlorine compounds are largely concentrated in the explosive cloud. On the other hand, molecular species of fluorine, which are more soluble than water in magmatic melts, strongly partition into this latter phase rather than into the fluid. As a consequence, fluorine compounds are normally present in small concentrations in fumarolic gases and are not expected to degas appreciably from quenched volcanic products.With reference to the influence of weathering processes, recent research has shown that unaltered volcanic glasses have lost but a minor quantity of fluorine as a result of secondary reactions. Because of this, analytical data for fluorine in fresh igneous rocks would not differ significantly from the actual values pertaining to the magmatic stage.The distribution of fluorine in samples from Italy (Vesuvius, Vulcano, Lipari, Roccamonfina, Phlegraean Fields) and Greece (Santorini) appears to be correlated with the concentration of potassium, which is in accordance with theoretical assumptions and analytical results in other areas.According to experimental data, the presence of fluorine in magmas of silicate composition considerably enhances the solubility of water.Higher concentrations of water would mean a higher potential explosivity, and the possibility that the observed concentrations of fluorine can serve as a measure of different degrees of stored energy is considered.Contribution of C.N.R. Centro di Studio per la Mineralogia e la Geochimica dei Sedimenti.