The study of F-rich mineral phases, namely fluorophlogopite and fluorapatite, occured in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive centre; ~15 ka BP) allowed us to define the physical and chemical crystallization conditions of such minerals. Textural evidence suggests a late-stage crystallization of the F-rich minerals, since fluorapatite is exclusively found in the groundmass and fluorophlogopite within lava vesicles. Furthermore, a colourless SiO2-rich amorphous phase, characterized by multi-stage deposition, has overgrown the fluorophlogopite crystals.
Comparison with simulations of crystal fractionation demonstrates that the benmoreitic lava characterized by the occurrence of F-dominant minerals is anomalously enriched in some major and trace elements (e.g., Ti, Fe, K, Ba and, to a minor extent, Rb and REEs). Even the modelling of crustal contamination, possibly caused by assimilation of the sedimentary basement underlying the volcano edifice, is poorly consistent with the geochemical features of the considered benmoreite. Chlorine and fluorine concentrations estimated for this lava sample are 0.20 and 0.34 wt% respectively, which are significantly higher than those of other Etnean prehistoric mugearites and benmoreites. The selective enrichment in major and trace elements, and particularly in halogens, has been therefore related to other rarely recognized differentiation processes acting in the feeding system. Specifically, volatile-induced differentiation, ruled by elemental transfer (as metal-halogen complexes) in a volatile phase, is able to account for the observed geochemical variations. Such a volatile influx might be released by more primitive, deeper and volatile-rich magmas while rising up towards shallower levels of the feeding system. Considering the solubility of fluorine in silicatic systems at low pressure higher than that of chlorine, we suggest that fluorapatite and fluorophlogopite were likely grown during syn- or post-eruption pneumatolytic stages, probably after open-system degassing when a gas phase characterized by a high Cl/F ratio was released. The paramount role played by volatiles is also consistent with the occurrence of SiO2-rich amorphous concretions surrounding the fluorophlogopite crystals. Indeed, large amounts of SiF4 in the gas phase can sublimate under cooling conditions into Si-rich amorphous concretions. On the grounds of our findings, the process here described could have significant implications to explain unexpected eruptive behaviours at Mt. Etna, such as highly explosive dynamics of extrusion or the rather low viscosity of highly evolved lavas.