Experimental constraints on the origin of pahoehoe cicirara lavas at Mt. Etna Volcano (Sicily, Italy)

We present results from phase equilibria experimentsconducted on the most primitive pahoehoe Bcicirara^trachybasaltic lava flow ever erupted at Mt. Etna Volcano.This lava is characterized by a pahoehoe morphology in spiteof its high content of phenocrysts and microphenocrysts(>40 vol%) with the occurrence of centimetre-sized plagioclases(locally named cicirara for their chick-pea-like appearance).Our experiments have been performed at 400 MPa,1100–1150 °C and using H2O and CO2 concentrations correspondingto the water-undersaturated crystallization conditionsof Etnean magmas. Results show that olivine does not crystallizefrom the melt, whereas titanomagnetite is the liquidusphase followed by clinopyroxene or plagioclase as a functionof melt–water concentration. This mineralogical featurecontrasts with the petrography of pahoehoe cicirara lavas suggestingearly crystallization of olivine and late formation oftitanomagnetite after plagioclase and/or in close associationwith clinopyroxene. The lack of olivine produces MgO-richmelt compositions that do not correspond to the evolutionarybehaviour of cicirara magmas.Moreover, in a restricted thermalpath of 50 °C and over the effect of decreasing water concentrations,we observe abundant plagioclase and clinopyroxenecrystallization leading to trace element enrichments unlikely fornatural products. At the same time, the equilibrium compositionsof our mineral phases are rather different from those ofnatural cicirara phenocrysts and microphenocrysts. The comparisonbetween our water-undersaturated data and those fromprevious degassing experiments conducted on a similar Etneantrachybasaltic composition demonstrates that pahoehoe ciciraralavas originate from crystal-poor, volatile-rich magmas undergoingabundant degassing and cooling in the uppermost part ofthe plumbing system and at subaerial conditions where most ofthe crystallization occurs after the development of pahoehoesurface crusts.