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

Abstract

We present results from phase equilibria experiments conducted on the most primitive pahoehoe "cicirara" trachybasaltic lava flow ever erupted at Mt. Etna Volcano. This lava is characterized by a pahoehoe morphology in spite of 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 degrees C and using H2O and CO2 concentrations corresponding to the water-undersaturated crystallization conditions of Etnean magmas. Results show that olivine does not crystallize from the melt, whereas titanomagnetite is the liquidus phase followed by clinopyroxene or plagioclase as a function of melt-water concentration. This mineralogical feature contrasts with the petrography of pahoehoe cicirara lavas suggesting early crystallization of olivine and late formation of titanomagnetite after plagioclase and/or in close association with clinopyroxene. The lack of olivine produces MgO-rich melt compositions that do not correspond to the evolutionary behaviour of cicirara magmas. Moreover, in a restricted thermal path of 50 degrees C and over the effect of decreasing water concentrations, we observe abundant plagioclase and clinopyroxene crystallization leading to trace element enrichments unlikely for natural products. At the same time, the equilibrium compositions of our mineral phases are rather different from those of natural cicirara phenocrysts and microphenocrysts. The comparison between our water-undersaturated data and those from previous degassing experiments conducted on a similar Etnean trachybasaltic composition demonstrates that pahoehoe cicirara lavas originate from crystal-poor, volatile-rich magmas undergoing abundant degassing and cooling in the uppermost part of the plumbing system and at subaerial conditions where most of the crystallization occurs after the development of pahoehoe surface crusts.