We present results from phase equilibria experi-ments conducted on the most primitive pahoehoeBcicirara^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 plagio-clases (locally named cicirara for their chick-pea-like appear-ance). Our experiments have been performed at 400 MPa,1100–1150 °C and using H2OandCO2concentrations corre-sponding to the water-undersaturated crystallization conditionsof Etnean magmas. Results show that olivine does not crystal-lize from 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 sug-gesting early 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 concen-trations, we observe abundant plagioclase and clinopyroxenecrystallization leading to trace element enrichments unlikely fornatural products. At the same time, the equilibrium composi-tions of our mineral phases are rather different from those ofnatural cicirara phenocrysts and microphenocrysts. The com-parison between 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 under-going abundant 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.
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|Titolo:||Experimental constraints on the origin of pahoehoe “cicirara” lavas at Mt. Etna Volcano (Sicily, Italy)|
|Data di pubblicazione:||2015|
|Appare nelle tipologie:||03.1 Articolo su rivista|