Unusual magma storage conditions at Mt. Etna (Southern Italy) as evidenced by plagioclase megacryst-bearing lavas: implications for the plumbing system geometry and summit caldera collapse

Abstract

At Mt. Etna volcano, the emission of plagioclase megacryst-bearing lavas, known locally as “cicirara”, has occurred rarely and generally in association with unusual volcanological phenomena. In this work, we interpret the magma chamber processes and the structural features of the plumbing system that led to the production of these peculiar volcanic rocks, based on a detailed study of plagioclase megacrysts, including their oscillatory zoning, sieve textures, and fluid inclusions. Patchy zoning suggests limited ascent in the deep levels of the plumbing system, based on the plagioclase nucleation threshold and the volatile saturation depth. At intermediate, water-undersaturated levels of the plumbing system ascent is faster, as indicated by crystals with coarse sieve textures. Storage at shallow, water-saturated levels (less than 6 km deep) is associated with oscillatory zoning with very small changes in An. Slightly larger An variations coupled with different wavelengths provide evidence of convection of crystals across distinct zones of the chamber. Stripes of melt inclusions formed at steps of magma ascent and volatile loss, whereas layers of fluid inclusions may be related to episodes of volatile flushing into the magma chamber. In contrast, strongly sieve-textured envelopes with An increase and constant FeO may be related to mixing with more volatile-rich magmas of similar composition. We interpret the repeated occurrence of “cicirara” lavas as evidence that the shallow portion of the plumbing system underwent a progressive coalescence of a complex network of dykes and sills in response to increasing rates of magma supply from depth. Major magma withdrawals from this larger reservoir may be linked to episodes of summit instability associated with major caldera collapses.