The 2011–2012 paroxysmal eruptions at Mt. Etna volcano: Insights on the vertically zoned plumbing system

Mt. Etna activity from January 2011 to April 2012 was characterized by 24 short-lasting (few to several hours) eruptions from the New South East summit crater. Despite the violence of the no appreciable geophysical signals were recorded during this period, except for an increase in the seismic tremors just minutes/hours before the occurrence of the paroxysm. This type represents a significant shift from the mainly effusive eruptions of 2004, 2006, 2008/2009 and from the lateral rift-related event of 2001 and 2002/2003. The 2011-2012 activity thus represent a unique opportunity to investigate the effects of chemical-physical variables (P-T-fO2) on the crystallization and fractionation processes occurring Etna open conduit feeding system. We investigated the petrographic and geochemical features of lava and scoria clasts from 10 paroxysmal events. Whole rock compositions plot inside the field with the typical etnean intraplate chondrite-normalized trace element distribution, characterized by positive U, Th and La and negative Rb, K, Nb anomalies. MELTs and mass balance fractional crystallization modelling suggest that most of the eruptive events were fed by magma differentiating along the conduit and by a deep basic magma during the 4/3/2012 event. Olivine (Ol), clinopyroxene (Cpx) and plagioclase (Plg) crystal-melt equilibrium conditions were checked before applying thermo-barometric, oxy-fugacity and hygrometer equations by the composition of phenocrysts with those of whole rock, glass and reconstructed composition of the basic magma. Results show that the erupted products are made up of a mixture of phenocrysts in equilibrium with the whole rock or disequilibrated toward more basic or more evolved compositions. Thermobarometric calculations indicate that ol is the first phase on the liquidus (~1270°C, up to 1200 MPa). Cpx crystallizes from 1200°C, at 700 Mpa in most basic melt (4/3/2012), to 100 Mpa. Plg nucleation is constrained by the dissolved amount of H2O in the melt and mostly occur above 250 Mpa. Hygrometer determinations indicate that basic magma contains up to H2O. The overall dataset suggests that the feeding system of 2011-2012 eruptive events was vertically extended without any significant ponding zone. The conduit is periodically filled with basic magma at +2 DFMQ average oxidation condition. The H2O degassing in the shallower portion of the magmatic column (P < 200 Mpa) induce a vertical differentiation. In the deeper the magmatic column at P > 300 MPa, the femic geochemical signature is preserved due to the low undercooling degree. The intermediate portion (300-200 Mpa) is repeatedly pushed H2O exsolution depth which promotes the plagioclase saturation and nucleation. In the shallower portion (< 200 Mpa) the efficient H2O exsolution result in a strong undercooling and promote massive plg nucleation and differentiation.