Physical constraints and magma dynamics of Mt. Etna rift systems

 

Mt. Etna features an articulated plumbing system characterized by a central open-conduit, culminating with the persistent degassing summit craters, three rift-related lateral systems (S-Rift, NE Rift and W Rift) and the eccentric feeding system, characterized by disperse monogenetic cones.

In the last twenty years, most the eruptive activity occurred at the summit central craters and by consequence most of the recent petrological studies focused on the parametrization of the central open-conduit system. In this study, we move the focus to the NE Rift system, whose last activity dates back to the 2002-2003 eruption. Rift-related events are potentially more dangerous since they are often accompanied by energetic precursor seismicity and increase the probability of lava effusion at low altitude where towns and infrastructures are concentrated. Samples from this last eruption were examined and the new chemical data integrated with a comprehensive whole rock and mineral chemistry dataset from pre-historical and historical events.

Textural observations of the NE Rift products highlight a greater variability compared to magmas erupted from the central craters, in spite of a comparable mineral assemblage made of Ol, Cpx, Plg and Ti-Mt. High and low porphyritic lavas coexist in the same event and appear frequently mingled. Similarly, whole-rock composition varies from hawaiite-trachybasalt to benmoreite, in contrast with the rather homogenous trachybasaltic composition of magmas erupted from the central craters. Plagioclase phenocrysts show partially resorbed rims associated with an increase in An content or alternatively, alignments of melt inclusions near the crystal rim, related to a decrease in An content.

Thermo-barometric estimates based on Ol-Liq and Cpx-liq equilibria suggest that most of Ol and Cpx phenocrysts equilibrated at temperature comprised between 1140 to 1000 °C and pressure ranging from 10 to 2 Kbar, with a remarkably higher DT/DP with respect to magmas erupted at the central craters. This suggests a magma crustal ponding zone between 4 and 2 kbar. These results have been integrated by thermodynamic modelling through the energy-constrained model Magma Chamber Simulator able to compute the evolution of the magma via fractional crystallization in a polybaric and polythermal volcanic plumbing system. Results highlights that fractionation occur along the Ol-Cpx-Plag liquid line of descent in a range of pressure equivalent to those determined by the crystal-melt geobarometry.