The Mt Etna December 2018 eruption: a two-magma mixing as evidenced by a geochemical study of melt and fluid inclusions

Mt Etna is one of the most investigated and monitored volcanoes in the world and produces summit and flank eruptions mostly controlled by magma rise in central conduits. In detail, flank eruptions are mostly driven by fracturing of the central conduits and radial magma drainage and produce lava flows of considerable volume. The eruption of 24-27 December 2018 at Mt. Etna is a flank eruption resulted from the intrusion of a deep dike that after an initial stage of lava fountains, proceeded with quiet lava effusion. Despite the low duration and the small lava volume emitted, the 2018 eruption was associated to a very strong seismic swarm that caused severe damages to neighbouring villages.

Major and trace element geochemistry of olivine-hosted melt inclusions (MIs) in volcanic products from Mt Etna December 2018 eruption, together with noble gas geochemistry of fluid inclusions (FIs) in olivines were investigated, with the aim to constrain the characteristics of the feeding magma.

We evidenced a geochemical variability in the major and trace element content of MIs (SiO₂=45.51-52.83 wt% MgO=3.83-6.02 wt% and CaO/Al₂O₃=0.34-0.72 and Ba/La =9.3-15.7, K/Nb =256 - 1037, Ce/Nb =1.98-3.39, Rb/La =0.37-1.6, Ba/Nb =10.87-25.8) that cannot be explained entirely by crystallization processes but that we interpreted with a mixing between two different terms:

(i) A first one (Type-1), is characterized by a more evolved major element composition, but a more primitive inprint of the trace elements; it is comparable to magma emitted during the flank eruptions of 2001 (from Upper Vents) and 2002-03 (from Northern fissures). This term is well represented by a HIMU + MORB heterogeneous source;

(ii) A second one (Type-2) is more evolved with regard to the trace element geochemistry but preserves a more primitive major element signature. This term, comparable to that emitted in 2001 from Lower Vents and 2002-03 from Southern fissures, was influenced by crustal fluid contamination and/or assimilation of plagioclase that modifies the primordial trace element geochemical marker of the source.

The helium isotopic ratio (³He/⁴He) from fluid inclusions entrapped into olivine phenocrysts shows a variability ranging between 6.5 and 6.6 Ra, which perfectly matches literature Etnean dataset and allows to hypothesize the influence of a bland crustal contamination.

Our results support a scenario where, some months before the 24 December eruption, a deep magma rose from depth and was contaminated by a crustal term before mixing with the magma ponding in a reservoir located at shallow depth. As the trace elements are more sensible geochemical tracers of magmatic processes with respect to major elements, they record the crustal contamination event whereas the major element geochemistry keep memory of the more primitive signature of the deep magma. The two magmas are stored into the reservoir long enough to mix widely their fluid content but not to homogenize the trace elements, which partially maintain the differences of the two magma types. The 2018 olivine-hosted MI appear to have captured two different end-members whose lack of complete homogenization may imply a very fast ascent.