Deep supply of volatile-rich high-Mg magmas changes explosivity of Mount Etna eruptions

Abstract

The volcanic hazard potential of Mount Etna volcano is presently nourished by long-lasting, powerful eruptions of basaltic magmas coupled with increased seismicity and ground deformation, and the world’s largest discharge of volcanic gases. The current evolutionary cycle of Mount Etna activity is consistent with subduction-related chemical modifications of the mantle source. Arrival of a new mantle-derived magma batch beneath the volcano has been hypothesized, but is still elusive among the erupted products. Here we demonstrate petrological and geochemical affinities between the magmas supplying modern eruptions and high-Mg, fall-stratified (“FS”) basalts ejected violently ~4 Kyr ago. The “FS” primitive magmas (~13 wt% MgO) are characteristically volatile-enriched (at least 3.8 wt% H2O and 3300 ppm CO2), and bear trace element signature of a garnet-bearing, metasomatised source (high Gd/Yb, K/La, U/Nb, Pb/Ce, Ca/Al). They started crystallizing olivine (Fo91), clinopyroxene (Mg# 92.5) and Cr33 spinel deep in the plumbing system (> 5 kb), contributing to the cumulate piles at depth and to differentiated alkaline basalt and trachybasalt magmas in the shallow conduit. Continuous influx of mantle-derived, volatile-rich magmas, such as those that supplied the “FS” fallout, provides a good explanation for major compositional and eruptive features of Mount Etna.