Melt inclusions track melt evolution and degassing of Etnean magmas in the last 15 ka

Abstract

We present major elements compositions and volatiles contents of olivine-hosted melt inclusions from Etna volcano (Italy), which extend the existing database with the aim of interpreting the chemical variability of Etnean magmas over the last 15 ka. Olivine phenocrysts were selected from the most primitive Fall Stratified (FS) eruptive products of picritic composition (Mg# = 67–70, Fo89–91), the Mt. Spagnolo eccentric lavas (Mg# = 52–64, Fo82–88) and among the more recent 2002–2013 eruptive products (Mg# = 33–53, Fo68–83). Crystal fractionation and degassing processes were modeled at temperatures of 1050–1300 °C, pressures <500 MPa, and oxygen fugacity between 1 and 2 log units above the nickel-nickel oxide buffer, in order to interpret melt inclusions data.

Melt inclusions show a great variability in major elements chemistry (e.g., 44–57 wt% SiO2, 3–16 wt% CaO, 4–13 wt% FeO, 2–12 wt% MgO, 1–6 wt% K2O), designating a continuous differentiation trend from FS toward 2013 entrapped melts, which is mostly reproduced by the fractional crystallization of olivine + spinel + clinopyroxene ± plagioclase, in order of appearance.

Volatile contents in the glass inclusions are also extremely variable, with maxima up to 6 wt% H2O and 0.6 wt% CO2 in FS melt inclusions, and up to 0.43 wt% S in Mt. Spagnolo inclusions. H2O and CO2 contents in the melt inclusions suggest minimum entrapment depths of 4–19 km (below crater level) for FS inclusions and <10 km for the 2002–2013 trachybasalts.

Petrological arguments coupled to the modeling of fractional crystallization and degassing processes concur to suggest that magmas from Mt. Spagnolo and the recent eruptions may be produced by differentiation from the most primitive volatile-rich FS magma along variable P-T paths, occasionally accompanied by secondary processes as crustal assimilation, mixing, and CO2 flushing. We do not exclude the occurrence of source processes at Etna, e.g., variable degrees of mantle melting and/or variable degrees of mantle contamination, already proposed by previous authors. Our data, nevertheless, suggest that the first-order features of the Etnean magmas erupted in the last 15 ka can be modeled by differentiation through fractional crystallization and degassing.