Reconstruction of magmatic variables governing recent Etnean eruptions: Constraints from mineral chemistry and P–T–fO2–H2O modeling

• Mineral compositions at Mt. Etna are clear indicators of magma crystallization conditions.
• Etnean magmas fractionate throughout the entire length of the vertically developed plumbing system.
• Magma mixing, volatile exsolution and degassing are key processes driving eruptions.

Petrological investigations of active volcanoes are often supported by mass balance, thermodynamic calculations and/or experiments performed at key conditions. Conversely, the compositions of mineral phases found in natural products are generally used as input data for predictive models calibrated to derive the intensive variables of the magmatic system. In order to evaluate the extent to which mineral chemistry records crystallization conditions, we have compared the compositions of olivine, clinopyroxene, plagioclase and titanomagnetite in 2001–2012 trachybasaltic lavas at Mt. Etna with those obtained through thermodynamic simulations and experiments conducted under anhydrous, water-undersaturated and water-saturated conditions. This systematic comparison allows us to track recent differentiation processes beneath Mt. Etna, as well as the P–T–fO2–H2O variables controlling the solidification path of magma. Two compositionally distinct populations of olivine and clinopyroxene phenocrysts are found in these lavas: Mg-rich and Mg-poor minerals formed at 600–1100 MPa and 1100–1250 °C, and 0.1–500 MPa and 1050–1175 °C, respectively. The oxygen fugacity varies by 1–2 log units suggesting water exsolution during magma ascent in the conduit and magma emplacement near the surface. The nucleation and growth of normally zoned plagioclases occur at P < 100 MPa, when the amount of H2O dissolved in the melt abruptly decreases from about 3.0 to 0.2 wt.% due to magma decompression and degassing. This leads to the conclusion that Etnean magmas fractionate throughout the entire length of the vertically developed plumbing system where magma mixing, volatile exsolution and degassing are the most important processes driving eruptions.