Sulphur behaviour and redox conditions in etnean magmas during magma differentiation and degassing

Abstract

Abstract Sulphur behaviour and variations in redox conditions during magma differentiation and degassing in Mt. Etna (Italy) volcanic system have been explored by integrating the study of olivine-hosted melt inclusions (MIs) with an experimental survey of sulphur solubility in hydrous basaltic magmas. Sulphur solubility experiments were performed at conditions relevant to the Etnean plumbing system (1200 °C, 200 MPa and oxygen fugacity between NNO + 0.2 and NNO + 1.7, with NNO being the Nickel-Nickel Oxide buffer), and their results confirm the important control of oxygen fugacity (fO2) on S abundance in mafic magmas and on S partitioning between fluid and melt phases (DSfluid/melt). The observed DSfluid/melt value increases from 51 ± 4 to 146 ± 6 when fO2 decreases from NNO + 1.7 ± 0.5 to NNO + 0.3. Based on the calculated DSfluid/melt and a careful selection of previously published data, an empirical model is proposed for basaltic magmas in order to predict the variation of DSfluid/melt values upon variations in P (25–300 MPa), T (1030–1200 °C) and fO2 (between NNO-0.8 and NNO + 2.4). Olivine-hosted melt inclusions (Fo89-91) from tephra of the prehistoric (4 ka BP) sub-plinian picritic eruption, named FS (“Fall Stratified”), have been investigated for their major element compositions, volatile contents and iron speciation (expressed as Fe3+/ƩFe ratio). These primitive MIs present S content from 235 ± 77 to 3445 ± 168 ppm, while oxygen fugacity values, estimated from Fe3+/ƩFe ratios, range from NNO + 0.7 ± 0.2 to NNO + 1.6 ± 0.2. Iron speciation has also been investigated in more evolved and volatile-poorer Etnean MIs. The only primitive melt inclusion from Mt. Spagnolo eruption (4–15 ka BP) presents a S content of 1515 ± 49 ppm and an estimated fO2 of NNO + 1.4 ± 0.1. The more evolved MIs (from 2002/2003, 2006, 2008/2009 and 2013 eruptions) have S content lower than 500 ppm, and their Fe3+/ΣFe ratios result in fO2 between NNO-0.9 ± 0.1 and NNO + 0.4 ± 0.1. Redox conditions and S behaviour in Etnean magmas during degassing and fractional crystallization were modelled coupling MELTS code with our empirical DSfluid/melt model. Starting from a FS-type magma composition and upon decrease of T and P, fractional crystallization of olivine, clinopyroxene, spinel and plagioclase causes a significant fO2 decrease. The fO2 reduction, in turn, causes a decrease in sulphur solubility and an increase in DSfluid/melt, promoting S exsolution during magma ascent, which further enhances the reduction of fO2. For the evolved melt inclusions of 2002–2013 eruptions, magma differentiation may therefore have played a crucial role in decreasing redox conditions and favouring efficient S degassing. Differently, during the unusual FS eruption, only limited melt evolution is observed and S exsolution seems to have been triggered by a major pressure decrease accompanied by H2O and CO2 exsolution during fast magmatic ascent.