Gennaro, E; Paonita, A; Iacono-Marziano, G; Moussallam, Y; Pichavant, M; Peters, N; Martel, C; (2020) Sulphur behaviour and redox conditions in etnean magmas during magma differentiation and degassing. Journal of Petrology , 61 (10) 10.1093/petrology/egaa095.

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Abstract

Sulphur behaviour and variations in redox conditions during magma differentiation and degassing in the 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 to predict the variation of DSfluid/melt values with 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+/ΣFe ratio). These primitive MIs present S content from 235 ± 77 to 3445 ± 168 ppm, and oxygen fugacity values, estimated from Fe3+/ΣFe 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 the 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 an 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 MIs 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.

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