Sulfur behavior and redox conditions in Etnean hydrous basalts inferred from melt inclusions and experimental glasses

Sulfur is an important volatile component of magmas that presents different oxidation states, depending on the redox conditions and on the phase of occurence: in silicate melts it is typically dissolved as S6+ and/or S2-, in the gas phase it occurs principally as SO2 (S4+) and H2S (S2-). Here, we adopt Mount Etna as a case study to investigate sulfur behavior in hydrous basaltic magmas during magma differentiation and degassing. Mount Etna is a complex magmatic system characterized by a certain variability in terms of both eruptive style and composition of erupted products, and in which magmatic redox conditions are poorly constrained. In order to improve knowledge about sulfur behavior and redox conditions in Etnean magmas, this research integrates the study of natural olivine-hosted melt inclusions with an experimental study on S solubility in hydrous alkali basalts at magmatic conditions. We have investigated major element compositions, volatile contents, and Fe speciation of olivine-hosted melt inclusions from 6 eruptions of the last 15 ky. Samples were selected from FS eruption, which is the most primitive (picritic composition, Fo91), Mt. Spagnolo (the oldest) and more recent eruptions (2002/2003, 2006, 2008/2009, and 2013). Experimental glasses were synthesized in the range of pressure (200 MPa), temperature (1200 °C), and oxygen fugacity conditions (NNO = +0.2 to +1.7) relevant to the Etnean system. Experimental results suggest the important control of fO2 on the S abundance in Etnean hydrous magma. The highest S content (6454 ppm) was achieved at fO2 = NNO+1.7, while at lower fO2 (NNO = +0.2 to +0.9) and at the sulfide saturation the maximum sulfur content was 2400 ppm. S content and Fe speciation of the experimental glasses are useful to interpret melt inclusions data. The melt inclusions were entrapped at different depths inside the magmatic system (up to ~ 18 km, below crater level, corresponding to a pressure of almost 500 MPa) as defined by H2O-CO2 contents. They delineate a continuous differentiation trend from the FS picrite toward the degassed 2013 basalt, marked by fractional crystallization. Sulfur content in Etnean melt is extremely variable and reaches 4150 ppm in the most primitive melt inclusions of Mt. Spagnolo (Fo87). XANES Fe3+/ΣFe spectra in some glass inclusions, resulted in the generally decreasing of Fe3+/ΣFe ratios from the most primitive (FS) to the most recent (2013) melts. MELTS software confirms that the Fe3+/ΣFe decrease is due principally to the melt differentiation process, enhanced to the S degassing at fO2 < NNO+1. Indeed, the magmas of Mt. Spagnolo and of the recent eruptions may be produced by differentiation from the oxidized, parental FS magma, accompanied occasionally by mixing processes. Magma reduction, in turn, induces the decrease of the sulfur solubility in the hydrous Etnean basalt, as well as of the sulfide saturation, and may constitute a possible enhancer of S exsolution, triggering the important S degassing observed in the last decades in Mt. Etna.