Program > Browse abstracts by speaker > Saracino Fabrizio

In this study, we performed high temperature (1500-1950 ℃) and high pressure (1.5−3 GPa) piston-cylinder experiments on Mercury-like mantle compositions with the aim of quantifying the effect of sulfur on depressing their liquidus temperature and understanding its role on phase equilibria. Indeed, the geochemical data provided by the NASA MESSENGER spacecraft unveiled a high amount of sulfur and low iron content on the surface of Mercury, which is explained by the reducing conditions of parental magmas. In reduced silicate melts, sulfur is dissolved in the silicate melt as S2- and is highly soluble because it replaces oxygen, forming MgS and CaS species in the melt. We prepared two Mercury mantle analogue samples to track the stability fields of olivine (high melt Mg/Si ratio) and orthopyroxene (low melt Mg/Si ratio) in both S-saturated and S-free systems. Reducing conditions were obtained by using different Si/SiO2 ratios in the mixes. S-saturated experiments show increasing S abundances in the silicate melts (~ 1 to 9 wt.%) as fO2 decreases (from IW-2.8 to IW-6, IW representing the iron-wüstite thermodynamic equilibrium). Using our experimental dataset, we propose an empirical model for predicting the liquidus depression as a function of the sulfur content in the melt. We show that 1 to 9 wt% S in the silicate melt causes the liquidus to decrease by ca. 20 to 190 °C, proving that the influence of S on melting temperatures is as strong as other volatiles (H2O, F, Cl). Our experiments also illustrate the role of sulfur in promoting the stability field of orthopyroxene over that of olivine, which has major implications for the crystallization of the Mercurian magma ocean and the primordial mineralogical stratification of the mantle.