Sulfidation kinetics of olivine and diopside at Mercury surface conditions

The surface of Mercury is enriched in sulfur, reaching concentrations of up to 4 wt.%, according to findings from NASA’s MESSENGER mission. One proposed explanation for this enrichment involves volcanic degassing, whereby reduced sulfur is released from the planet’s interior and subsequently initiates sulfidation reactions with the surface rocks. However, the underlying mechanisms involved in the sulfidation of silicate minerals remain poorly understood. In this study, we explore the kinetics and mechanisms of the reaction between reduced gaseous sulfur and two common silicate minerals on Mercury’s surface – olivine and diopside. Our approach included high-temperature experiments, in which the silicate mineral – with a polished surface – and sulfur powder were placed separately into graphite crucibles and subsequently loaded into an evacuated silica glass tube. The samples were subjected to temperatures between 800 and 1250 °C. Under these conditions, the sulfur powder forms a gas which fills the tube, enabling a reaction with the silicate mineral. The duration of the experiments was varied between 1 hour and 1 month. The highly reducing conditions on Mercury were simulated by buffering the oxygen fugacity through the graphite-CO reaction. Our results reveal a fundamental contrast in the mechanism of the sulfidation reaction between olivine and diopside. In olivine, the reaction is primarily limited by diffusion, whereas diopside develops intermediate reaction layers at its surface, including Ca-depleted pyroxene. These findings offer new insights into sulfidation processes on Mercury, which has implications for observations by the BepiColombo mission currently en route to the planet.