AN EXPERIMENTAL STUDY IN THE MG2SIO4-FE1-XS-H2O SYSTEM: IMPLICATIONS FOR GANYMEDE'S INTERIOR

The discovery of a strong magnetic field emanating from Jupiter's moon Ganymede spurred a flourish of research regarding the interior of the body. Many have speculated on the thermal history and current state of the interior. Previous studies have determined the reaction products and kinetics of serpentine, Mg₃Si₂O₅(OH)₄, dehydration at high temperatures. However, few have combined that with the presence of an iron-bearing sulfide, a commonly hypothesized material for the core of Ganymede, while conducting in situ X-ray studies of the two for the purpose of assessing the stability of an Fe-S phase during serpentine dehydration. This study tested the hypothesis that an iron-bearing sulfide undergoes alteration at the core-mantle boundary of Ganymede as a result of serpentine dehydration. Synchrotron X-ray radiation and X-ray powder diffraction techniques were utilized to establish the presence and reactivity of an iron-bearing sulfide (pyrrhotite in this study) in the presence of dehydrating serpentine and Energy Dispersive Spectroscopy (EDS) was used to estimate the exchange of iron between the sulfide and the Mg-rich silicates. The results indicate that the serpentine thermal decomposition correlated well with the voluminous previous research, except at ~2 GPa and 600 °C where serpentine dehydration, and thus forsterite formation, was observed. Pyrrhotite was found to be present throughout the study at all P-T conditions observed (up to 5 GPa and 600-800°C). Additionally, EDS data showed no indication of extensive iron exchange from the iron-bearing sulfide into the forsterite. These data indicate that iron-bearing sulfides are a possible core material for Ganymede and a forsterite layer will likely form from the dehydration of serpentine and may act as a thermal boundary layer between the core and a convective hydrous silicate mantle. Due to the absence of exchange of Fe from the FeS phase with the Mg-rich silicates during the experiments, it is unlikely that extensive exchange of Fe would take place at the core-mantle boundary due to the observed processes. If these experiments are indicative of the core-mantle boundary of Ganymede, it is likely that the observed, internally generated, magnetic field is a product of a dynamo within the core.