Carbon as a key driver of super-reduced explosive volcanism on Mercury: Evidence from graphite-melt smelting experiments

Here we present the results of experiments designed to reproduce the interaction between super-solidus mercurian magmas and graphite at high temperatures (ramped up from ambient temperature to 1195–1390°C) and low pressure (10 mbar). The compositions of resultant gases were measured in situ with a thermal gravimeter/differential scanning calorimeter connected to a mass spectrometer configured to operate under low pressures and reducing conditions. Solid run products were analyzed by electron microprobe and Raman spectroscopy. Three magma starting compositions were based on the composition of the Borealis Planitia region (termed NVP for the Northern Volcanic Plains) on Mercury ± alkali metals, sulfur, and transition metal oxides. Smelting between FeOmelt and graphite was observed above 1100°C, evidenced by the generation of CO and CO2 gas and the formation of Fe-Si metal alloys, which were found in contact with residual graphite grains. Experiments with transition metal oxide-free starting compositions did not produce metal alloys and showed no significant gas production. In all runs that produced gas, C-O-H±S species dominated the degassing vapor. Our results suggest that the consideration of graphite smelting processes can significantly increase calculated eruption velocities and that gas produced by smelting alone can account for >75% of the pyroclastic deposits identified on Mercury. A combination of S-H-degassing and CO-CO2 production from smelting can explain all but the single largest pyroclastic deposit on Mercury.