Explosive Volcanic Eruptions on Mercury: Eruption Conditions, Magma Volatile Content, and Implications for Mantle Volatile Abundances

During its first flyby of Mercury, MESSENGER imaged several candidate volcanic centers inside the rim of the Caloris impact basin. The most prominent of these features is a broad shield-like structure over 100 km in diameter, with a near-central, irregularly shaped depression surrounded by a bright, spectrally distinctive deposit interpreted to have a pyroclastic origin. The candidate pyroclastic deposit on Mercury has a mean radius of ~24 km, equivalent in size to the third largest lunar pyroclastic deposit when mapped to lunar gravity conditions. From the extent of the candidate pyroclastic deposit, it is possible to characterize the eruption parameters of the event that emplaced it, including vent speed and estimates of magmatic volatile content for candidate volatile species. The minimum vent speed is found to be ~300 m/s, and the pre- eruptive magmatic volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million (ppm) by weight for volatiles of the type that typically propel volcanic eruptions on other planetary bodies (e.g., CO, CO2, H2O, SO2). On Earth, measurements of the exsolution of volatiles (H2O, CO2, S) from basaltic magmas during eruptive episodes at Kilauea volcano, Hawaii, indicate values of ~1300-6500 ppm for the mantle source. Evidence for the presence of significant amounts of volatiles in partial melts derived from the crust or mantle of Mercury is an unexpected result given that most models for the formation of the innermost, metal-rich planet predict extreme volatile depletion.