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Lava in the time of Carbon: Explosive volcanism on Mercury

Kayla Iacovino, Jacobs/NASA Johnson Space Center

The planet Mercury is a fascinating example of what happens when a rocky planet gets geochemically weird. About 15% of Mercury’s surface is made up of “low reflectance material” terrane, possibly exposed portions of a primordial graphite flotation crust formed during differentiation. Pyroclastic volcanism was common on Mercury, with >150 likely pyroclastic deposits identified from orbit, but what drives explosive eruptions there is unknown. Given the possible presence of a relatively global graphite layer near the hermean surface, this begs the question: can the volatilization of carbon by an ascending magma generate sufficient explosive force to explain the pyroclastic deposits observed? We performed experiments to reproduce the interaction between supersolidus hermean 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 measured in situ and solid run products demonstrate the generation of CO and CO₂ gas and the formation of Fe-Si metal alloys, which were found in contact with residual graphite grains. Results are attributed to natural graphite-melt smelting via the generalized reaction FeO_melt + C_graphite = CO_gas + Fe⁰. Modeling coupling composition to eruption velocities indicates that gas produced by smelting alone can account for >75% of the pyroclastic deposits identified on Mercury. A combination of magmatic S-H-degassing and CO-CO₂ production from smelting can explain all but the single largest pyroclastic deposit on Mercury.