Carbon as the primary driver of super-reduced explosive volcanism on
Mercury: Evidence from graphite-melt smelting experiments
Abstract
Multiple volcanic deposits, both pyroclastic and effusive, have been
identified on the surface of Mercury. The modeling of volcanic processes
on Mercury, particularly with respect to the amount and composition of
volcanic volatiles, is hindered by a lack of existing experiments
performed at or near Mercurian conditions. Most notable is Mercury’s
extremely reducing nature (3–8 log units below the iron-wüstite
buffer), which is well beyond the range of fO2s for
which existing thermodynamic models are calibrated. The high carrying
capacity of sulfur compared to carbon or hydrogen in reduced magmas,
combined with remarkably high S concentrations in Mercurian surface
materials, has led to the assumption that S is an important driver of
volcanic activity on the planet. However, evidence for a primary
graphite floatation crust as well as graphite present within Mercury’s
regolith provide a mechanism for C-rich gas production via
magma-graphite smelting reaction. Smelting, in which graphite is
oxidized to CO and CO2 gas and melt oxide species are
reduced to metal (e.g., Cgraphite +
FeOmelt = COgas +
Fe0metal), would also serve to remove
O from the silicate melt, consistent with the production of a remarkably
reduced surface environment. We carried out experiments to emulate
conditions for graphite-induced smelting of three Mercurian magma
compositions at high temperatures (ramped from ambient to 1195–1390 °C)
and low pressures (8–10 mbar). The compositions of resultant gases were
measured in situ via an evolved gas analyzer, and solid run products
were analyzed by electron microprobe. Degassing vapor was always
dominated by C-O-H species, and S degassing was not detected in any
experiments. No significant C releases were measured in experiments
using transition metal oxide-free starting silicate compositions,
suggesting that transition metal reduction may be required to oxidize
graphite to gas. Experiments that produced vapor formed Fe-Si metal
alloy blebs, which were always in contact with residual graphite,
strongly supporting metal and gas production via smelting between
graphite and melt. Our results indicate that CO and CO2
are likely the most dominant volcanic degassers (and thus drivers of
explosive volcanism) on Mercury, and that S degassing plays only a
subordinate role, contrary to what has been hitherto assumed.