A Low O/Si Ratio on the Surface of Mercury: Evidence for Silicon Smelting?

• Published in: Journal of geophysical research. Planets Vol. 122; no. 10; pp. 2053 - 2076
• Main Authors: McCubbin, Francis M., Vander Kaaden, Kathleen E., Peplowski, Patrick N., Bell, Aaron S., Nittler, Larry R., Boyce, Jeremy W., Evans, Larry G., Keller, Lindsay P., Elardo, Stephen M., McCoy, Timothy J.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-10-2017
• Subjects: Aerospace environments; Atmosphere; explosive volcanism; Ferrous alloys; Gamma rays; Geochemistry; Graphite; Iron; Magma; Melts; Mercury; Mercury (planet); Mercury atmosphere; Mercury surface; MESSENGER; MESSENGER Mission; Metallurgy; oxygen fugacity; Partial pressure; Regolith; Silicon base alloys; Silicon dioxide; Smelting; Space weathering; Spacecraft; Weathering
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1002/2017JE005367

Data from the Gamma‐Ray Spectrometer (GRS) that flew on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12–20% of the surface materials on Mercury are composed of Si‐rich, Si‐Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite‐induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4–0.9 wt % CO from the reduction of FeO to Fe0—CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite‐entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si‐rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5–4.1 Ga timespan over which the planet was resurfaced through volcanic processes.
Key Points
Mercury's surface has 12–20% metallic phases produced by space weathering and graphite‐induced smelting of boninitic and komatiitic lavas
If smelting occurred, it would have produced substantial abundances of CO that may have facilitated explosive volcanic processes on Mercury
The S and Fe abundances on the surface of Mercury are sourced from volcanic processes and indicate a mantle that is at IW −3.2 to −4.3