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

Abstract

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.

Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

“…The highly reduced nature and mineralogy of enstatite‐rich meteorites is similar to the f O 2 of Mercury estimated using data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft (McCubbin et al. , ; Zolotov et al. ).…”

“…Although we do not have any confirmed mercurian samples in our collections, based on their similar f O 2 and inferred mineralogies (McCubbin et al. ; Namur and Charlier ; Vander Kaaden et al. ; McCoy et al.…”

“…A unique characteristic of Mercury from a geochemical standpoint is the extremely low oxygen fugacity at its surface (IW−7.3 to IW−2.6) and within its interior (~IW−3) compared to Earth, the Moon, and Mars (McCubbin et al. , ; Zolotov et al. ).…”

“…; McCubbin et al. , ; Namur et al. ; Vander Kaaden and McCubbin ), and experimental data in these highly reducing systems are limited.…”

“…For example, smelting reactions have been predicted to occur on Mercury through the interaction of graphite and low‐FeO silicate melts under low pressure (McCubbin et al. ). The presence of graphite in enstatite chondrite impact melts can be used as a natural analog to understand whether or not smelting reactions can be expected to occur, and if so, how much Si is incorporated into the resulting metallic phases.…”