  Mercury, the innermost planet, formed under highly reduced conditions, based
mainly on surface Fe, S, and Si abundances determined from MESSENGER mission
data. The minor element Cr may serve as an independent oxybarometer, but only
very limited Cr data have been previously reported for Mercury. We report Cr/Si
abundances across Mercury's surface based on MESSENGER X-Ray Spectrometer data
throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio
ranges from 0.0015 in the Caloris Basin to 0.0054 within the high-magnesium
region, with an average southern hemisphere value of 0.0008 (corresponding to
about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at
least 30%, but relative variations are more robust. By combining experimental
Cr partitioning data along with planetary differentiation modeling, we find
that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the
planet's core and differentiation must have occurred at log fO2 in the range of
IW-6.5 to IW-2.5 in the absence of sulfides in its interior, and a range of
IW-5.5 to IW-2 with an FeS layer at the core-mantle boundary. Models with large
fractions of Mg-Ca-rich sulfides in Mercury's interior are more compatible with
moderately reducing conditions (IW-5.5 to IW-4) owing to the instability of
Mg-Ca-rich sulfides at elevated fO2. These results indicate that if Mercury
differentiated at a log fO2 lower than IW-5.5, the presence of sulfides whether
in the form of a FeS layer at the top of the core or Mg-Ca-rich sulfides within
the mantle would be unlikely.
