^1,2Larry R. Nittler,^1,3Asmaa Boujibar,^4,5Ellen Crapster-Pregont,¹Elizabeth A. Frank,⁶Timothy J. McCoy,⁷Francis M. McCubbin,^8,9Richard D. Starr,^4,10Audrey Vorburger,^1,11Shoshana Z. Weider
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007691]
¹Earth and Planets Laboratory, Carnegie Institution of Washington, Washington, DC, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³Geology Department, Department of Physics & Astronomy, Western Washington University, Bellingham, WA, USA
⁴Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, USA
⁵Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA
⁶National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
⁷Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, USA
⁸Physics Department, The Catholic University of America, Washington, DC, USA
⁹Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
¹⁰Physics Institute, University of Bern, Bern, Switzerland
¹¹Agile Decision Services, Washington, DC, USA
Published by arrangement with John Wiley&Sons

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.