Chromium on Mercury: New Results From the MESSENGER X‐Ray Spectrometer and Implications for the Innermost Planet's Geochemical Evolution

Author:

Nittler Larry R.12ORCID,Boujibar Asmaa13,Crapster‐Pregont Ellen45,Frank Elizabeth A.1,McCoy Timothy J.6,McCubbin Francis M.7ORCID,Starr Richard D.89ORCID,Vorburger Audrey410,Weider Shoshana Z.111ORCID

Affiliation:

1. Earth and Planets Laboratory Carnegie Institution of Washington Washington DC USA

2. School of Earth and Space Exploration Arizona State University Tempe AZ USA

3. Geology Department Department of Physics & Astronomy Western Washington University Bellingham WA USA

4. Department of Earth and Planetary Sciences American Museum of Natural History New York NY USA

5. Department of Earth and Environmental Sciences Columbia University New York NY USA

6. National Museum of Natural History Smithsonian Institution Washington DC USA

7. Astromaterials Research and Exploration Science Division NASA Johnson Space Center Houston TX USA

8. Physics Department The Catholic University of America Washington DC USA

9. Solar System Exploration Division NASA Goddard Space Flight Center Greenbelt MD USA

10. Physics Institute University of Bern Bern Switzerland

11. Agile Decision Services Washington DC USA

Abstract

AbstractMercury, 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 3.6 × 10−5 in the Caloris Basin to 0.0012 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.

Publisher

American Geophysical Union (AGU)

Subject

Space and Planetary Science,Earth and Planetary Sciences (miscellaneous),Geochemistry and Petrology,Geophysics

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