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

Abstract

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 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.

Key Points

• Chromium abundance is useful as a probe of redox conditions during planet formation and evolution

• We used MESSENGER x-ray fluorescence data to determine whether surface Cr abundance is heterogeneous on Mercury with an average value of ∼200 ppm

• Experimental data and differentiation modeling put constraints on Mercury's oxidation state, depending on the presence of mantle sulfides

Plain Language Summary

Data returned by NASA's MESSENGER mission, which orbited Mercury from 2011 to 2015, have shown that the innermost planet formed under highly reducing (relatively low-oxygen) conditions, compared to the other terrestrial planets, but estimates of Mercury's oxidation state are highly uncertain. Chromium, a minor element in planetary materials, can exist in a wide range of oxidation states and its abundance thus can provide information about the chemical conditions under which it was incorporated into rocks. We used data from MESSENGER's X-ray Spectrometer instrument to map the Cr/Si ratio across much of Mercury and found that Cr is heterogeneously distributed. By comparing the average measured Cr abundance to the results of planetary differentiation models (informed by experimental data on how Cr partitions between different phases under different planetary differentiation conditions), we placed new constraints on Mercury's oxidation state and show that further refinement of this quantity could be used to place limits on the presence of sulfides in the planet's deep interior.