Manganese Mobility in Gale Crater, Mars: Leached Bedrock and Localized Enrichments

In Gale crater on Mars, the rover Curiosity has discovered evidence of fluid mobilization of the redox-sensitive element manganese. We present results for Mn from Curiosity's Alpha Particle X-ray Spectrometer (APXS), which show that the average MnO concentration in mudstone-dominated sedimentary units (0.22 wt%) is about one-half of the concentration in the average Mars crust (0.44 wt%). Geochemical trends indicate that Mn in the sedimentary bedrock, most of which has a basaltic provenance, was leached by chemical alteration and dissolution. In >350 vertical meters of mudstone-dominated strata, the apparent leaching of Mn and retention of Fe in Fe-O-H phase(s) resulted in the fractionation of Fe and Mn, indicating relatively moderate Eh-pH fluid conditions that were not highly alkaline, reducing, or oxidizing. Exceptions are fracture-associated, silica-rich haloes where both Mn and Fe were leached by low pH fluids. The rover also discovered Mn-rich veins, nodules, and patchy, dark coatings on rock surfaces, which are variably associated with enrichments in Fe, P, Cl, and/or Zn. These Mn-rich features represent ∼1% of the 1029 APXS measurements acquired over ∼25 km of rover traverse. A thermochemical model shows that dissolved Mn²⁺ could have been concentrated via evaporation, sublimation, and/or freezing. Manganese was then likely precipitated in localized features when >99.99% of the Mn²⁺-bearing water was removed from the system. These findings indicate that Mn was mobile in Gale crater and therefore bioavailable as a potential energy source for life.