Manganese Phase and Oxidation State Assessment in Analog Sample Analysis at Mars: Evolved Gas Analyzer Measurements

Manganese (Mn) enrichments in Gale crater, Mars, have been identified by the Curiosity rover in fracture fills, coatings, and nodules and can provide information about past geochemical conditions (e.g., oxidizing vs. reducing fluids). However, the oxidation state and phase of the Mn is unresolved. The goal of this study was to determine if the Sample Analysis at Mars-Evolved Gas Analyzer (SAM-EGA) can constrain the Mn-bearing phase and oxidation state.

68 Mn-bearing samples (synthetic and natural) were analyzed using a thermal gravimetry (TG)/differential scanning calorimetry (DSC) instrument connected to a quadrupole mass spectrometer, configured to operate similarly to SAM (e.g., heating rate, furnace pressure, carrier gas). Results demonstrated that lower valence (Mn2+) oxides did not produce significant O2 peaks below 1000°C and therefore would not be detectable with SAM-EGA. Higher valence (Mn3+, Mn4+) oxides evolved O2 releases between 460-1000°C and would be detectable with SAM-EGA if present above SAM’s O2 detection limit of 0.01 wt. %. Laboratory EGA results for amorphous Mn oxides are consistent with their crystalline counterparts, indicating that higher valence Mn oxides, potentially in the X-ray amorphous component of drill samples, would also be detectable by SAM-EGA.

Mn carbonate, Mn sulfate/sulfide, Mn perchlorate, and Mn oxalate are detectable by SAM-EGA, showing distinct CO2, SO2, HCl, and H2O, CO, and CO2 evolutions, respectively. In contrast, hydrated Mn phosphates would be difficult to discern with SAM-EGA, unless abundant, because their water releases overlap with more abundant water-evolving phases such as hydrated salts or clays.

SAM-EGA would be able to detect most but not all Mn-bearing phases if present above the instrument detection limit. SAM-EGA is effective for detecting higher valence Mn oxides. Therefore, drill samples containing Mn oxides (detected by other rover instruments) that did not evolve O2 must be Mn(II) oxides or below detection. In oxidizing conditions Mn could precipitate into multiple phases detectible by SAM, and if detected would constrain Eh-pH to a greater degree than Fe alone.