Abstract
We report the discovery of indigenous Mn-oxides in Martian regolith breccias Northwest Africa (NWA) 7034 and 7533. These Mn-oxides occur in Mn-rich regions as nanocrystals mixed with silicates, FeOOH, and possible phosphates. The Mn-rich regions contain up to 34 wt% Mn and typically display large chemical gradients on the scale of 10–20 μm. The Martian origin of Mn-oxides was established by the presence of Mn-rich glass (4.8–5.6 wt% Mn) in the fusion crust that crosscuts a Mn-oxides-bearing monzonite clast and by the absence of Mn-oxides on the environmentally exposed surfaces (exterior and fractures) of the meteorites. Manganese K-edge X-ray absorption spectrum (XAS) of the Mn-rich glass in the fusion crust indicated that this glass included high-valent Mn species. Synchrotron micro-X-ray diffraction of a Mn-rich region in a basalt clast and XAS of Mn-rich regions in three monzonite clasts indicate Mn-oxides in these regions are dominantly hollandite-structured with 67–85 mol% of the total Mn being Mn4+. The fact that Mn-rich regions are present in diverse petrological associations but are absent in the matrix of the breccias indicates that the Mn-oxides formed through surface alteration prior to the final brecciation event that assembled NWA 7034 and 7533. Thus, the age of the Mn-oxides is older than the lithification age (arguably 1.35 Ga) of NWA 7034 and 7533. Together with findings of Mn-rich phases within Noachian and Hesperian sedimentary strata in Endeavour and Gale craters, our results suggest that Mn-oxides are a common weathering product on Mars, suggesting aqueous environment on the Martian surface with high redox potential.
Highlights
Manganese (3+, 4+)-(hydr)oxides are important indicators of envi ronmental conditions because their formation from Mn2+ indicates an environment of high redox potential (Crerar et al, 1980)
Our results show that Mn-rich materials are mixtures of maghemite- and hollandite [Ba (Mn4+Mn2+)O16]-structured oxides, providing concrete support for the view that manganese oxides form on Mars (Lanza et al, 2016)
On the basis that the fine-grain matrix in these breccia clasts lack the macroscopic textures consistent with a quenched melt (McCubbin et al, 2016), we refer to these breccia clasts as protobreccias since they formed before their subsequent entrapment into Northwest Africa (NWA) 7034 and 7533
Summary
Manganese (3+, 4+)-(hydr)oxides are important indicators of envi ronmental conditions because their formation from Mn2+ indicates an environment of high redox potential (Crerar et al, 1980). Together with the slow kinetics of oxidizing Mn2+(aq) into insoluble Mn oxides (Morgan, 2005), enriching Mn in aqueous solutions to form significant quantities of Mn4+,3+-oxides typically requires longlasting aqueous environments. Oxidation of Mn2+(aq) into insoluble MnO2 or Mn2O3 occurs at high redox potentials (Eh > +0.5 V) for surface fluids of pH = 7 and a total Mn concentration of 1 μmol/L to 1 mmol/L total Mn (Crerar et al, 1980; Noda et al, 2019). The most readily high-Eh materials for Mn oxidation are dissolved O2 species in aqueous solutions (atmospheric O2 or its photochemical products)
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