Terrestrial formation of calcium sulfate and carbonate assemblages in Atacama CO chondrites: Implications for Martian evaporitic environments

Gabriel A. Pinto , Vinciane Debaille, Jolantha Eschrig, Alexandre Corgne, kevin Soto, Thierry Leduc, Sophie Decrée, Steven Goderis

Evaporites are frequently reported in carbonaceous chondrites from hot and cold deserts, yet their origin remains debated between formation on the parent body or by post-fall terrestrial alteration. Here, we present a systematic characterization of Ca sulfate and Ca carbonate assemblages in four CO carbonaceous chondrites from different dense collection areas of the Atacama Desert (Los Vientos 123, El Médano 464, Calama 031, Paposo 088). We combine backscattered electron imaging, EDS, X-ray compositional mapping, Raman spectroscopy, and modal point counting to assess the distribution, mineralogy, and formation context of evaporites. Evaporites occur mainly as pore- and vein-filling phases and as replacements of Fe sulfides. Los Vientos 123 and El Médano 464 contain high abundances of Ca sulfates (~2.5 ± 0.35 vol%), Calama 031 is dominated by Ca carbonate veins (1.4 ± 0.26 vol%) with minor Ca sulfate, and Paposo 088 shows only low Ca sulfate contents (0.47 ± 0.15 vol%). These phases are systematically associated with Fe oxyhydroxides, jarosite-like phases, and strongly altered sulfides. The sulfate- and carbonate-rich assemblages in CO chondrites correlate with local soil geochemistry and microclimates. Limestone bedrock and more rain-influenced inland set different evaporite assemblages compared to coastal areas characterized by marine aerosols and salt-rich soils. Raman spectra indicate that the dominant Ca sulfate polymorph is anhydrite, lacking OH-stretching bands, consistent with precipitation from low-water activity, chloride-nitrate-rich brines and limited subsequent hydration. Disordered carbonaceous matter locally sheltered within sulfate-rich areas suggests that secondary evaporites can trap and preserve organic material, even if non-biological. Our results thus support (i) a terrestrial origin for Ca sulfates and Ca carbonates in Atacama CO chondrites; (ii) the stability of anhydrite as an indicator of extremely low water activity; and (iii) process analogues for evaporite formation in Martian settings, where anhydrite regions may be key targets to reconstruct aqueous conditions and assess organic preservation on Mars.