The Coprecipitation of Akaganeite and Jarosite and Its Implications for the Late‐Stage Diagenetic Fluids in the Vera Rubin Ridge, Gale Crater, Mars.

Akaganeite and jarosite were detected in two mudstone drill samples from Vera Rubin ridge (VRR), Gale crater by Chemistry & Mineralogy X‐Ray Diffraction (CheMin). The co‐occurrence of these two minerals is quite rare in both terrestrial and Martian aqueous environments. In order to confine the chemical conditions of paragenetic akaganeite and jarosite, and provide insight into late‐stage diagenetic alterations at VRR, we synthesized akaganeite and jarosite with varying SO42− concentrations and initial pH levels. Synthetic samples were characterized using Field Emission Scanning Electron Microscopy, X‐ray powder diffraction and Raman spectroscopy. Our study reveals that akaganeite and jarosite exist in equilibrium in the solution with 0.011–0.028 M SO42− with respect to 0.6 M Cl− and an initial pH of 1.3–2.2. In combination with the CheMin detection results, the chemistry and pH values of the fluids at VRR can be further constrained. Considering the absence of goethite and the relative higher portion of akaganeite than jarosite in the drill samples, the pH values should be 1.4–2 and the S/Cl molar ratio should be within the range of 0.018–0.042. Based on our laboratory results, we hypothesize that the presence of akaganeite and jarosite at VRR represents an individual episode of acidic groundwater activity. During the late‐stage diagenetic process at VRR, upwelled acidic groundwater dissolved the local chlorides to form the Cl−‐dominated fluids. Subsequent evaporation further concentrated the acid saline fluids and therefore resulted in an extremely acidic environment (1.4 ≤ pH＜2 with S/Cl molar ratio of 0.018–0.042), which produced akaganeite and jarosite. Plain Language Summary: Akaganeite and jarosite have been detected in drill samples of Gale crater. But their co‐occurrence has only been found in two drill samples Stoer and Rock Hall at VRR. The formation conditions of akaganeite and jarosite at VRR remain unknown. To solve this issue, we conducted synthesis experiments under different SO42− concentrations and initial pH levels. We found that akaganeite and jarosite coprecipitate in the solution with 0.011–0.028 M SO42− with respect to 0.6 M Cl− and an initial pH of 1.3–2.2. In conjunction with the CheMin data and our experimental results, the chemistry and pH values of the fluids at VRR can be further constrained. The formation of akaganeite and jarosite at VRR likely occurred in a Cl−‐dominating and SO42−‐bearing solution (S/Cl molar ratio of 0.018–0.042) under extremely acidic conditions (pH range of 1.4–2). We hypothesize that the coprecipitation of akaganeite and jarosite represents an individual episode of acidic groundwater activity at VRR. The upwelling acidic groundwater possibly dissolved the local chlorides to form a Cl‐dominated fluid with trace amounts of SO42−. Evaporation further concentrated the fluid to extreme acidic and thus formed akaganeite and jarosite. Key Points: Akaganeite and jarosite at Vera Rubin ridge (VRR) formed in a Cl‐dominated solution with trace amounts of SO42− under extremely acidic conditionThe formation of akaganeite and jarosite at VRR represents an individual episode of acidic groundwater activityThe abundant Cl in the fluids is possibly of local origin while the extreme acidity is accomplished by evaporation [ABSTRACT FROM AUTHOR]