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Comparative Study of Manganese Oxidation in Martian Fluids: Oxygen v. Oxyhalogens v. Nitrate.

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe (Posters)

Presenting Author:

Lauren Ann Malesky

Authors:

Malesky, Lauren Ann¹, Brancaleon, Elena², Mitra, Kaushik³

(1) Department of Earth and Planetary Science, University of Texas at San Antonio, San Antonio, TX, USA, (2) Department of Earth and Planetary Science, University of Texas at San Antonio, San Antonio, Texas, USA, (3) Department of Earth and Planetary Science, University of Texas at San Antonio, San Antonio, Texas, USA,

Abstract:

Oxidized manganese [Mn(III/IV)] deposits discovered in veins and fractures are key indicators of past redox conditions and potential habitability on Mars. On Earth, manganese oxidation is primarily driven by molecular oxygen (O₂) in alkaline conditions or catalyzed by microbial activity, processes that became abundant during the Great Oxidation Event (~2.3 Ga). However, ancient Mars lacked a substantial O₂ atmosphere, with estimated concentrations ranging from 0.05 to 10^-5 bar, making the mechanism of Martian manganese oxidation a subject of debate. While some previous experimental data on Mn(II) oxidation by O₂ or simple oxyhalogen systems exist, the accurate determination of the rate and products of such oxidation in Mars-relevant conditions, specifically by O₂ in hydrothermal closed and open systems, mixed chlorate-bromate anions, and nitrate, remains underexplored. This study experimentally investigates oxygen (O₂), mixed oxyhalogens (chlorate and bromate), and nitrate (NO₃^-) to determine the most effective and plausible oxidant responsible for manganese oxidation on Mars in Mars-relevant fluids.

We conducted laboratory experiments in batch reactors (50 mL glass serum bottles) simulating a wide range of environmental conditions, including pH (~2 to 11), temperature (25 to 75°C), oxidant concentrations, and background fluids (MgCl₂ and MgSO₄). The resulting solid-phase products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ion chromatography (IC), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Together, these analyses will determine the oxidation extent, product mineralogy, and final fluid composition to identify the most effective Mn(II) oxidant. Ultimately, this will allow us to constrain the specific conditions (oxidant, pH, temperature) under which manganese oxide formation is favored.

Preliminary results from the experiments have demonstrated substantially faster rates of Mn(II) oxidation in mixed chlorate and bromate systems across all pH conditions. Nitrate, on the other hand, showed no detectable Mn(II) oxidation in any experimental solution investigated. Oxygen showed substantial Mn(II) oxidation at high pH conditions. Scanning electron microscopy reveals that the mineral morphologies of the oxidized products are substantially different in case of Mn(II) oxidation by O₂ as compared to oxyhalogens. By comparing Mn(II) oxidation across multiple chemical systems, we constrain the most plausible pathways, oxidants, and environmental conditions responsible for manganese oxide formation on early Mars.