Siderite and ferric oxyhydroxides imply interlinked carbon, iron, and halogen cycles on Mars

• 成果类型: Article
• 署名作者: Mitra, Kaushik; Malesky, Lauren A.; Thorpe, Michael T.; Stevanovic, Ana
• 署名单位: University of Texas System; University of Texas at San Antonio; National Aeronautics & Space Administration (NASA); University of Texas System; University of Texas at San Antonio
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-9186
• DOI: 10.1073/pnas.2504674122
• 发表日期: 2025-06-24
• 关键词: martian soil; gale crater; perchlorate; chlorate; geochemistry; oxidation; HISTORY; climate; brines; rocks

摘要

Pure siderite [FeIICO3] was recently discovered in abundant quantities (4.8 to 10.5 wt.%) by the Curiosity rover at Gale crater, Mars. Diagenetic alteration of siderite likely caused the carbonate-sequestered CO2 to be released back into the atmosphere and consequently produced ferric [Fe(III)] oxyhydr(oxide) minerals. Here, using laboratory experimentation, we demonstrate that while closed system acid diagenesis-as proposed for Gale crater-is incapable of effective siderite alteration in Mars-relevant fluids, oxyhalogen compounds (chlorate and bromate) can weather siderite not only at acidic pH but also in near-neutral Mars-relevant solutions. The ferric oxyhydroxide minerals produced as a consequence are controlled by the diagenetic fluid composition. While photooxidation is possible, the mutually exclusive products of alteration-magnetite (Fe3O4) during ultraviolet irradiation and ferric oxyhydroxide (FeOOH) by oxyhalogens-demonstrate that siderite at Gale crater underwent chemical weathering by chlorate and bromate brines owing to the complete absence of magnetite in drill samples containing siderite. We propose a top-down oxyhalogen brine percolation model to explain the iron mineralogy of the sulfate-rich unit at Gale crater. We conclude that siderite alteration by acidic fluids alone cannot explain the redox disequilibrium witnessed in Gale crater sediments as promulgated before and siderite weathering by oxyhalogen brines is the most likely explanation. It is highly likely that the halogen cycle on Mars is interlinked to the iron and the carbon cycle on early and current Mars. Significance The overall rarity of carbonate deposits on Mars have been interpreted to be the result of acid dissolution. We show that siderite is stable in Mars-relevant fluids from pH 7 to 2 and its alteration to ferric minerals- similar to those recently discovered at Gale crater-is not possible by acidic diagenesis alone. Our study reveals that chlorate and bromate, the dominant form of chlorine and bromine on Mars, can oxidatively weather siderite to form ferric hydroxide minerals observed on Mars. Siderite weathering by oxyhalogen brines not only operates in acidic solutions but is also effective at near-neutral conditions.