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Oxidative Alteration of Siderite to Goethite and Akaganeite By Chlorate & Bromate: Implications for Recent Siderite Discovery at Gale Crater on Mars.

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

Presenting Author:

Kaushik Mitra

Authors:

Mitra, Kaushik¹, Malesky, Lauren Ann², Thorpe, Michael Terrance³, Stevanovic, Ana⁴

(1) Department of Earth & Planetary Sciences, University of Texas at San Antonio, San Antonio, TX, USA, (2) UTSA, San Antonio, TX, USA, (3) NASA Goddard Space Flight Center CRESST II, Greenbelt, MD, USA, (4) KAMC, UTSA, San Antonio, Texas, USA,

Abstract:

Ferrous iron carbonate, siderite [Fe^IICO₃], was discovered in the sulfate-rich Mirador formation at Gale crater. The discovery of pure siderite of high crystallinity likely represents a transition event in the iron and carbon geochemical cycle on Mars. Other ferric minerals detected with siderite likely formed by later diagenetic events. The relatively uncommon presence of siderite and other carbonate minerals on Mars has been hypothesized to be due to acid dissolution. Similarly, an acid alteration hypothesis was suggested to explain the juxtaposition of ferrous and ferric minerals in Gale crater sediments. Here, we conduct laboratory experiments in a closed system environment to determine if acidic dissolution alone and oxyhalogen brines could weather siderite to produce ferric oxyhydroxide in different Mars-relevant fluids at ambient conditions (24°C, 1 atm).

We designed laboratory experiments to evaluate the oxidative weathering of siderite at Gale crater as a function of oxidant type [NaClO₃ and NaBrO₃], background fluid composition [MgCl₂ and MgSO₄], pH (~2.5, 4.5, and 7), and time [35 and 100 days]. All experiments were conducted inside an anaerobic chamber (N₂ = 97%, H₂ = 3%; O₂ < 1 ppmv). The 50 mL glass serum bottle reactors used in the experiments were wrapped in Al foil to prevent any inadvertent influence of ultraviolet radiation on siderite dissolution and oxidation. 50 mL solutions containing 0.2 g siderite (~4 g L^-1; W:R ~ 250) were allowed to react for about 100 days with ~100 mmol L^-1 NaClO₃ or NaBrO₃. At the end of the experiments, all reactors were filtered, final solution pH was measured, and the filtrates and the supernatant fluids were analyzed by a suite of analytical instruments.

Contrary to popular hypothesis, siderite survived extensive acidic diagenesis. On the other hand, chlorate- and bromate-containing experimental solutions were able to substantially alter siderite in all types of fluid conditions, including near-neutral solutions. The primary product of alteration was found to be goethite and akaganeite forming in acidic Cl-rich fluids. Chlorate and bromate can oxidatively weather siderite to form ferric hydroxide minerals, similar to those observed at Gale crater, Mars. We propose a top-down alteration model to explain the iron mineralogy in the sulfate-rich section at Gale crater, and our ongoing experiments at lower temperatures (4 and 0°C) will be able to investigate siderite alteration as a function of temperature.