TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 6), pp. 495-505, 2017

Removal of organic contaminants from iron sulfides as a pretreatment for mineral-mediated chemical synthesis under prebiotic hydrothermal conditions

Manabu Nishizawa,1* Masafumi Saitoh,2,3 Yohei Matsui,2,3 Yoichi Usui,3,4 and Takazo Shibuya1,2,3

1Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan
2Research and Development (R&D) Center for Submarine Resources, JAMSTEC, Yokosuka 237-0061, Japan
3Project Team for Development of New-generation Research Protocol for Submarine Resources, JAMSTEC, Yokosuka 237-0061, Japan
4Department of Deep Earth Structure and Dynamics Research, JAMSTEC, Yokosuka 237-0061, Japan

(Received December 1, 2016; Accepted April 20, 2017)

Abstract: Iron sulfides are thought to play critical roles in prebiotic chemistry as a synthesis reactant or a mineral catalyst to form key organic compounds and as an active center for proto-enzymes. Laboratory experiments with iron sulfides and other reactants at geologically reasonable low concentrations are essential to test this hypothesis. Therefore, organic contaminants potentially attached to iron sulfides should be removed as much as possible prior to experiments. This study measured amounts of organic contaminants in troilite and pyrite that are/were likely present in extraterrestrial and Hadean Earth hydrothermal environments, respectively. In addition, we evaluated the effectiveness of the anoxic heating (450–1000°C and 0.1 MPa), hydrothermal processing (300°C and 50 MPa), and wet chemical treatment (acid and/or organic solvents) of these reagents as pretreatments. Commercially available iron sulfide reagents contain considerable amounts of organic matter with up to 1560 ppm carbon and 30 ppm nitrogen. With the best cleaning procedure, anoxic heating removed 66% and 53% of the carbon and nitrogen from troilite, respectively, whereas it removed 25% and <10% of the carbon and nitrogen from pyrite, respectively. Anoxic heating and hydrothermal processing of the troilite reagents reduced the metallic iron impurity contents but produced small amounts of iron oxides. Clearly, future work is needed to establish more efficient protocols to substantially remove persistent organic components. However, appropriate sample selection combined with anoxic heating and mineral separation would allow the preparation of troilite or pyrite fractions with a relatively small amount of refractory organic contaminants (≤40 ppm C, ≤2 ppm N) that could not contribute to the synthesis of ammonia and polymeric biomolecules in simulated extraterrestrial and Hadean Earth hydrothermal conditions, respectively.
Key words: iron sulfides, chemical evolution, hadean, extraterrestrial objects, hydrothermal activity

*Corresponding author E-mail: m_nishizawa@jamstec.go.jp

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