Manganese mineralogy and diagenesis in the sedimentary rock record. (15th January 2016)

• Record Type: Journal Article
• Authors: Johnson, Jena E.; Webb, Samuel M.; Ma, Chi; Fischer, Woodward W.
• Is Part Of: Geochimica et cosmochimica acta. Volume 173(2016:Jan. 15)
• Page Start: 210
• Page End: 231
• Publication Date: 2016-01-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.10.027
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Oxidation of manganese (II) to manganese (III, IV) demands oxidants with very high redox potentials; consequently, manganese oxides are both excellent proxies for molecular oxygen and highly favorable electron acceptors when oxygen is absent. The first of these features results in manganese-enriched sedimentary rocks (manganese deposits, commonly Mn ore deposits), which generally correspond to the availability of molecular oxygen in Earth surface environments. And yet because manganese reduction is promoted by a variety of chemical species, these ancient manganese deposits are often significantly more reduced than modern environmental manganese-rich sediments. We document the impacts of manganese reduction and the mineral phases that form stable manganese deposits from seven sedimentary examples spanning from modern surface environments to rocks over 2 billion years old. Integrating redox and coordination information from synchrotron X-ray absorption spectroscopy and X-ray microprobe imaging with scanning electron microscopy and energy and wavelength-dispersive spectroscopy, we find that unlike the Mn(IV)-dominated modern manganese deposits, three manganese minerals dominate these representative ancient deposits: kutnohorite (CaMn(CO3 )2 ), rhodochrosite (MnCO3 ), and braunite (Mn(III)6 Mn(II)O8 SiO4 ). Pairing these mineral and textural observations with previous studies of manganese geochemistry, we develop a paragenetic model of post-depositional manganese mineralization with kutnohorite and calcian rhodochrosite as the earliest diagenetic mineral phases, rhodochrosite and braunite forming secondarily, and later alteration forming Mn-silicates.