Manganese mineralogy and diagenesis in the sedimentary rock record

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(CO 3 ) 2 ), rhodochrosite (MnCO 3 ), and braunite (Mn(III) 6 Mn(II)O 8 SiO 4 ). 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.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Whole-rock REE+Y patterns and their interpretations are in good agreement with those of the aforementioned Hamersley IFs of Australia (Bau and Dulski, 1996;Planavsky et al, 2010a). Manganese -deposited as discrete Mnoxide layers in the stratigraphically younger Hotazel Formation -is incorporated entirely in iron 57 carbonate minerals in both the Griquatown and Koegas IFs, where its bulk-rock abundance locally reaches percent levels (Tsikos and Moore, 1997;Johnson et al, 2016;Kurzweil et al, 2016).…”

Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars

“…To precipitate and concentrate Mn in rocks and sediments, high‐potential oxidants (much higher than that needed for Fe or S) are required to oxidize Mn to insoluble, high‐valence oxides. Consequently, Mn‐rich rocks on Earth closely track the rise of atmospheric oxygen [ Johnson et al ., ; Maynard , ; Hazen et al ., ; Kirschvink et al ., ; Johnson et al ., ].…”

Real-Time Manganese Phase Dynamics during Biological and Abiotic Manganese Oxide Reduction

“…The rock record of manganese is dominated by Mn-bearing carbonates (rhodochrosite, MnCO 3 , or kutnohorite, MnCa(CO 3 ) 2 ) and a Mn(III) phase (braunite, Mn(III) 6 Mn(II)SiO 12 ). 23–26 The occurrence of Mn(II+III) minerals in ancient sediments is notable because manganese is deposited primarily as Mn(IV) oxides. 1,27–32 These phases confirm modern observations 5,6,12–14,33 that reductive processes occur frequently in sediments after Mn(IV) oxide deposition, but the diagenetically stable mineral products associated with the various reduction reactions are not well-known.…”

“…Many previous workers have hypothesized that ancient Mn-carbonates were once Mn(IV) oxides that were secondarily reduced by organic carbon, potentially in microbially mediated reactions. 25,26,34–37 Rhodochrosite has been observed as a product of microbial respiration of Mn oxides previously 2,10,22 although it was not studied extensively for the requisite conditions of precipitation. Other studies have measured Mn-carbonate production during microbial sulfate reduction or thiosulfate disproportionation from secondary abiotic interactions between sulfide and Mn-oxides.…”