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Published January 15, 2016 | public
Journal Article

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)_6Mn(II)O_8SiO_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.

Additional Information

© 2015 Elsevier B.V. Available online 9 November 2015. We are grateful for manuscript comments and analytical support from George Rossman, Alex Sessions, and Victoria Orphan, and the careful feedback provided by five anonymous reviewers. We thank the David and Lucile Packard Foundation and a NSF-GRFP Grant No. DGE-1144469 to J.E.J. for support. We acknowledge the Lewis and Clark Foundation for funding field work for J.E.J., the Agouron Institute for supporting both the Agouron South Africa Drilling Project which provided core samples of the Koegas Subgroup and the Agouron Field Excursions that allowed J.E.J. to sample the Kungarra Formation, Victoria Orphan and Benjamin Harris for donating part of a deep-sea ferromanganese nodule, J.B. Maynard for contributing many Molango deposit samples, Joseph Kirschvink for donating a well-preserved sample of the Hotazel Formation, Thiago Piacentini for samples of the Santa Cruz Formation, and Nic Beukes and Bertus Smith for assistance in the field. The Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. SEM, EBSD and EPMA analyses were carried out at the Caltech GPS Division Analytical Facility, which is supported, in part, by NSF Grants EAR-0318518 and DMR-0080065.

Additional details

Created:
August 20, 2023
Modified:
October 25, 2023