Oxygen isotope analyses of chemically and microbially produced manganese oxides and manganates

Abstract

Understanding the formation of metal deposition in the geological record depends in part on understanding some of the basic reactions that could have occurred when those ore deposits were formed. For manganese oxides and manganates, the b 180 value might reflect the conditions that influenced the oxidation pathway during deposition, which include temperature, the δ 180 values of H 2O or dissolved O 2, and microbial catalysis. Mn (IV) 10 Å manganates were, therefore, prepared by three different procedures with different sources of water having distinct δ 180H 20 values, while keeping the δ 1800 2 constant. Manganates were prepared (1) chemically (i.e., abiotically) under strong alkaline conditions at 3°C, (2) biologically with Mn(II) -oxidizing, dormant spores of a marine bacterium, Bacillus sp. strain SG-1, and (3) biologically with metabolically active cells of a Mn(II)-oxidizing marine bacterium, strain S185-9A1. The SG-1- and SI85-9A1-produced manganates were formed under environmentally relevant conditions of pH (7.6) and temperature (20–25°C) in natural filtered seawater containing 100 μM Mn (II) . X-ray diffraction analysis and oxidation state measurements demonstrated that the chemically precipitated mineral is characterized by a collapsible (and expandable) 10 A peak (buserite), whereas those produced by SG-1 and S185-9A1 had noncollapsible 10 A peaks. Oxygen isotope analysis of the Mn(IV) minerals revealed significant incorporation of molecular O2 from each synthesis (32–50%), which is consistent with direct oxidation of Mn(II) to Mn(IV) in a single step, rather than by disproportionation of lower valence state intermediates. Estimates of the kinetic isotope fractionation values for molecular O2 for the chemical and SG-1 manganates were −13 and −22‰, respectively, whereas values of −5 and +1‰ were estimated for H 2O. In contrast, the SI85-9A1-produced minerals showed only a small negative fractionation for either oxygen source. Application of these results to natural deposits of manganese oxides indicates a microbial origin for a freshwater manganese nodule from Oneida Lake, NY, USA, as well as a 50% dissolved oxygen signal. Oxygen isotopic values of Mn (IV) manganates from the Kaikata seamount, however, appear to reflect the δ 180 value of seawater exclusively. Therefore, Mn (IV) manganates in nature may not always contain a dissolved oxygen signal, presumably a result of different oxidation pathways or postdepositional alteration.