Relationships between Fe redistribution and Po2 during mineral dissolution under low O2 conditions

Iron-bearing olivine was dissolved at different partial pressures of atmospheric oxygen (Po2) in a glove box to clarify the relationships between Fe redistribution and Po2 during mineral dissolution and to provide a deeper understanding of weathering under low oxygen conditions. The olivine was first dissolved at pH 4.5-6.0 and 15-55 °C under the ambient air using a modified flow-through system. Some portion of Fe2+ released from olivine was oxidized and precipitated as Fe3+ oxides/hydroxides, and remained in the system, while the other portion flowed out of the system as Fe2+. Then, a kinetic model, considering mineral dissolution, water flow and Fe2+ oxidation rates, was made to estimate Fe behavior at different pHs, temperatures and dissolved O2 concentrations during dissolution. The kinetic model was applied to the experimental results under the ambient air; the validity of the model was confirmed by good agreement between the observed and calculated Ф values where Ф is a ratio of the amount of Fe3+ precipitated to the total amount of Fe2+ actually dissolved from olivine, an indicator of the oxidation state of a mineral dissolution system. Then, by using the kinetic model, we analyzed the results of the olivine dissolution experiments at 0.20 atm to 8.7 × 10-5 atm of Po2 and at constant pH and temperature. While the dissolution rates of olivine were not affected significantly by Po2, the olivine dissolution has indicated that Ф decreases with decrease in Po2 and the Ф variation at low Po2 cannot be explained by the classic rate law of Fe2+ oxidation. The calculations of Ф based on the above kinetic model have revealed that the observed Ф variation is well explained by the decrease in the power of [O2] (x) with decrease in [O2] for the Fe2+ oxidation rate, -d[Fe2+]/dt = k[Fe2+][O2]x[OH-]y (k: the rate constant, 0 ⩽ x ⩽ 1, and y ∼ 2); the x value decreased from 1.0 to 0.76 at 0.20 to 8.7 × 10-5 atm of Po2, respectively. The Φ−Po2 relationships imply that the Φ value in a paleosol (fossil weathering profile) increases with increase in Po2 in the Paleoproterozoic.