TY - JOUR
T1 - Iron oxidation state of a 2.45-Byr-old paleosol developed on mafic volcanics
AU - Utsunomiya, Satoshi
AU - Murakami, Takashi
AU - Nakada, Masami
AU - Kasama, Takeshi
N1 - Funding Information:
The authors are grateful to J. F. Banfield for useful discussion, to T. Tachikawa, H. Yoshida, and Y. Imazu for technical assistance, and to M. Hailstone at Ontario Geological Survey, who helped us while collecting samples. The authors are also grateful to N. Yanase and T. Ohnuki at the Japan Atomic Energy Research Institute, who kindly permitted us to use ICP-MS. R. C. Ewing at University of Michigan kindly allowed S.U. to complete this work at the current address. The authors also thank K. Traexler for reviewing an earlier version of this manuscript. The useful comments of H. D. Holland, L. R. Kump, and G. M. Young have greatly improved the quality of the paper. The electron microscopy was performed in the Electron Microbeam Analysis Facility for Mineralogy at the Department of Earth and Planetary Science, University of Tokyo. This work was partially supported by the Science Grant of the Ministry of Education, Science and Culture.
PY - 2003/1/15
Y1 - 2003/1/15
N2 - The 2.45-Byr-old weathering profile developed on early Proterozoic mafic volcanics located near Cooper Lake, Ontario, Canada, was examined geochemically and mineralogically for a better understanding of the atmospheric oxygen evolution. Ferrous to ferric ion, Fe(II) and Fe(III), respectively, ratios of the bulk rock samples were analyzed by Mössbauer spectrometry. The total Fe (Fe(T)) and Fe(II) concentrations decrease from 12.0 and 11.2 wt.% to 1.85 and 0.89 wt.%, respectively, from the bottom to the top of the weathering profile. The Fe(T) and Fe(II) concentrations normalized to Ti and Zr, as well as the Fe(II)/Fe(III) ratio of raw data, linearly decrease with depth toward the top, while the Fe(III) concentration remains nearly constant throughout the profile. The linear decrease of Fe(II), accompanied by the nearly constant distribution of Fe(III), is difficult to be explained by the scenario of oxidizing weathering and subsequent reducing hydrothermal alteration. The behaviors of Fe(II) and Fe(III) can be simply explained by anoxic weathering. The anoxic weathering suggests that the 2.45-Ga atmosphere was anoxic. The slight increase of Fe/(Fe) in the octahedral sites of chlorite toward the top and no Ce anomaly in the REE patterns are also consistent with anoxic weathering.
AB - The 2.45-Byr-old weathering profile developed on early Proterozoic mafic volcanics located near Cooper Lake, Ontario, Canada, was examined geochemically and mineralogically for a better understanding of the atmospheric oxygen evolution. Ferrous to ferric ion, Fe(II) and Fe(III), respectively, ratios of the bulk rock samples were analyzed by Mössbauer spectrometry. The total Fe (Fe(T)) and Fe(II) concentrations decrease from 12.0 and 11.2 wt.% to 1.85 and 0.89 wt.%, respectively, from the bottom to the top of the weathering profile. The Fe(T) and Fe(II) concentrations normalized to Ti and Zr, as well as the Fe(II)/Fe(III) ratio of raw data, linearly decrease with depth toward the top, while the Fe(III) concentration remains nearly constant throughout the profile. The linear decrease of Fe(II), accompanied by the nearly constant distribution of Fe(III), is difficult to be explained by the scenario of oxidizing weathering and subsequent reducing hydrothermal alteration. The behaviors of Fe(II) and Fe(III) can be simply explained by anoxic weathering. The anoxic weathering suggests that the 2.45-Ga atmosphere was anoxic. The slight increase of Fe/(Fe) in the octahedral sites of chlorite toward the top and no Ce anomaly in the REE patterns are also consistent with anoxic weathering.
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U2 - 10.1016/S0016-7037(02)01083-9
DO - 10.1016/S0016-7037(02)01083-9
M3 - Article
AN - SCOPUS:0037439136
SN - 0016-7037
VL - 67
SP - 213
EP - 221
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
IS - 2
ER -