Iron oxidation state of a 2.45-Byr-old paleosol developed on mafic volcanics
Journal
Geochimica et Cosmochimica Acta
Journal Volume
67
Journal Issue
2
Start Page
213
End Page
221
ISSN
0016-7037
Date Issued
2003-01
Author(s)
Abstract
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.
Subjects
Canada
iron
oxidation
paleoatmosphere
paleosol
Proterozoic
volcanic rock
weathering
Publisher
Elsevier BV
Type
journal article