Anoxic dissolution processes of biotite: implications for Fe behavior during Archean weathering

Details Anoxic dissolution processes of biotite: implications for Fe behavior during Archean weathering Anoxic dissolution processes of biotite: implications for Fe behavior during Archean weathering

Jul 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004e%26psl.224..117m&link_type=abstract

Earth and Planetary Science Letters, Volume 224, Issue 1-2, p. 117-129.

Computer Science

3

Precambrian, Atmospheric Evolution, Biotite Dissolution, Paleosol, Anoxic Weathering

Scientific paper

Iron-rich biotite (Fe/Mg=5) dissolution experiments were carried out in a batch system under O2-deficient conditions (PO2<3×10-5 atm; referred to as `anoxic' conditions) at 1 atm of PCO2, pH 4.6, and 100 °C for 7-120 days for a better understanding of `anoxic' weathering processes and Fe behavior during weathering before 2.2 Ga. For comparison, oxic Fe-rich biotite dissolution experiments were conducted under present atmospheric conditions at pH 4.7 and 100 °C for 7-80 days (referred to as oxic conditions) by using the same starting biotite as that for the `anoxic' experiments. The concentrations of Fe in solution after the dissolution experiments were larger by one to more than two orders of magnitude under `anoxic' conditions than under oxic conditions. High-resolution scanning and transmission electron microscopy revealed that Fe(II)-rich vermiculite or smectite was precipitated as a secondary mineral at the edge of biotite under `anoxic' conditions, in contrast to the formation of Fe(III)- and Al-(hydr)oxides under oxic conditions. The results suggest that part of Fe(II) is released to water as `anoxic' weathering proceeds, explaining the decrease of Fe content in pre-2.2 Ga paleosols relative to their parent rocks. The Fe/Mg molar ratio of the secondary vermiculite or smectite was more than 7 while the starting Fe-rich biotite had a value of about 5. The Fe/Mg molar ratio was less than 2.5 in solution. The results of the `anoxic' experiments suggest that Fe(II)-rich vermiculite or smectite could be the precursor of the chlorite preserved in pre-2.2 Ga paleosols. This is further corroborated by the increase in Fe/Mg molar ratios in chlorite with decreasing depth in some Precambrian paleosols.

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