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

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.