『Abstract
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℃ 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℃ 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 corroborates by the increase in Fe/Mg
molar ratios in chlorite with decreasing depth in some Precambrian
paleosols.
Keywords: Precambrian; atmospheric evolution; biotite dissolution;
paleosol; anoxic weathering』
1. Introduction
2. Experimental and analytical methods
2.1. Samples
2.2. Anoxic experiments
2.3. Oxic experiments}
2.4. Analytical methods
3. Results
4. Discussion
4.1. Dissolution and weathering of biotite under anoxic conditions
4.2. Weathering processes under anoxic conditions and implication
for Fe behavior during pre-2.2 Ga weathering
Acknowledgements
References