TY - JOUR
T1 - Anoxic dissolution processes of biotite
T2 - Implications for Fe behavior during Archean weathering
AU - Murakami, Takashi
AU - Ito, Jun Ichi
AU - Utsunomiya, Satoshi
AU - Kasama, Takeshi
AU - Kozai, Naofumi
AU - Ohnuki, Toshihiko
N1 - Funding Information:
The authors are grateful to G. Kamei at the Japan Nuclear Cycle Development Institute for the use of the glove boxes and to B. Sreenivas for discussion. We thank A. Monkawa for the XANES data, and T. Tachikawa and T. Takeshige for technical assistance. The reviews by D.R. Peacor and M.-P. Turpault improved the original version of the manuscript. The electron microscopy was performed in the Electron Microbeam Analysis Facility for Mineralogy at the Department of Earth and Planetary Science, the University of Tokyo. This work was supported by the Science Grant of the Ministry of Education, Science and Culture. [EB]
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.
PY - 2004/7/30
Y1 - 2004/7/30
N2 - 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.
AB - 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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U2 - 10.1016/j.epsl.2004.04.040
DO - 10.1016/j.epsl.2004.04.040
M3 - Article
AN - SCOPUS:3242671813
VL - 224
SP - 117
EP - 129
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
SN - 0012-821X
IS - 1-2
ER -