Potassium-bearing manganese oxides, cryptomelane, K1-2(Mn3+Mn4+)8 O-16 . xH2O, and hollandite, (K,Ba)1-2 (Mn3+,Mn4+)8 O-16 . xH2O, are often authigenically precipitated in weathering profiles. The presence of structural potassium in manganese oxides makes these minerals datable by the K-Ar and 40Ar/39Ar methods. Dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes. Detailed age resolution of the weathering history allows us to use deep weathering profiles as land-based stratigraphic records of past climatic and geomorphic events, complementing evidence from the marine sedimentary and paleontological record. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, 39Ar loss by recoil during neutron irradiation, contamination by primary minerals resistant to weathering, the presence of multiple generations of manganese oxides, and proportionately low radiogenic Ar yield due to adsorbed atmospheric gases are addressed. The K-Ar and 40Ar/39Ar analytical results indicate that Ar and/or K losses, excess 40Ar, and 39Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that fine scale, laser-probe 40Ar/39Ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and 40Ar/39Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 +/- 6 Ma. Petrographic, electron microscope, and electron microprobe investigation 1reveal multiple generations of manganese oxide precipitation. K-Ar and 40Ar/39Ar laser-probe step-heating results independently confirm the presence of these various generations of manganese oxides. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 +/- 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting from changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and 40Ar/39Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean. Potassium-bearing manganese oxides, cryptomelane, K1-2(Mn3+Mn4+)8 O-16 . xH2O, and hollandite, (K,Ba)1-2 (Mn3+,Mn4+)8 O-16 . xH2O, are often authigenically precipitated in weathering profiles. The presence of structural potassium in manganese oxides makes these minerals datable by the K-Ar and 40Ar/39Ar methods. Dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes. Detailed age resolution of the weathering history allows us to use deep weathering profiles as land-based stratigraphic records of past climatic and geomorphic events, complementing evidence from the marine sedimentary and paleontological record. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, 39Ar loss by recoil during neutron irradiation, contamination by primary minerals resistant to weathering, the presence of multiple generations of manganese oxides, and proportionately low radiogenic Ar yield due to adsorbed atmospheric gases are addressed. The K-Ar and 40Ar/39Ar analytical results indicate that Ar and/or K losses, excess 40Ar, and 39Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that fine scale, laser-probe 40Ar/39Ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and 40Ar/39Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 +/- 6 Ma. Petrographic, electron microscope, and electron microprobe investigation 1reveal multiple generations of manganese oxide precipitation. K-Ar and 40Ar/39Ar laser-probe step-heating results independently confirm the presence of these various generations of manganese oxides. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 +/- 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting from changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and 40Ar/39Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean. Potassium-bearing manganese oxides, cryptomelane, K1-2(Mn3+Mn4+)8 O-16 . xH2O, and hollandite, (K,Ba)1-2 (Mn3+,Mn4+)8 O-16 . xH2O, are often authigenically precipitated in weathering profiles. The presence of structural potassium in manganese oxides makes these minerals datable by the K-Ar and 40Ar/39Ar methods. Dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes. Detailed age resolution of the weathering history allows us to use deep weathering profiles as land-based stratigraphic records of past climatic and geomorphic events, complementing evidence from the marine sedimentary and paleontological record. Potential problems in manganese oxide dating, such as Ar and/or K losses, excess argon, 39Ar loss by recoil during neutron irradiation, contamination by primary minerals resistant to weathering, the presence of multiple generations of manganese oxides, and proportionately low radiogenic Ar yield due to adsorbed atmospheric gases are addressed. The K-Ar and 40Ar/39Ar analytical results indicate that Ar and/or K losses, excess 40Ar, and 39Ar recoil seem not to pose problems in manganese oxide dating. This investigation suggests that fine scale, laser-probe 40Ar/39Ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles. K-Ar and 40Ar/39Ar dating of supergene K-bearing manganese oxides formed during lateritization of Archean and Proterozoic bedrocks in the Carajas Region, Amazonia, Brazil, indicates that weathering started before 72 +/- 6 Ma. Petrographic, electron microscope, and electron microprobe investigation 1reveal multiple generations of manganese oxide precipitation. K-Ar and 40Ar/39Ar laser-probe step-heating results independently confirm the presence of these various generations of manganese oxides. Age clusters at 65-69, 51-56, 40-43, 33-35, 20, 24, 12-17 Ma, and zero-age (0.2 +/- 0.2 Ma) suggest episodic precipitation of K-Mn oxides resulting from changing weathering conditions in the Amazon throughout the Cenozoic. K-Ar and 40Ar/39Ar dating of supergene cryptomelane from weathering profiles in eastern Minas Gerais, southeastern Brazil, suggests continuous weathering from 10 to 5.6 Ma ago, possibly reflecting local climatic conditions due to the proximity with the Atlantic Ocean.
