Age and Duration of Weathering by 40K-40Ar and 40Ar/39Ar Analysis of Potassium-Manganese Oxides

Supergene cryptomelane [K1–2(Mn3+Mn4+)8O16� χH2O] samples from deeply weathered pegmatites in southeastern Brazil subjected to 40K-40Ar and 40Ar/39Ar analysis yielded 40K-40Ar dates ranging from 10.1 � 0.5 to 5.6 � 0.2 Ma (million years ago). Laser-probe 40Ar/39Ar step-heating of the two most disparate samples yielded plateau dates of 9.94 � 0.05 and 5.59 � 0.10 Ma, corresponding, within 2 σ, to the 40K-40Ar dates. The results imply that deep weathering profiles along the eastern Brazilian margin do not reflect present climatic conditions but are the result of a long-term process that was already advanced by the late Miocene. Weathering ages predate pulses of continental sedimentation along the eastern Brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes.

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