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

Supergene cryptomelane [K 1–2 (Mn 3+ Mn 4+ ) 8 O 16 ⋅ χH 2 O] samples from deeply weathered pegmatites in southeastern Brazil subjected to 40 K- 40 Ar and 40 Ar/ 39 Ar analysis yielded 40 K- 40 Ar dates ranging from 10.1 ± 0.5 to 5.6 ± 0.2 Ma (million years ago). Laser-probe 40 Ar/ 39 Ar 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 40 K- 40 Ar 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.