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Science 16 October 1992:
Vol. 258. no. 5081, pp. 451 - 455
DOI: 10.1126/science.258.5081.451
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Articles

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

Paulo M. Vasconcelos 1, Tim A. Becker 2, Paul R. Renne 2, and George H. Brimhall 1

1 Department of Geology and Geophysics, University of California, Berkeley, CA 94720
2 Institute of Human Origins Geochronology Center, 2453 Ridge Road, Berkeley, CA 94709

Supergene cryptomelane [K1–2(Mn3+Mn4+)8O16· khgrH2O] 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 sgr, 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.

Submitted on April 20, 1992
Accepted on July 27, 1992


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Quaternary continental weathering geochronology by laser-heating 40Ar/39Ar analysis of supergene cryptomelane.
Y. Feng and P. Vasconcelos (2001)
Geology 29, 635-638
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Chronology of Exotic Mineralization at El Salvador, Chile, by 40Ar/39Ar Dating of Copper Wad and Supergene Alunite.
T. I. Mote, T. I. Mote, T. A. Becker, P. Renne, and G. H Brimhall (2001)
Economic Geology 96, 351-366
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