Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.

Site Tools

  • AAAS
  • Subscribe
  • Feedback

Site Search

Search Advanced

Science 16 October 1992:
Vol. 258. no. 5081, pp. 451 - 455
DOI: 10.1126/science.258.5081.451
Prev | Table of Contents | Next

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
   Abstract »    Full Text »    PDF »
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
   Abstract »    Full Text »    PDF »


To Advertise     Find Products

Science. ISSN 0036-8075 (print), 1095-9203 (online)