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Published May 2012 | public
Journal Article

Cosmogenic ^(3)He in hematite and goethite from Brazilian "canga" duricrust demonstrates the extreme stability of these surfaces

Abstract

Helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts (canga) developed on banded iron formations. These samples uniformly have high ^(3)He concentrations which must arise from long periods of cosmic ray exposure. From coexisting phases from the Quadrilátero Ferrífero in east central Brazil, we determined the ratio of cosmogenic ^(3)He in hematite to that of ^(21)Ne in quartz to be 3.96 ± 0.19. Combined with best current estimates of the ^(21)Ne production rate in quartz, this ratio implies a sea-level high latitude (SLHL) ^(3)He production rate in hematite of 68.1 ± 8.1 atoms/g/yr; from the chemical composition we estimate the ^(3)He production rate in goethite to be ~5% higher. We use these production rate estimates to interpret ^(3)He concentrations measured in goethite and hematite from a ~10 m depth profile collected from a surface canga in Carajás, in the Amazon basin of Brazil. We find that the Carajás canga has experienced a very low rate of surface erosion (~0.16–0.54 m/Myr) over at least the last few millions of years. This iron-rich canga surface is remarkably resistant to erosion despite its location in a wet tropical environment. Details of the depth profile suggest that despite its stability, the canga has also been internally dynamic (translocation of material; solution and reprecipitation) over million-year timescales.

Additional Information

© 2012 Elsevier B.V. Received 1 November 2011. Revised 9 February 2012. Accepted 17 February 2012. Available online 21 March 2012. Editor: B. Marty. We thank Vale S.A. for providing us access to the sampling sites at Carajás, Carlos Spier (then at MBR) for providing access and field support for sampling that the Pico Mine, and M. Honda and P.-H. Blard for constructive reviews of the manuscript. Funding was provided by the following sources: UQ-AGES for funding the field sampling exercises at Carajás and Pico, grants NSF- EAR-0738474 (to DLS) and NSF- EAR-0921295 (to KAF), and the Ann and Gordon Getty Foundation.

Additional details

Created:
August 22, 2023
Modified:
October 17, 2023