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

Helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts (canga) developed on banded iron formations. These samples uniformly have high He-3 concentrations which must arise from long periods of cosmic ray exposure. From coexisting phases from the Quadrilatero Ferrifero in east central Brazil, we determined the ratio of cosmogenic He-3 in hematite to that of Ne-21 in quartz to be 3.96 +/- 0.19. Combined with best current estimates of the Ne-21 production rate in quartz, this ratio implies a sea-level high latitude (SLHL) He-3 production rate in hematite of 68.1 +/- 8.1 atoms/g/yr; from the chemical composition we estimate the He-3 production rate in goethite to be similar to 5% higher. We use these production rate estimates to interpret He-3 concentrations measured in goethite and hematite from a similar to 10 m depth profile collected from a surface canga in Carajas, in the Amazon basin of Brazil. We find that the Carajas canga has experienced a very low rate of surface erosion (similar to 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. (C) 2012 Elsevier B.V. All rights reserved.