(U–Th)/He geochronology of goethite and the origin and evolution of cangas

Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia

“…A loss between 5 and 30% of He by diffusion is generally estimated for goethite (e.g. Shuster et al, 2005;Vasconcelos et al, 2013;Monteiro et al, 2014;Riffel et al, 2016). A possible loss by diffusion in our samples will be estimated in the result section.…”

“…6A. This result attests to the homogeneity of the samples, as the Th/U ratio can be used to trace any difference in iron oxide populations or the presence of inclusions (Monteiro et al, 2014;Riffel et al, 2016). However, the JIC sample that comes from São Gabriel da Cachoeira region yielded a more dispersed range of raw (UTh)/He ages, from 0.9 ± 0.1 to 12.7 ± 0.6 Ma (n = 7).…”

“…In addition, ferruginous duricrusts from this region were also dated by (U-Th)/He and yielded ages ranging from 70 to 1 Ma (Shuster et al, 2012;Vasconcelos et al, 2015). Besides the similar ages recorded by 40 (Carmo and Vasconcelos, 2006;Monteiro et al, 2014;Shuster et al, 2012). In addition, old cosmogenic 3 He isotopic ages suggested a long stability with respect to erosion since 20 Ma (Shuster et al, 2012).…”

“…Shuster et al (2005) and more recent references (e.g. Vasconcelos et al, 2013;Monteiro et al, 2014;Riffel et al, 2016) also applied a 5 to 30% correction for goethite (UTh)/He ages based on diffusion data measured directly on the dated crystals. We also used an error of 15% that represents the sum of analytical error and the error on He diffusion loss estimation.…”

“…One can also notice that the Th/U ratio is also quite scattered and can trace any difference in iron oxide populations or the presence of inclusions (e.g. Monteiro et al, 2014;Riffel et al, 2016).…”