Short-lived U and Th isotope distribution in a tropical laterite derived from granite (Pitinga river basin, Amazonia, Brazil): Application to assessment of weathering rate

We have analyzed samples of a 15 m thick profile weathered from the Madeira granite, located in the Pitinga basin river, north of Manaus, in the state of Amazonia, Brazil. This profile consists essentially of a yellow-red saprolite covered by a soil. U and Th concentrations are particularly high in the granite (20 and 80 mu g/g respectively). Normalized element to Th concentrations indicate that Th is most resistant to chemical weathering, except to some extent in the top soil. Higher concentrations in the saprolite compared to the granite comprise a relative enrichment, resulting from a loss of mass. The saprolites are initially generated by a descending weathering front which alters the granite to a yellow-red saprolite, a second front, close to the top, turns the saprolite into a soil. Weathering has led to leaching of U. The U-234/U-238 and Th-230/U-238 isotopic ratios are in radioactive disequilibrium. Numerous nodules are present and apparently started to form at the base of the saprolite. These nodules achieve more developed form during their relative ascent until they are reached by the descending top front where they undergo dissolution. The Th and Pb are concentrated in the nodules close to the top front. The U, being more mobile, is strongly leached by the first front, and most of the remainder, freed by the second, engages in a descending flux which supplies the underlying saprolite. Using the data an attempt is made to model the isotopic distribution in the profile. We conclude that the first front has descended at a rate of 5 cm/1000 yt, and that the time needed to create the saprolite must have been around 300,000 yr.