SHORT-LIVED U AND TH ISOTOPE DISTRIBUTION IN A TROPICAL LATERITE DERIVED FROM GRANITE (PITINGA RIVER BASIN, AMAZONIA, BRAZIL) - APPLICATIONTO ASSESSMENT OF WEATHERING RATE

We have analyzed samples of a 15 m thick profile weathered from the Ma deira 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 p articularly high in the granite (20 and 80 mu g/g respectively). Norma lized 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 rel ative enrichment, resulting from a loss of mass. The saprolites are in itially generated by a descending weathering front which alters the gr anite to a yellow-red saprolite, a second front, close to the top, tur ns 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 diseq uilibrium. Numerous nodules are present and apparently started to form at the base of the saprolite. These nodules achieve more developed fo rm during their relative ascent until they are reached by the descendi ng top front where they undergo dissolution. The Th and Pb are concent rated 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, fr eed by the second, engages in a descending flux which supplies the und erlying saprolite. Using the data an attempt is made to model the isot opic 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 crea te the saprolite must have been around 300,000 yr.