Strategies for successful U-Th dating of paleolake carbonates: An example from the Bolivian Altiplano

Abstract

We report over 90 U-Th dates from carbonates deposited around paleolakes on the Bolivian Altiplano. Petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow (1) assessment of possible diagenetic effects, (2) a development of a strategy for selection of carbonate samples with low initial Th contents, and (3) assessment of the uncertainty due to initial Th. This strategy allows us to produce precise U-Th dates from lacustrine carbonates. The principal consideration in dating such carbonates is the composition and quantity of initial Th incorporated into the carbonate, and additional uncertainty is introduced because this initial Th may have two sources in lacustrine deposits. Isochron plots, measured (230Th/232Th), and X-ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial Th in carbonates from the Bolivian Altiplano, a situation that simplifies single-sample dating of Altiplano carbonates.