LEAD ISOTOPIC SYSTEMATICS OF MAJOR RIVER SEDIMENTS: A NEW ESTIMATE OF THE PB ISOTOPIC COMPOSITION OF THE UPPER CONTINENTAL CRUST

Abstract

The lead isotopic composition of river sediments is reported in the present work for the Earth's major river basins, from old cratonic to young orogenic areas and from subarctic to tropical climates. Sediment samples from these large river basins provide a useful tool to calculate the average upper crustal composition because they are large-scale integrated samples of the weathering products of the present-day Upper Continental Crust (UCC). Two different and complementary calculations were done to estimate the average lead isotopic composition of the UCC. The first, based on the flux weighted average of particulate lead delivered by the rivers, gave values of 19.07, 15.74 and 39.35 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, respectively. To avoid over-estimating the contribution of orogenic areas, which produces a bias (because the flux of particulate lead depends strongly on the physical erosion rate), a second calculation was done by averaging with drainage areas of each river basin. This gave values of 18.93, 15.71 and 39.03 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, respectively. These direct calculations of the lead isotopic composition of the UCC are similar and are in agreement with previous estimates made using an indirect approach.