PB ISOTOPE COMPOSITIONS OF MODERN DEEP SEA TURBIDITES

Abstract

Modern deep sea turbidite muds and sands collected from Lamont piston cores represent a large range in age of detrital sources as well as a spectrum of tectonic settings. Pb isotope compositions of all but three of the 66 samples lie to the right of the 4.56 Ga Geochron, and most also lie along a slope consistent with a time-integrated κ (232Th/238U) between 3.8 and 4.2. Modern deep sea turbidites show a predictable negative correlation between both Pb and Sr isotope ratios and εNd and εHf, clearly related to the age of continental sources. However, the consistency between Pb and Nd isotopes breaks down for samples with very old provenance (εNd<-20) that are far less radiogenic than predicted by the negative correlation. The correlations among Sr, Nd and Hf isotopes also become more scattered in samples with very old provenance. The unradiogenic Pb isotopic character of modern sediments with Archean Nd model ages is consistent with a model where Th and U abundances of the Archean upper crust are significantly lower than the post-Archean upper crust.