Natural fractionation of 238 U / 235 U

The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the U/U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/ nodules and BIFs) of 1.3‰, exceeding by far the analytical precision of our method ( 0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed U–U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display dU of 0.54‰ to 0.62‰ and for three of four analyzed Banded Iron Formations, which have dU of 0.89‰, 0.72‰ and 0.70‰, respectively. High dU values are observed for black shales from the Black Sea (unit-I and unitII) and three Kupferschiefer samples (Germany), which display dU of 0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated dU ( 0.41‰ to 0.16‰) compared to seawater, which has dU of 0.41 ± 0.03‰. Granites define a range of dU between 0.20‰ and 0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (dU = 0.29‰). Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble U to insoluble U probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that U/U varies in nature also has implications for the precision and accuracy of U–Pb dating. The total observed range in U isotope compositions would produce variations in Pb/Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old. 2007 Elsevier Ltd. All rights reserved.