Relationships between Lu-Hf and Sm-Nd isotopic systems in the global sedimentary system

We report new Hf (and Nd) data for more than 100 sedimentary samples, recen t to Archean in age, from a wide range of depositional environments. These data document the behavior of Lu-Hf and Sm-Nd isotopic systems in the globa l sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf-Nd isotopic behavio r in the crust and mantle. Lu/Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to conc entration of low Lu/Hf, low Hf-176/Hf-177, Hf-rich zircons in mature sands. In active margin settings, Lu-Hf fractionation due to the 'zircon effect' is minor due to the less weathered and more juvenile character of the sedim ents. Nd isotopic compositions are not highly fractionated by sedimentary s orting because heavy minerals, also rich in REEs, do not fractionate Sm-Nd efficiently. The lack of a large and systematic fractionation at active mar gins means that no significant Hf-Nd decoupling occurs here. This is import ant because sediments at active margins are the most likely to be recycled to the mantle. Hf-Nd isotopic data for all terrestrial samples fall along a single coherent trend (epsilon(Hf) = 136 epsilon(Nd) + 2.95) which we call the 'terrestrial array'. This array is composed of two complementary compo nents: a mantle array (epsilon(Hf) = 133 epsilon(Nd) + 3.19, defined by all oceanic basalts; and a crustal array (epsilon(Hf) = 1.34 epsilon(Nd) + 2.8 2), defined by sediments, continental basalts, granitoids, and juvenile cru stal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf-Nd decoupling occurs between the crust and mantle. The co herency of the terrestrial Hf-Nd array implies mixing within the mantle, du e to stirring processes, and also within the crust, due to homogenization b y collective sedimentary processes. in addition, tight Hf-Nd covariation ma y also imply that efficient crust to mantle recycling has modulated isotopi c correlation in the silicate Earth. All Hf-Nd arrays, including the terres trial array, lie significantly above (2-3 epsilon(Hf) units) the BSE (bulk silicate Earth) reference. This would appear to require a hidden reservoir in the Earth, heretofore unsampled, to balance the Hf-Nd isotopic compositi on of the terrestrial array. However, the discrepancy between the terrestri al array and BSE may simply be due to differences in the way the CHUR (chon dritic uniform reference) values were determined for the Lu-Hf and Sm-Nd is otope systems. (C) 1999 Elsevier Science B.V. All rights reserved.