Computer Science
Scientific paper
Apr 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.168...79v&link_type=abstract
Earth and Planetary Science Letters, Volume 168, Issue 1-2, p. 79-99.
Computer Science
104
Scientific paper
We report new Hf (and Nd) data for more than 100 sedimentary samples, recent 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 global sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf-Nd isotopic behavior in the crust and mantle. Lu/Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to concentration of low Lu/Hf, low 176Hf/177Hf, 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 sediments. Nd isotopic compositions are not highly fractionated by sedimentary sorting because heavy minerals, also rich in REEs, do not fractionate Sm-Nd efficiently. The lack of a large and systematic fractionation at active margins means that no significant Hf-Nd decoupling occurs here. This is important 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 (ɛHf=1.36ɛNd+2.95) which we call the `terrestrial array'. This array is composed of two complementary components: a mantle array (ɛHf=1.33ɛNd+3.19, defined by all oceanic basalts; and a crustal array (ɛHf=1.34ɛNd+2.82), defined by sediments, continental basalts, granitoids, and juvenile crustal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf-Nd decoupling occurs between the crust and mantle. The coherency of the terrestrial Hf-Nd array implies mixing within the mantle, due to stirring processes, and also within the crust, due to homogenization by collective sedimentary processes. In addition, tight Hf-Nd covariation may also imply that efficient crust to mantle recycling has modulated isotopic correlation in the silicate Earth. All Hf-Nd arrays, including the terrestrial array, lie significantly above (2-3 ɛHf 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 composition of the terrestrial array. However, the discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu-Hf and Sm-Nd isotope systems.
Albarède Francis
Blichert-Toft Janne
Jonathan Patchett P.
Vervoort Jeff D.
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