A hafnium isotope and trace element perspective on melting of the depleted mantle Catherine Chauvel a;b; *, Janne Blichert-Toft c a Ge ¨osciences Rennes, Campus de Beaulieu, F-35042 Rennes Cedex, France b LGCA, Campus de St Martin d'He ©res, P.O. Box 53, F-38041 Grenoble Cedex 09, France c Laboratoire des Sciences de la Terre, Ecole Normale Supe ¨rieure de Lyon, 46 Alle ¨e d'Italie, 69364 Lyon Cedex, France Received 10 January 2001; received in revised form 8 May 2001; accepted 15 May 2001 Abstract New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform ( 176 Hf/ 177 HfW0.28313^0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109^123 ; Patchett, Lithos 16 (1983) 47^51]. Atlantic MORB have 176 Hf/ 177 Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077^1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low 176 Hf/ 177 Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf^Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB) : ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions ^ the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] ^ can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Hf-177/Hf-176; trace elements; mid-ocean ridge basalts; mantle; spinel; garnet group 1. Introduction Patchett [2,3], who ¢rst measured the Hf iso- topic compositions of mid-ocean ridge basalts (MORB), showed that the Hf isotopic composi- tions of these basalts vary independently of their Sr and Nd isotopes. Hf and Nd isotopic compo- sitions correlate well in ocean island basalts (OIB), whereas Hf isotopic compositions of MORB scatter widely at almost constant Nd. 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00379-X * Corresponding author. E-mail address: cchauvel@ujf-grenoble.fr (C. Chauvel). Earth and Planetary Science Letters 190 (2001) 137^151 www.elsevier.com/locate/epsl