A hafnium isotope and trace element perspective on melting of the depletedmantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) fro m the Pacific Ocean basin are remarkably uniform (Hf-176/Hf-177 approximate to0.28313-0.28326) and comparable to previously published data [Salters, E arth Planet. Sci. Lett. 141 (1996) 109-123; Patchett, Lithos 16 (1983) 47-5 1]. Atlantic MORB have Hf-176/Hf-177 ranging from 0.28302 to 0.28335 confir ming the wide range originally identified by Patchett and Tatsumoto [Geophy s. 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 Hf-176/Hf-177 ratios, together with t heir trace element characteristics, require the presence of unusual plume-t ype material beneath the Indian ridge. All other Indian MORB have uniform H f 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 I ndian MORB is best explained by the presence in Indian depleted mantle of o ld recycled oceanic crust and pelagic sediments. Sm/Hf` ratios calculated f rom 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 c onstant in OIB, but decrease with increasing Sm contents in MORB. The const ancy of Sm/Hf` in OIB is probably due to an overwhelming influence of resid ual garnet during melting. By contrast, the decrease of Sm/Hf` in MORB is d ue to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relat ionship between Sm/Nd and Lu/Hf ratios in MORB does not require the presenc e 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 T rans. Am. Geophys. Union 70 (1989) 510] - can be explained by melting domin antly in the spinel field at shallow depths beneath mid-ocean ridges. (C) 2 001 Elsevier Science B.V. All rights reserved.