HF ISOTOPE CONSTRAINTS ON MANTLE EVOLUTION

The similarity of the Lu-Hf and Sm-Nd isotope system during most mantl e differentiation processes makes the combination of Hf-176/Hf-177 and Nd-143/Nd-144 a very sensitive indicator of a select number of proces ses. This paper present new Hf-isotope data for a large number of ocea n islands and examines the Hf-Nd-Pb isotope relations of oceanic volca nics. Except for HIMU islands, St. Helena and Tubaii, the Hf and Nd is otope ratios in ocean island basalts (OIB) are extremely well correlat ed. It is argued that crustal recycling (by either continental or ocea nic sediments) most likely did not cause the Hf-Nd variations. The cor related Hf-176/Hf-177-Nd-143/Nd-144 variations in OIB most likely repr esent the time integrated fractionations which are the result of melti ng in the presence of garnet. The Hf-isotope systematics of HIMU-type OIB are consistent with these basalts representing recycled oceanic cr ust and thus support the earlier hypothesis on the origins of HIMU bas alts. Chondrites form an array that is at high angle with the OIB arra y. This allows a choice in the Nd-143/Nd-144 and( 176)Hf/Hf-177 values for chondritic bulk earth. With a choice of bulk earth at the extreme end of the OIB array the shift of OIB to higher Hf-176/Hf-177 can be explained by either isolation of a significant amount of basalts from the mantle for several billions of year or by fractionation and isolat ion of small amounts (< 1%) of perovskite during the early history of the Earth. The large range in Hf-isotopes for a limited range in Nd-is otopes in MORE can be explained by addition of small amounts (much les s than 1%) of the perovskite material back into the MORE mantle. If ch ondritic BE is chosen to be internal to the OIB the well correlated Hf -Nd isotope characteristics in OIB argue against the fractionation of perovskite during the early history of the Earth and the shape of the MORE field remains an enigma. (C) 1998 Elsevier Science B.V. All right s reserved.