Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time

The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic pro cesses has long been recognized, but the: details of this behavior in the d epleted mantle reservoir have not been adequately examined. We report new w hole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid -Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenl and. Hf and Nd isotopic compositions of the juvenile rocks are well correla ted, with the best fit corresponding to the equation epsilon(Hf) = 1.40 eps ilon(Nd) + 2.1, and is similar to the collective Hf-Nd correlation for terr estrial samples of epsilon(Hf) = 1.36 epsilon(Nd) + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in epsilon(Nd) value s ((-)4.4 to (+)14.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The epsilon(Hf) values for these rocks are consistently po sitive and have much less variation (0 to (+)3.4) than their epsilon(Nd) co unterparts. The information from the Hf isotopic compositions of the West Greenland gne isses portrays an early Archean mantle that is relatively isotopically homo geneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an e nriched reservoir. The Hf isotopic data an in stark contrast to the Nd isot opic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West G reenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks. Although Hf and Nd isotopic data do not uniquely constrain either the natur e of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle i n the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive epsilon(Hf) and epsilon(Nd) values for the early Archean rocks, this crust must have had a short residence time at the surf ace of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred millon years. The follo wing period from 3.5 to 2.7 Ga may mark a transition during which this earl y formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 3.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An im portant aspect of crustal growth and evolution, therefore, may be the trans formation of the enriched reservoir from being predominantly mafic in the e arly Earth to becoming progressively more sialic through time. Copyright (C ) 1999 Elsevier Science Ltd.