THE LU-HF ISOTOPE GEOCHEMISTRY OF CHONDRITES AND THE EVOLUTION OF THE MANTLE-CRUST SYSTEM

Abstract

We report analyses of the 176Hf177Hf ratio for 25 chondrites from different classes of meteorites (C, O and E) and the 176Lu177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of 176Hf177Hf = 0.282772 +/- 29 and 176Lu177Hf = 0.0332 +/- 2. The 176Hf177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 +/- 29. Because the mantle array lies above the Bulk Silicate Earth in a 143Nd144Nd versus 176Hf177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high 3He4He ratios. Comparison of observed HfNd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.