The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system

We report analyses of the ¹⁷⁶Hf/ ¹⁷⁷Hf ratio for 25 chondrites from different classes of meteorites (C, O, and E) and the ¹⁷⁶Lu/ ¹⁷⁷Hf 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 ¹⁷⁶Hf/ ¹⁷⁷Hf= 0.282772 ± 29 and ¹⁷⁶Lu/ ¹⁷⁷Hf= 0.0332 ± 2. The ¹⁷⁶Hf/ ¹⁷⁷Hf 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 ¹⁴³Nd/ ¹⁴⁴Nd versus ¹⁷⁶Hf/ ¹⁷⁷Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high ³He/ ⁴He ratios. Comparison of observedHf/Nd 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.