The Nd and Hf isotopic evolution of the mantle through the Archean. results from the Isua supracrustals, West Greenland, and from the Birimian terranes of West Africa

• Published in: Geochimica et cosmochimica acta 1999-11, Vol.63 (22), p.3901-3914
• Main Authors: Blichert-Toft, Janne, Albarède, Francis, Rosing, Minik, Frei, Robert, Bridgwater, David
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(99)00183-0

Evidence for early differentiation events in the Earth’s mantle similar to those that demonstrably affected the Moon and Mars seems to be embedded in the existence of largely super-chondritic 143Nd/ 144Nd ratios in the Early Archean terrestrial mantle. Samples from the ca. 3.8 Ga old rocks from the Isua supracrustal belt, Greenland, and from the ca. 2.1 Ga old juvenile segment of Birimian crust, West Africa, were measured for Nd and Hf isotopic compositions in order to evaluate whether Hf systematics may help enlighten this debate. The determination of Nd and Hf initial isotopic compositions of these terranes is entangled with the uncertainty of which geological events isochron ages and ion probe zircon U-Pb ages actually record, and by metasomatic fractionation of parent/daughter element ratios. It is shown that Nd and Hf mobility has affected the samples for which the Sm/Nd and Lu/Hf ratios depart from the range of modern volcanic rocks. The most conspicuous effect is a preferential leaching of Hf—presumably accompanied by Zr—from mafic metavolcanic rocks. Precipitation of Zr from metasomatic fluids may explain the significantly younger overgrowth zones on zircons from the felsic rocks. For Isua, the pooled whole-rock isochron ages are 3.712 ± 26 Ga (Nd) and 3.593 ± 15 Ga (Hf), while for the Birimian the respective ages are 2.135 ± 29 Ga (Nd) and 2.093 ± 16 Ga (Hf). Elemental mobility and the unsupported assumption of minimum variance underlying the isochron model, however, render the significance of these ages questionable. In an attempt to restore possible ancient mantle-crust arrays, the trajectory of each sample in the (ε Nd[T], ε Hf[T]) space for variable values of the age T are reconstructed. Our best estimate for the isotopic properties of the mantle protolith is ε Nd[3.85] ≈ +2 ± 2 and ε Hf[3.85] ≈ +4 ± 2 for Isua and ε Nd[2.15] ≈ +3 ± 1 and ε Hf[2.15] ≈ +6 ± 2 for the Birimian. We therefore agree with the earlier suggestion that some samples from the Isua supracrustal belt were extracted from a mantle that went through strong parent/daughter fractionation very early in the Earth’s history. This event may be the original differentiation of a largely molten mantle in the presence of majorite garnet. In addition, the contrast between the isotopically inferred long-term Lu/Hf fractionation and the actual Lu/Hf ratio of the rocks requires that garnet was present as a residual mineral (the ‘Hf paradox’). It indicates that melting must have started in