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

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 Nd-143/Nd-144 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 se gment of Birimian crust, West Africa, were measured for Nd and I-if isotopi c compositions in order to evaluate whether Hf systematics may help enlight en this debate. The determination of Nd and Hf initial isotopic composition s of these terranes is entangled with the uncertainty of which geological e vents isochron ages and ion probe zircon U-Pb ages actually record, and by metasomatic fractionation of parent/daughter element ratios. It is shown th at Nd and Hf mobility has affected the samples for which the Sm/Nd and Lu/H f ratios depart from the range of modem volcanic rocks. The most conspicuou s 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 fels ic 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 t he unsupported assumption of minimum variance underlying the isochron model , however, render the significance of these ages questionable. In an attemp t to restore possible ancient mantle-crust arrays, the trajectory of each s ample in the (epsilon(Nd)[T], epsilon(Hf)[T]) space for variable values of the age T are reconstructed. Our best estimate for the isotopic properties of the mantle protolith is epsilon(Nd)[3.85] approximate to +2 +/- 2 and ep silon(Hf)[3.85] approximate to +4 +/- 2 for Isua and epsilon(Nd)[2.15] appr oximate to +3 +/- 1 and epsilon(Hf)[2.15] approximate to +6 +/- 2 for the B irimian. We therefore agree with the earlier suggestion that some samples f rom the Isua supracrustal belt were extracted from a mantle that went throu gh strong parent/daughter fractionation very early in the Earth's history. This event may be the original differentiation of a largely molten mantle i n the presence of majorite garnet. In addition, the contrast between the is otopically inferred long-term Lu/Hf fractionation and the actual Lu/Hf rati o of the rocks requires that garnet was present as a residual mineral (the 'Hf paradox'). It indicates that melting must have started in the Archean i n plumes at a very deep level, thereby providing a mechanism to create thic k buoyant oceanic plateaux that would act as nuclei for proto-continents, C opyright (C) 1999 Elsevier Science Ltd.