High-precision Nd/Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth s mantle

We present new ultra-high precision Nd/Nd measurements of early Archaean rocks using the new generation thermal ionization mass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the Nd/Nd ratio can be determined with external precision of 2 ppm (2r), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement lies in the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fractionation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland craton display positive Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial eNd value of 1.9 ± 0.6 eunits, coupled Sm–Nd and Sm–Nd chronometry constrains mantle differentiation to 50–200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earth s mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing Nd and Nd isotopic evolution of depleted mantle during the subsequent evolution of the crust–mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7–1 Ga in order to produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle–crust system, we model the evolution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of Nd/Nd and Nd/Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earth s mantle to 100– 250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts. 2005 Elsevier Inc. All rights reserved.