High-precision Nd-142/Nd-144 measurements in terrestrial rocks: Constraints on the early differentiation of the Earth's mantle

We present new ultra-high precision Nd-142/Nd-144 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-142/Nd-144 ratio can be determined with external precision of 2 ppm (2 sigma), 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-142 anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial epsilon(143)Nd value of 1.9 +/- 0.6 epsilon-units, coupled Sm-147-Nd-143 and Sm-146-Nd-142 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-142 and Nd-143 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-142/Nd-144 and Nd-143/Nd-144 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. (c) 2005 Elsevier Inc. All rights reserved.