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

We present new ultra-high precision 142 Nd/ 144 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 142 Nd/ 144 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 142 Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial e 143 Nd value of 1.9 ± 0.6 eunits, coupled 147 Sm– 143 Nd and 146 Sm– 142 Nd chronometry constrains mantle differentiation to 50–200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earths mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing 142 Nd and 143 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 142 Nd/ 144 Nd and 143 Nd/ 144 Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earths mantle to 100– 250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts.