High-precision 142Nd/144Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth " s mantle146Sm -- 142Nd systematics of terrestrial rocks --IntroductionRegional geology and sampling strategySouthern West GreenlandMetamorphosed clastic sediments from the Isua greenstone beltMetamorphosed basaltic rocks from the Isua greenstone beltGneisses of magmatic origin (orthogneisses)Enclave in 3.82Ga gneissesAcasta gneisses, Slave Province, NWT, CanadaBarberton komatiite, Kaapvaal craton, South AfricaAnalytical techniquesReproducibility of Nd isotope measurementsExternal precision of Nd standard measurements from march 2001 to August 2002External precision of Nd standard measurements between July 2003 and January 2004Nd chemical separationMass spectrometryResultsSouthern West GreenlandMetamorphosed basaltic rocks from the Isua greenstone beltGneisses of magmatic origin (orthogneisses)Metamorphosed clastic sediments from the Isua greenstone beltBarberton komatiiteAcasta gneissesChronological constraints from coupled 146Sm ndash 142Nd and 147Sm ndash 143Nd systematicsA test for crustal growth models using 146Sm ndash 142Nd systematicsModeling Nd isotopic evolution in a continuously interacting mantle ndash crust systemA box model for early mantle differentiationEstimating model parametersConstraints on the lifetime of early Earth rsquo s crustMantle mixing and thermal state of the early EarthConstraining isotopic heterogeneity of the early Earth rsquo s mantle using 146Sm ndash 142Nd and 147Sm ndash 143Nd systematicsModeling isotopic heterogeneityMantle mixing in the early EarthConclusionsAcknowledgmentsReferencesHigh-precision142Nd/144Nd measurements in terrestrial rocks:Constraints on the early differentiation of the EarthÕs mantleGuillaume Caroa,*, Bernard Bourdona, Jean-Louis Bircka, Stephen MoorbathbaLaboratoire Ge´ochimie-Cosmochimie (UMR 7579 CNRS), Institut de Physique du Globe de Paris, Universite´Paris 7 Denis Diderot,4 place Jussieu, 75252 Paris Cedex 05, FrancebDepartment of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UKReceived 29 July 2004; accepted in revised form 25 August 2005AbstractWe present new ultra-high precision142Nd/144Nd measurements of early Archaean rocks using the new generation thermal ionizationmass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the142Nd/144Nd ratio can be determinedwith external precision of 2 ppm (2r), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement liesin the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fraction-ation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland cratondisplay positive142Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial e143Nd value of 1.9 ± 0.6 e-units, coupled147Sm–143Nd and146Sm–142Nd chronometry constrains mantle differentiation to 50–200 Ma after formation of the solarsystem. This chronological constraint is consistent with differentiation of the EarthÕs mantle during the late stage of crystallization of amagma ocean. We have developed a two-box model describing142Nd and143Nd isotopic evolution of depleted mantle during the sub-sequent evolution of the crust–mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7–1 Ga in orderto produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle–crust system, we model the evo-lution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of142Nd/144Nd and143Nd/144Nd ratios observed in early Archaean rocks, we constrain the stirring time of early EarthÕs 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.1. IntroductionShort-lived isotope systems182Hf–182Wand129I–244Pu–129Xe provide precise chronological constraints on the veryearly differentiation of the EarthÕs core (Halliday, 2004;Kleine et al., 2002; Scho¨nberg et al., 2002; Yin et al.,2002) and the formation of terrestrial atmosphere (Kunzet al., 1998; Ozima and Podosek, 1999; Staudacher andAlle`gre, 1982). In contrast, there are considerable uncer-tainties about the early history of the EarthÕs mantle andthe nature of the EarthÕs earliest crust. In particular, theexistence of continental crust during the first half billionyears of Earth history (e.g., Armstrong, 1968, 1981), andthe extent to which terrestrial mantle may have experiencedlarge-scale differentiation following crystallization of ahypothetical magma ocean (e.g., Agee and Walker, 1989;Kato et al., 1988, 1989a,b; Ohtani, 1988), are matters ofdebate.With the exception of rare detrital zircons discoveredwithin the Yilgarn craton, Australia (Froude et al., 1983;Compston and Pidgeon, 1986; Wilde et al., 2001), crustalmaterial older than 4.0 Ga is missing from the present-day continental rock record; this precludes direct observa-tion of crust–mantle evolution during the Hadean era(4.0–4.5 Ga). However, insights into early mantle historycan be obtained indirectly from the geochemical signaturespreserved by isotopic tracers of mantle–crust differentiation.www.elsevier.com/locate/gcaGeochimica et Cosmochimica Acta 70 (2006) 164–1910016-7037/$ - see front matter  2005 Elsevier Inc. All rights reserved.doi:10.1016/j.gca.2005.08.015*Corresponding author. Present address: Division of Geological andPlanetary Sciences, California Institute of Technology, Pasadena, CA91125, USA.E-mail address: [email protected] (G. Caro).Application of long-lived147Sm–143Nd isotopic systemindicates that early Archaean rocks are characterized byinitial143Nd/144Nd ratios higher than chondritic, therebyrequiring form ation of these rocks from a mantle reservoirwith super-chondritic147Sm/144Nd ratio. This, in turn, sug-gests that the EarthÕs mantle may have experienced large-scale depletion in incompatible elements several hundredmillion years prior to the emplacement of the most ancientcontinental rocks, and more than a billion yea r prior to themain crustal grow th stages (e.g., Chase and Patchett, 1988;Galer and Goldstein, 1991; Smith and Ludden , 1989; Ver-voort et al., 1996).In the