| Summary: | Combined high precision Sm-Nd, Sm-Nd data have been obtained for two early Archaean (pre-3.8 Ga) samples (a 4.0 ± 0.2 Ga meta-komatiite and an approximately 3.9 Ga orthogneiss) from the northern Labrador segment of the North Atlantic Craton. The samples have ∈ values at 3.8 Ga of -4.0 and +1.4, respectively, however ∈ for both samples is indistinguishable from the bulk earth value. For the sample with ∈ - 4.0, ∈ is +0.03 ± 0.14, which suggests that it was derived from a source which differentiated from the primitive mantle at between 4.1 and 4.3 Ga. Sm-Nd and Sm-Nd data are also presented for a kyanite eclogite xenolith from a 350 Ma kimberlite from Yakutia, Siberia. Nd/Nd for this sample is also within error of the bulk earth value. Eclogite xenoliths, such as those from kimberlite pipes, are, therefore, unlikely to be residues of crystallization of a pre-4.4 Ga magma ocean, but instead may represent fragments of post-4.3 Ga oceanic crust incorporated into the subcontinental lithospheric mantle. The absence of widespread variations in Nd/Nd in early Archaean rocks supports models in which the volume of the depleted mantle has grown through time. Continuous growth of the depleted mantle and continental crust at the expense of the primordial mantle at between 4.56 and 4.0 Ga can generate variations in ∈ and account for the absence of detectable variations in Nd/Nd in most early Archaean rocks.
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