147Sm 143Nd, 146Sm 142Nd systematics of early Archaean rocks and implications for crust-mantle evolution

Abstract

Combined high precision 147Sm 143Nd, 146Sm 142Nd 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 ϵ t 143Nd values at 3.8 Ga of −4.0 and +1.4, respectively, however ϵ t 142Nd 142Nd for both samples is indistinguishable from the bulk earth value. For the sample with ϵ t 143Nd − 4.0, ϵ t 142Nd 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. 14Sm 143Nd and 146Sm 142Nd data are also presented for a kyanite eclogite xenolith from a 350 Ma kimberlite from Yakutia, Siberia. 142Nd/ 144Nd 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 142 Nd 144 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 ϵ t 143Nd and account for the absence of detectable variations in 142 Nd 144 Nd in most early Archaean rocks.