SM-147-ND-143, SM-146-ND-142 SYSTEMATICS OF EARLY ARCHAEAN ROCKS AND IMPLICATIONS FOR CRUST-MANTLE EVOLUTION

摘要

Combined high precision Sm-147-Nd-143, Sm-146-Nd-142 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 epsilon(t)(143Nd) values at 3.8 Ga of -4.0 and +1.4, respectively, however epsilon(t)(142Nd) for both samples is indistinguishable from the bulk earth value. For the sample with epsilon(t)(143Nd) - 4.0, epsilon(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. Sm-147-Nd-143 and Sm-146-Nd-142 data are also presented for a kyanite eclogite xenolith from a 350 Ma kimberlite from Yakutia, Siberia. Nd-142/Nd-144 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-142/Nd-144 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 epsilon(t)(143Nd) and account for the absence of detectable variations in Nd-142/Nd-144 in most early Archaean rocks. [References: 43]