Nd and Sr isotopic crustal contamination patterns in an Archaean meta-basic dyke from northern Labrador

Samples from the core to the margin of a 20 cm wide meta-dolerite dyke are sequentially enriched in K, Rb, Sr, and the light REE's. Rb-Sr and Sm-Nd compositional and isotopic profiles in the dyke are interpreted to be the result of selective contamination with components of country rock derivation, rather than the result of simple bulk mixing. ${}^{87}\text{Rb}{}^{86}\text{Sr}$ ratios are higher at the edge of the dyke than at its centre, although they are somewhat irregular, due probably to the effects of subsequent alteration. This profile and one shown by unsupported ⁸⁷Sr are both consistent with contamination of the dyke by a fluid phase derived by the breakdown of biotite. Common Sr shows a parallel, albeit weaker, contamination profile which is interpreted to reflect the contribution of a Sr-bearing phase such as plagioclase. ${}^{147}\text{Sm}{}^{144}$Nd ratios and ¹⁴⁴Nd concentrations increase and decrease respectively from the margin to the core of the dyke. In addition, the margin of the dyke is significantly less radiogenic than the interior. This contrasts with the relatively radiogenic character of an adjacent pegmatite vein. As this sample does not lie on an anticipated contamination profile between the Uivak gneisses and the dyke it is concluded that the REE contamination of the dyke occurred by the addition of a REE-enriched fluid phase which gained access to the dyke by flow along the dyke-pegmatite interface. If it is assumed that both the Rb-Sr and Sm-Nd contamination profiles are the result of diffusion limited processes, then the observations of scale made in this paper suggest that the rate of diffusion of Nd is an order-of-magnitude slower than that for Sr. In view of the scale and nature of these profiles, ages obtained from isotopic data for such mafic dykes must be interpreted with some care. Nevertheless, in spite of these limitations the ?_Nd values for the least contaminated specimens provide a clear indication that the Saglek dykes were derived from a depleted mantle source with ?_Nd? +2.