SR-ND-PB ISOTOPE RELATIONSHIPS IN LATE ARCHEAN CARBONATITES AND ALKALINE COMPLEXES - APPLICATIONS TO THE GEOCHEMICAL EVOLUTION OF ARCHEAN MANTLE

Isotopic studies of strontium, neodymium, and lead in young carbonatites from several continents have shown that the data generally plot within the fields of ocean island basalts in isotope correlation diagrams. These and other data suggest that the magmas originate in mantle sources and generally pass through continental crust with minimal contamination. Further studies have shown that initial isotopic ratios in carbonatites and alkaline complex rocks from the Canadian Shield spanning an age range from 0.1 to 2.7 Ga appear to trace the evolution of ''depleted'' subcontinental mantle over the past 2.7 Ga. However, the data were based upon carbonatites between 0 and 1.9 Ga, and substituted syenites at 2.7 Ga due to lack of known carbonatites of that age. This raised the question of possible crustal contamination in the syenites, particularly for lead. Recently two carbonatite bodies with ages of 2.68 Ga have been identified at the Lac Shortt mine and Dolodau dykes in south-central Quebec. Lead, strontium, and neodymium isotope data from these carbonatites fit the evolution patterns already established from the syenites, with slightly more radiogenic neodymium and slightly less radiogenic strontium and lead. The initial epsilon(Nd) from eight carbonatite samples is +2.8 +/- 0.3; epsilon(Sr) = -0.3 +/- 0.3 (Sr-87/Sr-86 = 0.70127 +/- 0.00002). Lead ratios from combined carbonatites and syenites define a regression line with a slope of 0.71 +/- 0.04 in a Pb-207-Pb-206 correlation diagram, corresponding to radiogenic lead evolved in a closed system between ca. 3.8 and 2.7 Ga; however, we ascribe this trend to mixing of mantle components rather than to crustal contamination processes. Data from young (<0.2 Ga) carbonatites from five continents closely fit a model corresponding to mixing between EM1 and HIMU mantle components in isotope correlation diagrams. Similar diagrams for the Canadian Shield 2.7 Ga samples exhibit no clear mixing trends, and Sr-87/Sr-86-Pb-206/Pb-204 ratios tend to cluster closely around model ''bulk silicate Earth'' in an isotope correlation diagram. Mantle differentiation processes appear to have changed in fundamental ways around 3 Ga ago, with most sialic continental crust produced after that time.