In situ U-Pb age determination and Nd isotopic analysis of perovskites from kimberlites in southern Africa and Somerset Island, Canada

摘要

Determination of the emplacement ages and initial isotopic composition of kimberlite by conventional isotopic methods using bulk rock samples is unreliable as these rocks usually contain diverse clasts of crustal- and mantle-derived materials and can be subject to post-intrusion sub-aerial alteration. In this study, 8 samples from 5 kimberlites in southern Africa and twelve samples from 7 kimberlites from Somerset Island, Canada have been selected for in situ perovskite U-Pb isotopic age determination and Nd isotopic analysis by laser ablation using thin sections and mineral separates. These fresh perovskites occur as primary groundmass minerals with grain-sizes of 10-100 μm. They were formed during the early stage of magmatic crystallization, and record data for the least contaminated or contamination-free kimberlitic magma U-Pb isotopic data indicate that the majority of the southern Africa kimberlites investigated were emplaced during the Cretaceous with ages of 88 ±3 to 97 ±6 Ma, although one sample yielded an Early Paleozoic age of 515 ± 6 Ma Twelve samples from Somerset Island yielded ages ranging from 93 ± 4 Ma to 108 ± 5 Ma and are contemporaneous with other Cretaceous kimberlite magmatism in central Canada (103-94 Ma). Although whole-rock compositions of the kimberlites from southern Africa have a large range of ε_(Nd)(t) values (-0.5 to + 5.1), the analysed perovskites show a more limited range of +1.2 to + 3.1. Perovsldtes from Somerset Island have ε_(Nd)(t) values of - 0.2 to +1.4. These values are lower than that of depleted asthenospheric mantle, suggesting that kimberlites might be derived from the lower mantle. This study shows that in situ U-Pb and Nd isotopic analysis of perovskite by laser ablation is both rapid and economic, and serves as a powerful tool for the determination of the emplacement age and potential source of kimberlite magmas.