Time constraints on the formation of lithospheric mantle beneath cratons : a Re-Os isotope and platinum group element study of peridotite xenoliths from northern Canada and Lesotho
Understanding the generation and evolution of cratonic lithospheric keels requires detailed knowledge of their age, timescale of formation, and the relationship between the crust and mantle part of the lithosphere. The Re-Os isotope system has been instrumental in providing an improved understanding of the timing of formation of cratonic lithospheric keels because of the large fractionation of Re from Os during mantle melting and the relative immunity of Os during mantle metasomatism. This study uses combined full PGE (Platinum Group Elements; Os, Ir, Ru, Pt, Pd) and Re-Os isotope analyses to place better constraints on the significance of Re-Os model ages in sub-continental lithospheric mantle suites from northern Canada and Lesotho, southern Africa. When combined with major-element studies, the coherence of major-elements with PGE fractionation trends, (Pd/Ir)(_n9), provide a firm basis for evaluating the significance of T(_RD) and T(_MA) model ages obtained from Re-Os isotope systematics. The fractionation of PGEs track melt depletion and melt enrichment end-member processes, and allow us to discern samples which have experienced Pd and Re enrichment at varying stages. The Re-Os isotope and PGE analyses of peridotite xenoliths from the Jericho kimberlite clearly indicate the presence of Archean mantle beneath the Slave craton (T(_RD) range from 3.1 to 2.6 Ga), extending down into the high-temperature garnet facies. Archean ages are also evident in peridotite xenoliths from Somerset Island in the Churchill Province (oldest T(_RD) ca. 2.8 Ga). This is the first indication of Archean sub continental lithosphere beneath this region that is characterised by early to mid Proterozoic crustal ages. Some peridotites from the Jericho and Somerset Island peridotite suites have noticeably disturbed Re-Os isotope and PGE systematics and produce anomalously young model ages for cratonic lithospheric mantle overall. Many of the young model ages observed at Jericho and Somerset Island are coincident with major tectonomagmatic events manifested in the overlying crust and may represent new lithospheric mantle added at this time. Depleted peridotite xenoliths from Lesotho (Kaapvaal craton) have very tightly clustered T(_RD) model ages (2.8 ± 0.1 Ga, n = 26). The model age of the lithospheric mantle beneath this region is significantly younger than the age of the oldest overlying crust on the Kaapvaal craton, but correlates well with the end of major crustal differentiation and stabilisation of the Kaapvaal craton. This indicates that large fractionation of lithospheric mantle beneath the Kaapvaal craton could have formed in the late-Archean.