Source and Mechanism of Archean Lithosphere Formation

Chemical analyses and Re-Os model ages have been obtained for approximately 140 peridotite xenoliths from on and off craton kimberlites widely distributed across southern Africa. Peridotites erupted through Proterozoic basement have slightly lower Mg/Fe and display no Archean Re-Os model ages compared to peridotites from on-craton kimberlites that generally are very Fe, Ca, and Al depleted with Re-Os model ages > 2.5 Ga. The one exception on craton is the Premier kimberlite where magmatism associated with formation of the Bushveld intrusion at 2.05 Ga introduced Proterozoic peridotite into the underlying mantle. The mean Re-depletion and Re-Os model ages for xenoliths from all on-craton localities, excluding Premier, are 2.59 +/- 0.49 Ga and 3.9 +/- 4.2 Ga, respectively. The large error on the Re-Os model age reflects recent Re addition to some samples due to host kimberlite contamination or metasomatism in the mantle. Excluding samples with 187Re/188Os > 0.1 (approximately half of the data set) gives mean Re-depletion and Re-Os model ages of 2.69 +/- 0.23 Ga and 2.99 +/- 0.25 Ga, respectively. Both ages are younger than the generally > 3.2 Ga age of the overlying crust. Re-Os model ages usually are calculated relative to a mantle evolving with chondritic Re/Os (present day 187Os/188Os = 0.1287; Meisel et al., Nature 383, 1996). If instead, the present day 187Os/188Os of the model mantle is 0.132, as seen in many modern ocean island basalts, the mean Re-Os model age of the low Re/Os southern African peridotites increases to 3.15 +/- 0.23 Ga. This result requires that: 1) disturbance of the Re-Os system during metasomatism and/or alteration is not negligible even for low Re/Os samples, 2) depletion of the lithospheric mantle occurred roughly 200 Myr after crust formation or, 3) the Kaapvaal lithosphere formed from mantle more like the enriched source of modern ocean island basalts rather than primitive mantle or depleted MORB mantle.