Pb isotope variations in Archaean time and possible links to the sources of certain Mesozoic-Recent basalts

Abstract

AbstractThe present-day Pb isotope ratios of mafic and felsic rocks, feldspars and ore samples (galenas) from Archaean cratons ranging in age from 3.8–3.7 Ga to 2.6–2.7 Ga have been used to investigate their initial 207Pb/204Pb ratios, and μ1 ratios (source U/Pb ratios). Two broad trends of initial 207Pb/204Pb evolution are observed, with samples from Greenland, Abitibi and SW India tending to have lower initial 207Pb/204Pb ratios through time than those from eastern India, Australia and southern Africa (Kaapvaal and Zimbabwe). The calculated μ1 values are in the range of 7–9 and they tend to be constant for a given craton through time, with Zimbabwe as a notable exception, for both mafic and felsic rock types. It follows that each craton is characterized by specific μ1 values, or initial 207Pb/204Pb ratios; μ1 for Greenland is 7.37 ± 0.27, and they are higher for Abitibi at 7.57 ± 0.13, which are similar to SW India, where μ1 = 7.57 ± 0.13. In Scotland μ1 = 7.94 ± 0.152, in Finland μ1 is 8.03 ± 0.08, in eastern India it is 8.03 ± 0.1, and for Australia μ1 = 8.07 ± 0.14, and there tend to be higher values of 8 ± 0.2 and 8.32 ± 0.29 in the Kaapvaal and Zimbabwe, respectively. Overall, there is therefore a strong provinciality of μ1 values in the Archaean areas, and unexpectedly the high μ1 Archaean terranes are close to the areas of high μ1 Mesozoic continental flood basalts (CFB), and the DUPAL anomaly in recent oceanic basalts. The cratons with high μ1 values are in the southern hemisphere near the 130–190 Ma CFB from the Gondwana supercontinent, and the maximum DUPAL anomalies at latitudes 30–45°S south of Africa and 0–15°S south of India. The variations in μ1 are based on 207Pb/204Pb ratios, and so they primarily reflect events that took place early in Earth history. In one model, the high μ1 values were initially generated at the core-mantle boundary, through processes linked to core formation and late accretion events, and they were mobilized in plumes responsible for at least some of the early Archaean mafic crust. Preservation of domains characterized by different U/Pb ratios is inferred to have been at relatively shallow levels, as they are preserved in both felsic and mafic rocks, and they appear to have survived for long periods of time, despite the effects of mantle convection and plate motion. Recycling of early Archaean lithosphere in the mantle by delamination might perpetuate these anomalies through time in a given area, perhaps even contributing to the modern DUPAL Pb anomalies. Delamination rather than subduction as the process of lithosphere recycling might explain why such regional domains are not observed in the kappa (κ1: source Th/U) ratios of these Archaean samples.