STABILIZATION OF ARCHEAN LITHOSPHERIC MANTLE - A RE-OS ISOTOPE STUDY OF PERIDOTITE XENOLITHS FROM THE KAAPVAAL CRATON

Os isotopic compositions of lithospheric peridotite xenoliths erupted by kimberlites in the Kaapvaal craton are almost exclusively less radiogenic than estimates of Bulk Earth (Os-187/Os-188 as low as 0.106) and require long-term evolution in low Re/Os environments. Using Re depletion model ages which assume complete Re removal during formation, the data indicate that cratonic lithosphere stabilisation occurred at, at least, 3.5 Ga, when the lithosphere was over 200 km thick. This thick lithosphere persisted into the Phanerozoic to be sampled by kimberlites. Younger, Proterozoic and Phanerozoic Re depletion ages are interpreted to be largely the result of open system behaviour involving Re addition by metasomatic processes. Some of the younger ages may represent the addition of new lithospheric material during periods of major igneous activity. A mid-Archaean age for the Kaapvaal cratonic mantle concurs with Archaean Re-Os ages found in samples of Siberian and Wyoming cratonic mantle. Both shallow (spinel facies) and deep (diamond facies) Kaapvaal peridotites have similar ages (3.3-3.5 Ga) suggesting that 150 km of mantle lithosphere may have accumulated very rapidly. Os isotope estimates for the timing of separation and stabilisation of Kaapvaal cratonic mantle overlap the main period of cratonic crust building and stabilisation (3.5-2.7 Ga). A similar overlap between crust and mantle stabilisation is evident for the Siberian craton. Archaean lithospheric mantle is compositionally different to that formed post-Archaean. The Kaapvaal peridotites have very low FeO compared to post-Archaean peridotites and show a large spread in Mg/Si. Some samples are anomalously Si-enriched compared with post-Archaean mantle samples. This compositional distinction and the varied Nd-Os isotope systematics are difficult to explain in terms of accepted models involving ancient melt depletion and subsequent metasomatism. Crystal segregation/cumulate processes have been suggested as a mechanism for forming the compositional range observed in Kaapvaal peridotites. This type of process may have occurred during harzburgite crystallisation from high-degree (> 50%) mantle melts associated with Archaean plume activity. A role for hot mantle plumes in generating the thick lithospheric keels beneath the Kaapvaal and Siberian cratons is supported by the possibility of their rapid formation and their thermal stability with respect to post-Archaean lithosphere. The coincidence of mid-Archaean cratonic mantle differentiation with periods of major crust building and stabilisation on the Kaapvaal and Siberian cratons suggests a link between crust generation and stabilisation and lithospheric mantle formation in the Archaean. Thermal energy from the plume may have been the impetus for major crust building at the time of Lithosphere stabilisation, possibly by underplating of basaltic magmas. Direct involvement of mantle plumes in episodes of major mantle and possibly crust differentiation would imply that modern style plate tectonics may not have been the primary mechanism of planetary differentiation in the early Earth. Archaean ages for peridotites originating up to 200 km deep suggest that the mechanical boundary layer beneath continents is at least this thick.