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 radi ogenic than estimates of Bulk Earth (Os-187/Os-188 as low as 0.106) an d require long-term evolution in low Re/Os environments. Using Re depl etion model ages which assume complete Re removal during formation, th e data indicate that cratonic lithosphere stabilisation occurred at, a t least, 3.5 Ga, when the lithosphere was over 200 km thick. This thic k lithosphere persisted into the Phanerozoic to be sampled by kimberli tes. Younger, Proterozoic and Phanerozoic Re depletion ages are interp reted to be largely the result of open system behaviour involving Re a ddition by metasomatic processes. Some of the younger ages may represe nt the addition of new lithospheric material during periods of major i gneous activity. A mid-Archaean age for the Kaapvaal cratonic mantle c oncurs with Archaean Re-Os ages found in samples of Siberian and Wyomi ng cratonic mantle. Both shallow (spinel facies) and deep (diamond fac ies) Kaapvaal peridotites have similar ages (3.3-3.5 Ga) suggesting th at 150 km of mantle lithosphere may have accumulated very rapidly. Os isotope estimates for the timing of separation and stabilisation of Ka apvaal cratonic mantle overlap the main period of cratonic crust build ing and stabilisation (3.5-2.7 Ga). A similar overlap between crust an d mantle stabilisation is evident for the Siberian craton. Archaean li thospheric mantle is compositionally different to that formed post-Arc haean. The Kaapvaal peridotites have very low FeO compared to post-Arc haean peridotites and show a large spread in Mg/Si. Some samples are a nomalously Si-enriched compared with post-Archaean mantle samples. Thi s compositional distinction and the varied Nd-Os isotope systematics a re difficult to explain in terms of accepted models involving ancient melt depletion and subsequent metasomatism. Crystal segregation/cumula te processes have been suggested as a mechanism for forming the compos itional range observed in Kaapvaal peridotites. This type of process m ay have occurred during harzburgite crystallisation from high-degree ( > 50%) mantle melts associated with Archaean plume activity. A role fo r 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 stabilisatio n on the Kaapvaal and Siberian cratons suggests a link between crust g eneration and stabilisation and lithospheric mantle formation in the A rchaean. Thermal energy from the plume may have been the impetus for m ajor crust building at the time of Lithosphere stabilisation, possibly by underplating of basaltic magmas. Direct involvement of mantle plum es in episodes of major mantle and possibly crust differentiation woul d imply that modern style plate tectonics may not have been the primar y mechanism of planetary differentiation in the early Earth. Archaean ages for peridotites originating up to 200 km deep suggest that the me chanical boundary layer beneath continents is at least this thick.