The age of continental roots

摘要

Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both periodtite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post=dating the depletion events that led to the formation and isolation of the periodotite from convecting mantle. These signatures document a history of melt- and fluid-rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re-Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystalisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric periodotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the obldest Re depletion model ages are Archean and bradly similar to major crust-forming events. Some locations, such as Premier in soluthern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lighospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that > 150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lighospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orgenic massifs show more systematic behaviour of Re-Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr-Nd-Pb-Hf-Os isotopic studies of massif peridotites and new kimberlite- and basalt-bonrne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.