The age of continental roots

Determination of the age of the mantle part of continental roots is essenti al to our understanding of the evolution and stability of continents. Datin g 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 th at 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 peridoti te xenoliths and inclusions in diamonds, requiring isolation of cratonic ro ots from the convecting mantle for billions of years. The enriched Nd and S r isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt- and f luid-rock interaction within the lithospheric mantle. In some suites of cra tonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re-Os isotope system is wall suited to dating lithospheric peridotites because of the compatible nature of Os and its rel ative immunity to post-crystallisation disturbance compared with highly inc ompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evoluti on in low Re/Os environments, probably as melt residues. Peridotite xenolit hs from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. S ome locations, such as Premier in southern 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 lithospher ic mantle roots. Few localities studied show any clear variation of age wit h depth of derivation, indicating that >150 km of lithosphere may have form ed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model a ges, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coup led since formation in these areas, for billions of years, despite continen tal drift. Orogenic massifs show more systematic behaviour of Re-Os isotope s, where correlations between Os isotopic composition and S or Re content y ield 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-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their su bsequent evolution. (C) 1999 Elsevier Science B.V. All rights reserved.