Tham khảo tài liệu 'earth as an evolving planetary system part 4', khoa học tự nhiên, địa lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Lithosphere 127 Modal Olivine Figure Mg number versus modal olivine in post-Archean mantle xenoliths and ophiolite ultra-mafics and in Archean lithosphere xenoliths from South Africa. Mg number Mg Mg Fe molecular ratio. Modified from Boyd 1989 . the Earth just after planetary accretion Sun and McDonough 1989 . Compared with primitive mantle incompatible element distributions in the mantle lithosphere and depleted mantle are distinct Table . The striking depletion in the most incompatible elements Rb Ba Th etc. in depleted mantle Fig. as represented by ophiolite ultramafics and mantle compositions calculated from ocean-ridge basalts reflects the removal of basaltic liquids enriched in these elements perhaps early in the Earth s history Hofmann 1988 . In striking contrast to depleted mantle lithosphere mantle shows prominent enrichment in the most incompatible elements. Spinel lherzolites from post-Archean lithosphere show a positive Nb-Ta anomaly suggesting that they represent plume material plastered on the bottom of the lithosphere McDonough 1990 . Archean garnet lherzolites commonly show textural and mineralogical evidence for metasomatism . modal metasomatism such as veinlets of amphibole micas and other secondary minerals Waters and Erlank 1988 . The high content of the most incompatible elements in modally metasomatized xenoliths Fig. probably records metasomatic additions Figure Primitive mantle PM normalized incompatible element distributions in subcontinental lithosphere and depleted mantle. Primitive mantle values from Sun and McDonough 1989 data from Nixon et al. 1981 Hawkesworth et al. 1990 Wood 1979 and miscellaneous sources. 128 The Mantle of these elements to the lithosphere. In some cases such as in the xenoliths from Kimberley in South Africa Hawkesworth et al. 1990 these additions occurred after the Archean. Thickness of Continental Lithosphere The continental lithosphere varies considerably in thickness depending on .
