title: The evolution of mantle mixing.
abstract: We present a geochemical overview of the canonic model that suggests a two-layer mantle for most of the Earth's history. A change in the Rayleigh number may have modified the convection and now allows the subduction of oceanic plates into the lower mantle, which was not the case in the past. The measurement of stirring time in the source of mid-ocean-ridge basalt, together with Xe- and Pb-isotopic ratios in the mid-ocean-ridge-basalt source, suggests that the upper mantle is separated into two domains, one above the 400 km discontinuity (asthenosphere) with rapid mixing and short residence time, and another between the 400 and 670 km discontinuities with sluggish mixing and a residence time of ca. 1.5 x 10(9) yr.
authors.affiliation: Laboratoire de Géochimie and Cosmochimie, Institut de Physique du Globe de Paris, Boite 89, 4 place Jussieu, 75252 Paris cedex 05, France. llegre@ipgp.jussieu.fr
authors.forename: Claude J
authors.initials: CJ
authors.lastname: Allègre
mesh_terms: D018534:Earth, Planet; D019418:Evolution, Chemical; D019419:Evolution, Planetary; D019015:Geologic Sediments / Q000032:analysis; D005844:Geology / Q000379:methods*; D007554:Isotopes / Q000032:analysis; D008962:Models, Theoretical; D005741:Noble Gases / Q000032:analysis*; D009792:Oceans and Seas; D012212:Rheology / Q000379:methods; D012680:Sensitivity and Specificity
chemical_list: D007554:Isotopes; D005741:Noble Gases