Evolution of the lithosphere and its interaction with the underlying mantle as inferred from noble gas isotopes

Abstract

Continental lithosphere is generally thicker than oceanic lithosphere and seems to have experienced different evolutionary processes in time and space. Based on studies of noble gas isotopes for ultramafic inclusions and related rocks, the evolution of the lithosphere and the interaction with the underlying mantle have been examined. In the 3He/4He‐40Ar/36Ar diagram, samples from oceanic areas indicate relatively high 3He/4He and 40Ar/36Ar ratios, which are similar to those of MORB. Those from the continental areas indicate similar or lower 3He/4HE ratios and generally lower 40Ar/36Ar ratios. The differing ratios appear to reflect differences in the average age and evolution of oceanic and continental lithosphere. Noble gas isotope signatures of ultramafic nodules in South African kimberlites imply that at least three isotopically different source materials might have contributed to the formation of the African lithosphere. The apparent differences in the variation of 3He/4He ratios with time among the Hawaiian, Icelandic and Reunion islands may be related to the speed of the moving plate and the supply rate of magmas from each hot spot. However, oceanic lithosphere seems to have formed with continuous addition of materials that are isotopically similar to those of MORB and probably came from the asthenosphere. Thus, noble gas isotopes suggest that the formation processes of the continental lithosphere seem to be more complicated than those of the oceanic lithosphere.