Benutzer:Christian b219/Kruste

Archaean continental mantle lithosphere is ultra-depleted after komatiite extraction, and it is thus intrinsically less dense than the surrounding asthenosphere. This low density was a controlling factor in its original stabilisation, allowing it to remain both thick and cool enough for diamonds to form at depths below 150 km [1,2,39].

In contrast, it is argued that post-Archaean, i.e. post-komatiite, mantle lithosphere is compositionally similar to that sampled recently as spinel peridotite inclusions in alkali basalts. It stabilised by conductive cooling, and thus it does not need to be compositionally distinct from the asthenosphere to remain convectively isolated. As a result, such post-Archaean lithosphere is fertile enough to contrib- ute significantly to the generation of CFBs, and it is also dense enough to be more readily de- laminated and incorporated into the astheno- sphere source region of oceanic basalts. Combin- ing the available data on mantle xenoliths and continental flood basalts with "old" isotope sig- natures suggests that the continental mantle lithosphere contains a maximum of 10% of the K, and 3.5% of the Sr and Nd in the crust/mantle system.

An alternative is to describe mantle heterogene- ities in terms of enrichment and depletion processes [58]. This seeks to encourage a view of the mantle as a dynamic system in which het- erogeneities may be created and destroyed within relatively short time periods as, for example, within the age of an ocean basin. How long such material survives then depends on the life span of the ocean and the efficiency of mixing. The processes described above generate considerable Rb/Sr and U/Pb fractionation, with the result that signifi- cant 87Sr/86Sr and 2°6pb/2°apb variations can evolve in a few 100 Ma. The same is not true for ta3Nd/144Nd and 2°Tpb/2°4pb, and so they offer better evidence for the introduction of older material. Current interpretations of the isotope ratios in oceanic basalts are still shrouded in uncertainty. However, the view that isotope variations largely reflect deep-seated heterogeneities, often due to recycled crust and mantle lithosphere, may in- creasingly be countered by the notion that enrich- ment processes are likely to develop significant heterogeneities at shallow levels in the Earth's mantle.

C.J. Hawkesworth, P.D. Kempton, N.W. Rogers, R.M. Ellam, P.W. van Calsteren, Continental mantle lithosphere, and shallow level enrichment processes in the Earth's mantle, Earth and Planetary Science Letters, Volume 96, Issues 3–4, January 1990, Pages 256-268, ISSN 0012-821X, 10.1016/0012-821X(90)90006-J. (http://www.sciencedirect.com/science/article/pii/0012821X9090006J)