Lithospheric mantle formation

Carlson, R. W., Pearson, D. G., Boyd, F. R., Shirey, S. B., Irvine, G., Menzies, A. H. Gurney, J. J. 1999. Regional age variation of the southern African mantle significance for models of lithospheric mantle formation. In Gurney, J. J., Gurney, J. L., Pascoe, M. D. Richardson, S. H. (eds) Proceedings of the 7 th International Kimberlite Conference, Cape Town. Red Roof Design, Cape Town, 99-108. 

Downes H (2001) Formation and modification of the shallow sub-continental lithospheric mantle a review of geochemical evidence from ultramafic xenolith suites and tectonically emplaced ultramafic massifs of Western and Central Europe. J Petrol 42 233-250... 

The wide diversity of the isotopic compositions in orogenic peridotites, and the anomalously depleted composition of several ophiolitic and abyssal peridotites have strong implications on the small-scale structure of the convective mantle, as well as on mantle processes such as decompression partial melting of mantle rocks, the formation of oceanic lithosphere and the thermomechanical and chemical erosion of lithospheric mantle by upwelling asthenosphere. We briefly review some of these important issues below. 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

Irvine G. J. (2002) Time constraints on the formation of lithospheric mantle beneath cratons a Re—Os isotope and Platinum Group Element study of peridotite xenoliths from Northern Canada and Lesotho, PhD Thesis, University of Durham. 

Archean emplacement of eclogitic components into the lithospheric mantle during formation of the Kaapvaal craton. Geophys. Res. Lett. 28, 2509-2512. 

Shirey, S. B., Carlson, R. W., Richardson, S. H. 5 others 2001. Emplacement of eclogite components into the lithospheric mantle during craton formation. Geophysical Research Letters, 28(13), 2509-25(2. 

Subsequent to the differentiation of the lithosphere, i.e., crust and mantle formation, and the cooling that promote formation of the hydrosphere, tectonic cycles and abiotic weathering are considered to have contributed to temperature regulation of prebiotic Earth. 

A potentially significant reservoir, and one which a number of authors have suggested is important in the context of continent formation, is the subcontinental lithospheric mantle (SCLM). Kramers (1987, 1988), suggested that the TTG magmas of the Archaean crust formed in an open-system magma layer in the early Earth, the cumulates from which are now preserved as the SCLM. More recently Abbott et al. (2000) proposed a model of continental growth founded upon the premise that the continental crust was extracted from the SCLM. 

The outer shell of the earth, consisting of the upper mantle and the crust (Figure I4. lO), is formed of a number of rigid plates. These plates are 20 in number and are shown in Figure 14.1 I. Of these, six or seven are major plates, as can be seen in the map. The edges of these plates define their boundaries and the arrows indicate the direction of their movement. These plates contain the continents, oceans and mountains. They almost float on the partially molten rock and metal of the mantle. The outer shell, known as the lithosphere, is about 70 to 1,50 km thick. It has already moved great distances below the etirth s surface, ever since the earth was formed and is believed to be in slow and continuous motion all the time. The plates slide on the molten mantle and move about lO to 100 mm a year in the direction shown by the arrows. The movement of plates is believed to be the cause of continental drifts, the formation of ocean basins and mountains and also the consequent earthquakes and volcanic eruptions. 

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