Physics
Scientific paper
Mar 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992izpse..27..682t&link_type=abstract
Izv. Acad. Sci. USSR, Phys. Solid Earth, Vol. 27, No. 8, p. 682 - 696
Physics
Scientific paper
Experimental data on the geochemistry of U-Pb, Sm-Nd, Rb-Sr, U-He, and K-Ar isotope systems are analyzed. A model of their evolution is described. Computer simulations of isotope-geochemical and evolutionary parameters are reported. Geochemical data on radiogenic isotopes provide evidence of the participation of the mantle and the upper and lower continental and oceanic crust in fractionation, mixing, and degassing of matter. Qualitative analysis of such data suggests early intensive fractionation/degassing and the importance of material cycles. Modern transport models describing the evolution of isotope systems postulate the following processes of mass transport and atom transport between the above-mentioned reservoirs: formation of the oceanic crust via fractionation, degassing, and contamination of the mantle matter; crustal subduction; partial melting of subducted material; formation of the continental crust from melts and their degassing/contamination; intracrustal magmatism and formation of the upper crust; and crust destruction/recycling. A match between experimental data and simulation results is obtained assuming extremely early and intensive fractionation of matter and degassing. The timeframe for these processes does not exceed 100 million years, i.e., close to the Earth's accretion range. Mass flows supporting early degassing attained 1019g/year. The continental crust and the complementary mantle reservoir were largely formed by the time 4.0 billion years ago. The lifetimes of the most incompatible elements on the mantle and the continental crust were 0.1 and 4.0 billion years, respectively. The proportion of radiogenic and stable isotopes in the mantle are largely determined by recycling of matter rather than by accumulation of radiogenic isotopes in situ.
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