Pb・Sr同位体とマントルの分化

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  • Fundamental Problems on the Differentiation of the Earth's Mantle as deducd from Sr and Pb Isotopes

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In comparison of the isotopic data obtained by many laboratories, they should be standardized considering the machine bias and the isotopic fractionation during the measurements. It is usually assumed that the isotopic equilibrium is attained and there is no isotopic fractionation of strontium and lead in the geologic processes. However, significant variation is observed in the isotopic composition of strontium in the differentiation process of the Skaergaard magma and that of strontium and lead in some volcanic rocks. It is possible that there may be the isotopic fractionation due to the isotopic disequilibrium during the magmatic differentiation. Care must be exercised in applying the observed abundances of elements in basalts to their source materials. It is necessary to accumulate the data on the partition relations of elements between magma and crystals. Isotopic composition of lead in basalts shows that the upper mantle is quite in homogeneous and the isotopic evolution in many samples can not be explained by the single-closed system model. It is necessary to study the isotopic relations in anceint basaltic rocks of the continental region in order to understand the evolution of the mantle in the light of the isotopic data.

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