隕石と地球物質の中のランタニド元素の存在度から見たマントルと地殻の生成(<特集>地球及び惑星の内部構造について,研究会報告)

Abstract

The difference in abundances of lanthanides among terrestrial materials, Norton County achondrite, and chondrites was dealt with mathematically and its implication to the development of the mantle and crust was presented. The difference, d, in relative partition coefficient to that for La was estimated to be 0.296 (see Figs.3, 4, and 5) by assuming that Norton County achondrite represents the solid-type material corresponding appoximately to a certain stage of solidifcation and by the use of a least square method. By introducing this value of difference in relative partition coefficient, the abundances (C_A=18.3 and C^O_A=0.32÷0.832, ppm) of La in Minami's shales and in average chondritic oxide phase (the average percentage of non-oxide components in chondrite was taken as 16.8%), and the slope coefficients in chondrite was taken as 16.8%), and the slope coefficient (s=1.17) of Fig.1 into Eq. (12), the partition coefficient k_A of La was evaluated to be 0.121. If the lanthanide abundances in shales determined by Minami (1935) are assumed to approach the average crustal abundances of lanthanides, the thickness of the crust can be calculated to be 34km. This it could be said that the crust and mantle as a whole originated from chondritic oxide melt, the crust represents the final residual liquid, while the mantle represents the aggregate of crystals separated from the liquid, and the composition of mantle is to vary gradually and steadily with the depth.