ABSTRACT

The REE abundances in one basalt and two andesites from Oshima-Oshima and -Kojima isles, Hokkaido, Japan, were determined precisely by mass-spectrometric stable isotope dilution technique. The abundance ratios between two distinct volcanic rocks, i.e., andesite (old somma) and basalt (young somma), from Oshima-Oshima isle show a linear function of atomic number, with breaks at Z = 68 and between 63 and 64. This can be interpreted as implying that the andesite and the basalt studied stand in a mutually conjugate relationship as possible, previously concurrent liquid and solid phases. Employing the abundance ratio function as an effective bulk partition coefficient function, the retrogressive fractionation was theoretically followed back by calculation for different F_W* values. The results of calculation suggest that there were two stages concerning the relevant bulk partition coefficient functions for the development of the andesite in question and that the terrace-shaped bulk partition coefficient function with a break at Z = 67 (Ho) worked prior to the tri-segment partition coefficient function obtained directly from the abundance ratio between the andesite and the basalt.