Significance of iron and cobalt partitioning between plagioclase and biotite for problems concerning the Eu2+/Eu3+ ratio, europium anomaly, and magnetite-/ilmenite-series designation for granitic rocks from the Inner Zone of southwestern Japan

Abstract

Abundances of REEs and other trace elements were determined for granitic rocks and their mineral separates from the Sanin, Sanyo, and Ryoke belts in the Inner Zone of southwest Japan. Among those data obtained, the Fe and Co partition coefficients between plagioclase and biotite were found to be specially important. The partition coefficients for Fe are negatively correlated with those for Co, and values for the magnetite-series granites from Sanin belt are clearly separated from those for the ilmenite-series granites of Ryoke belt. The Sanyo belt samples, which include both types of granites, fall within the range of the Ryoke ilmenite-series granites. The variation of Fe and Co partition coefficients can be interpreted by the difference in redox states, crystal chemistry of these minerals, and kinetic disequilibrium effect. The Sanyo belt data suggest that the ilmenite-series granites were converted from a magma with magnetite-series affinity during later solidification stages. Moreover, the Eu2+/Eu3+ ratios in minerals were calculated by the Philpotts' method in which the equivalence of Eu2+ and Sr2+ partitioning is assumed. The Eu2+/Eu3+ ratio in plagioclase from the Sanyo and Ryoke belt granites seems to be correlated with the whole-rock Fe2+/Fe3+, though the Sanin belt granites tend to show high plagioclase Eu2+/Eu3+ ratios probably due to a disequilibration caused by rapid solidification. Furthermore, effect of redox states on the size of the Eu anomaly in plagioclase/melt partition coefficient was examined from the plagioclase Eu2+/Eu3+ data of the granites.