Crystallization trend of chromian spinel in some alkali basalts and calc-alkali andesites from Japanese island (arc?).

Abstract

Chromian spinels in the alkali basalts from the Misasa district, W Honshu, and in the calc-alkali andesites from the Chokai volcano, NE Honshu, have been analysed, as well as their host rocks. These spinels are the most aluminous of those yet reported from the island-arc volcanic rocks except those from xenoliths; the Cr/(Cr + Al) ratio ranges 0.255-0.592 and 0.173-0.668 in the Misasa spinel and the Chokai spinel, respectively. The crystallization trends of the spinels show increasing Fe3+ content with increase in the Cr/(Cr + Al) ratio. On the basis of these data and those previously published it is concluded that the composition of spinel is controlled by the chemistry of the host magma as well as P, and therefore the Al2O3 content of chromian spinel is rather inadequate as an indicator of depth of crystallization of magma at early stages in the different provinces. -M.Ak.