Plagioclases from the basalt of Okinawa Trough and its petrogenesis significance

There are three different generations of plagiclases in the basalt, of Okinawa Trough: phenocryst, microphenocryst and groundmass plagioclase. The average size of phenocryst is 100 mum microphenocryst 13 mum+/- and with groundmass plagioclase 2 mum Electron microprobe analysis shows that there are distinctive compositional differences between core and edges of the phenocryst, that is, a higher platform in the core and a lower platform in the edges. The An values of the higher platform in cores vary from 80 to 90, and for the lower platform in edges the An values are between 60 and 70, which well agree with the chemical composition of microphenocryst. Based on the plagioclase geothermometer, the temperatures of phenocryst core and edge were found to be about 1250degreesC and 1050degreesC, respectively. The temperature of groundmass plagioclase is just about 950degreesC. We thus consider that these characteristic features of plagioclases record the dynamic process of the basaltic magma: the core of phenocryst reflecting the features of magma source, while the edges of phenocryst and microphenocryst indicating the features of the magma chamber, and the groundmass plagioclase recording the eruption process.