Discordant Io-ages and the uranium and thorium distribution between zircon and host rocks

抄録

Io-ages obtained for glass-zircon pairs which were not consistent with expected ages were discussed on the basis of uranium and thorium distribution between zircon and glass. Uranium and thorium distribution between zircon and host rocks from dacites and granites were also discussed. Discordant Io-ages obtained for glass-zircon pairs with normal (Th/U)_zircon/(Th/U)_glass ratio are explained by early stage crysallization of the zircon in the magma. Discordant Io-ages with abnormal (Th/U)_zircon/(Th/U)_glass ratios suggest that the zircon was captured in the magma as xenoryst. The discrepancy between (Th/U)_zircon/(Th/U)_glass ratios (about 0.19) for dacites and (Th/U)_zircon/(Th/U)_{whole rock} ratios (about 0.12) for granites could be explained by crystallization of granitic zircon from liquid having chemical composition different from that of the whole rock of granite.