姫島火山岩のストロンチウム同位体組成 マグマの混合および非平衡なホルンブレンド斑晶

Abstract

Strontium isotope compositions were determined for 1 dacite, 2 low-silica rhyolites, 1 high-silica rhyolite and 2 phenocrystic hornblendes from Hime-shima volcano, Southwest Japan. The whole rocks increase in Rb and 87Sr/86Sr and decrease in Sr with increasing SiO2. The high-silica rhyolite composition can be closely reproduced in terms of Rb, Sr and 87Sr/86Sr by model calculations of assimilation-fractional crystallization of the dacite magma: the material to be assimilated is granodiorite which is presumably spread beneath the volcano. The low-silica rhyolites have compositions close to a mixing line between the dacite and high-silica rhyolite in a Sr-87Sr/86Sr relation diagram, suggesting that they are mixing products of the dacite and high-silica rhyolite magmas. Hornblende phenocryst and its host dacite have virtually identical 87Sr/86Sr. On the other hand, hornblende in the low-silica rhyolite has 87Sr/86Sr between its host rock and the dacite. It is likely that the hornblende was derived from the end-member dacite magma and on the way to be in re-equilibrium with the melt phase of mixed rhyolitic magma when the magma erupted. Because the hornblende in rhyolite shows no sign of re-equilibration with respect to chemical composition, the tracer diffusivility of 87Sr/86Sr is likely to be higher than the chemical diffusivility.