Magmatic processes of Unzen volcano revealed by excess argon distribution in zero-age plagioclase phenocrysts

摘要

Excess argon (~(40)Ar) is one of the most important problems in K-Ar geochronometry, because it renders incorrect the simple assumption that all initial argon in rock samples is derived from the atmosphere. Using the laser-microprobe analytical method we investigate and reveal the source and trapping site of excess argon in zero-age plagioclase phenocrysts extracted from the lavas erupted during 1990-1995 activity of the Unzen volcano, Kyushu, Japan. By combining the results of laser-microprobe analysis and conventional stepwise heating of separated plagioclase phenocrysts, we reveal that the majority of the excess argon is retained in melt inclusions in the dusty zone of the phenocrysts. We propose a model for the isotopic evolution of argon in the phenocryst-bearing magma of the Unzen volcano by taking into consideration the noble gas data from Unzen volcanic gases and the chemical composition of the melt inclusions in the phenocrysts. In our model, plagioclase phenocrysts crystallize from, or are captured by a low-temperature (T) rhyodacitic magma. This magma has a comparatively high ~(40)Ar/~(36)Ar ratio for an arc magma due to assimilation of old Cretaceous granitic wall rock. Mantle-derived argon is replenished in the rhyodacitic magma over the life of the volcano when the magma chamber is recharged at intervals by high-T juvenile magma. The plagioclase phenocrysts crystallize rapidly, trapping excess argon (~(40)Ar/~(36)Ar>970) during successive dissolution/crystallization events caused by intrusion of the high-T magma or water-rich fluids. The phenocryst-bearing mixed magma ascends at a relatively rapid rate following a final injection of the high-T magma, and interacts with shallow groundwater causing the ~(40)Ar/~(36)Ar ratio of the magma to decrease to the atmospheric ratio. Contamination of the phenocrysts by atmospheric argon in the surrounding magma is enhanced by rapid "short-circuit" diffusion in which dislocations and/or micro-cracks in the plagioclase crystals are used for argon transport. The remnant magma has been continuously degassing for a decade since the recent (1990-1995) eruptive activity, without significant change to ~3He/~4He and ~(40)Ar/~(36)Ar ratios.