ABSTRACT

The usefulness of noble gas isotope systematics for understanding the volcanic systems is demonstrated in the case of Unzen Volcano, Japan. Noble gas concentrations and Ar isotope ratios were determined for coexisting plagioclase, hornblende and biotite in two dacitic volcanic rocks. Different species of minerals coexisting in a magma show different ⁴⁰Ar/³⁶Ar, and the ⁴⁰Ar/³⁶Ar is correlated to the grain sizes of the phenocrysts, indicating that they had crystallized in an evolving system of the magma. The variation of ⁴⁰Ar/³⁶Ar observed among samples can be explained by a two-component mixing: magmatic Ar and atmosphere-derived Ar. In order to explain the variation of ⁴⁰Ar/³⁶Ar, we propose a magmatic model in which the ⁴⁰Ar/³⁶Ar of magma had gradually decreased due to addition of atmosphere-derived Ar and, in the same sequence of time, phenocrysts had crystallized one after another as cooling of the magma had proceeded. Groundwater is the most probable candidate as carrier of atmosphere-derived Ar although crustal contamination is not completely excluded.