ABSTRACT

Ca, Sr and Ba contents of basalts and andesites from Miyakejima, Onoharajima, Mikurajima and Inambajima volcanoes, the Izu Islands, Japan, have been determined by an inductively coupled plasma optical emission spectrometer. The data reveal that Miyakejima, Onoharajima and Mikurajima volcanoes gave three different series of Sr/Ca-Ba/Ca systematics, while an Inambajima andesite belongs to Mikurajima systematics. The difference is considered to reflect different degrees of partial melting of a common mantle material with chondritic Sr/Ca and Ba/Ca ratios. The degree of partial melting decreases from Oshima to Miyakejima and is constant from Miyakejima to Mikurajima along the arc, while it decreases from Miyakejima to Onoharajima across the arc. Therefore, there seems to be no relationship between the “degree” of partial melting and the distance from the Izu-Ogasawara Trench in this region.