Lawrence Berkeley National Laboratory

An Eocene submarine boninite series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types, boninite, bronzite andesite, dacite, quartz dacite, and rhyolite, were distinguished within the boninite volcanic sequence on the basis of petrographic and geochemical observations. Boninite lavas contain high magnesium, nickel, and chromium contents indicative of primitive melts, but have high silica contents relative to other mantle-derived magmas. All boninite series lavas contain very low incompatible element concentrations, and concentrations of high-field strength elements in primitive boninite lavas are less than half of those found in depleted mid-ocean ridge basalts. Abundances of large-ion lithophile elements are relatively high in boninite series lavas, similar to the enrichments observed in many island arc lavas. Trends for both major and trace element data suggest that the more evolved lavas of the boninite magma series were derived primarily through high-level fractional crystallization of boninite. Textural features, such as resorption and glomeroporphyrocrysts, and reverse chemical zonations suggest that magma mixing contributed to the development of the quartz dacite lavas.