Petrochemistry, Age and Isotopic Composition of Alkali Basalts from Ponape Island, Western Pacific

Eleven analyzed lava samples from Ponape Island are alkali olivine basalt, basanite and basanitoid. Most lavas are aphyric or sparsely phyric (\u3c 10% phenocrysts) and have phenocrysts of olivine (Fo77–80), clinopyroxene and titanomagnetite, and microphenocrysts of plagioclase (An53–68) in a fine-grained groundmass of olivine, clinopyroxene, plagioclase, opaques, potassic oligoclase, ± nepheline and accessary phases. Oxygen isotope and Fe2O3FeO data suggest that most samples are fresh, although H2O contents are high. Xenoliths of chromite-bearing harzburgites and dunites, both with cumulate textures occur in one locality. Major- and trace-element concentrations are similar to other oceanic volcanic islands. Most major elements and compatible trace elements vary systematically with respect to the Mg number [100Mg(Mg + Fe2+)]. In contrast, the incompatible trace elements do not correlate with the Mg number, but do covary with other incompatible elements. Simple closed-system shallow fractionation cannot be invoked to explain the observed chemical variation in the lavas. Derivation of the fractionated lavas (Mg number = 66-48) probably involved polybaric crystal fractionation from a high-Mg-number parental liquid. In addition, variable-source concentration of a trace-element-rich minor phase is postulated. However, the mantle was homogeneous with respect to the ratio of 87Sr86Sr. New KAr age data are not consistent with the hypothesis that Ponape and the Caroline Ridge represent a simple “hot spot”