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

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Eleven analyzed lava samples from Ponape Island are alkali olivine basalt, basanite and basanitoid. Most lavas are aphyric or sparsely phyric (< 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 KAr age data are not consistent with the hypothesis that Ponape and the Caroline Ridge represent a simple “hot spot”.

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Chemical Geology, v. 43, issue 1, p. 1-28