Origin of Hawaiian Tholeiites: Trace Element Constraints

We report here geochemical studies of Hawaiian tholeiites and ultramafic xenoliths from Salt Lake Crater, Oahu. We focus attention on tholeiitic basalts that comprise the bulk of Hawaiian volcanoes. When the samples are screened to include only those lying neat the log-MgO (about 7 percent) end of olivine-control lines (Wright, 1971), tholeiites from individual volcanoes are remarkably uniform. On this basis, we show that, for tholeiites from six volcanoes, systematic geochemical differences exist that cannot be attributed to differentiation of these magmas from a common parental magma. Apparently there have been important differences in the processes of magma generation, source composition, or source mineral constitution. Partial melting calculations based on REE contents emphasize these distinctions, but unique melting models are not presented. In these models, relative REE abundances in the source material is a major uncertainty. Nd isotopic studies of Hawaiian basalts require systematic differences in Sm/Nd for the source material of each volcano. Furthermore, the time-integrated Sm/Nd of the sources must be less than that in chondrites. REE analyses of Hawaiian garnet lherzolite xenoliths show that they have chondritic to light REE-enriched relative abundances with absolute contents (for light REE) about 3 to 8 times chondrites. These data obviously conflict with interpretations of the Nd isotopic data. Several possibilities follow: (1) the available xenoliths are not parental to tholeiite, (2) our simple interpretation of the Nd isotopic data is wrong, and (3) the source regions may have been invaded at geologically recent times by a light REE-enriched phase, in which case the xenoliths may represent the course material. If the xenoliths are characteristic of the source, partial melting calculations indicate that the tholeiites may be generated by 15 to 20 percent melting of garnet lherzolite and at the sane tune conform to constraints imposed by the REE and Ni contents and the partitioning of Fe and Mg between melts and residues. We propose that the primary tholeiitic magmas contain no more than about 12 percent MgO, and that erupted magmas probably fractionated less than 10 to 15 percent of olivine during ascent and storage in high-level chambers