Volcanism at the Edge of the Hawaiian Plume: Petrogenesis of Submarine Alkalic Lavas from the North Arch Volcanic Field

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Abstract
Submarine lavas erupted onto the Hawaiian arch 200–400 km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises 25 000 km2 of ∼0·5–1·15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe–Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere–plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume.