Petrology and Geochemistry of Volcanic Rocks from the South Kaua'i Swell Volcano, Hawai'i: Implications for the Lithology and Composition of the Hawaiian Mantle Plume

The South Kaua`i Swell (SKS) volcano was sampled during four JASON dives and three dredge hauls recovering rocks that range from fresh pillow lavas to altered volcanic breccias. Two geochemical groups were identified: shield-stage tholeiites (5 center dot 4-3 center dot 9 Ma) and rejuvenation-stage alkalic lavas (1 center dot 9-0 center dot 1 Ma). The young SKS ages and the coeval rejuvenated volcanism along a 400 km segment of the Hawaiian Islands (Maui to Ni`ihau) are inconsistent with the timing and duration predictions by the flexure and secondary plume melting models for renewed volcanism. The SKS tholeiites are geochemically heterogeneous but similar to lavas from nearby Kaua`i, Ni`ihau and Wai`anae volcanoes, indicating that their source regions within the Hawaiian mantle plume sampled a well-mixed zone. Most SKS tholeiitic lavas exhibit radiogenic Pb isotope ratios (Pb-208*/Pb-206*) that are characteristic of Loa compositions (> 0 center dot 9475), consistent with the volcano's location on the west side of the Hawaiian Islands. These results document the existence of the Loa component within the Hawaiian mantle plume prior to 5 Ma. Loa trend volcanoes are thought to have a major pyroxenite component in their source. Calculations of the pyroxenitic component in the parental melts for SKS tholeiites using high-precision olivine analyses and modeling of trace element ratios indicate a large pyroxenite proportion (a parts per thousand yen50%), which was predicted by recent numerical models. Rejuvenation-stage lavas were also found to have a significant pyroxenite component based on olivine analyses (40-60%). The abundance of pyroxenite in the source for SKS lavas may be the cause of this volcano's extended period of magmatism (> 5 Myr). The broad distribution of the Loa component in the northern Hawaiian Island lavas coincides with the start of a dramatic magma flux increase (300%) along the Hawaiian Chain, which may reflect a major structural change in the source of the Hawaiian mantle plume.