Noble gas systematics of submarine alkalic lavas near the Hawaiian hotspot

Abstract

Noble gas isotopic ratios were determined for submarine alkalic volcanic rocks distributed around the Hawaiian islands to constrain the origin of such alkalic volcanism and hence understand the details of mantle upwelling beneath Hawaii. Samples were collected by dredging or using submersibles from the Kauai Channel between Oahu and Kauai, north of Molokai, northwest of Niihau, Southwest Oahu, South Arch, and North Arch volcanic fields. Sites located downstream from the center of the hotspot have 3He/ 4He ratios close to MORB at about 8 Ra (Ra; atmospheric ratio). North Arch samples have neon isotope ratios that lie on the MORB array in a 21Ne/ 22Ne– 20Ne/ 22Ne diagram. The noble gas isotope evidence demonstrates that the magmas erupted at these sites had minimum contribution of volatiles from a mantle plume. In contrast, South Arch located upstream of the hotspot on the Hawaiian Arch has 3He/ 4He ratios between 17 and 21 Ra, indicating a strong plume influence. Differences in noble gas isotopic characteristics between alkalic volcanism downstream and upstream of the hotspot imply that upstream volcanism contains incipient melts from an upwelling mantle plume, having primitive 3He/ 4He. In combination with lithophile element isotopic data, we conclude that the most likely source of the upstream magmatism is depleted asthenospheric mantle that has been metasomatised by incipient melt from a mantle plume. After major melt extraction from the mantle plume during production of magmas for the shield stage, the plume material is highly depleted in noble gases and moderately depleted in lithophile elements. Partial melting of the depleted mantle impregnated by melts derived from this volatile depleted plume source may explain the isotopic characteristics of the downstream alkalic magmatism. Lavas from the Southwest Oahu volcanic field have intermediate 3He/ 4He ratios of about 10 Ra, which suggests that some melt was fed to the source region of the volcano from a mantle plume, in contrast with downstream side alkalic volcanism.