The Kea- and Loa- trends and magma genesis in the Hawaiian mantle plume

The Hawaiian-Emperor volcanic island and seamount chain has been created by a hot mantle plume located beneath the Pacific lithosphere. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear parallel trends, termed _eKea_Eand _eLoa_E(Jackson et al., 1972). Lavas from these two trends are commonly believed to have different geochemical characteristics (Tatsumoto, 1978; Frey et al., 1994; Hauri, 1996; Lassiter et al., 1996; Abouchami et al., 2005). The Kea- and Loa- geochemical trends within the Hawaiian shield volcanoes have been interpreted to reflect melting above a compositionally concentrically zoned (Hauri, 1996; Lassiter et al., 1996; Kurz et al., 1996; DePaolo et al., 2001) or compositionally left-right asymmetrically zoned mantle plume (Abouchami et al., 2005). In order to evaluate the homogeneity of the mantle plume source sampled by the Kea- and Loa- trends, we analyzed major and trace element compositions of olivine-hosted melt inclusions from Hawaiian shield lavas, using EPMA and Laser ICP-MS. We selected lava samples form submarine Hana Ridge, Haleakala volcano (Kea trend) and submarine exposures of the Makapuu stage, Koolau volcano (Loa trend), respectively. We found both Kea- and Loa-like major and trace element compositions from olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes, even within single rock samples. We infer from these data that although one mantle source component may dominate a single lava flow, the two (or more) mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. On the basis of whole rock geochemical characteristics (Ren et al., J. pet., 2004; 2005) combined with the melt inclusion data (Ren et al., 2005, Nature), we propose a Hawaiian mantle plume characterized by more random heterogeneity than would be present in a simple compositionally zoned mantle plume. The geochemical differences in the shield volcanoes likely reflect different mixing proportions of subducted recycled gabbroic oceanic crust and peridotite from the lower mantle. The dominant component sampled at a given shield volcano is likely controlled by the thermal structure. As the volcano grows, it migrates away from the hot plume axis with plate motion. In the plume core, higher temperatures are able to generate melts with Kea-like composition from the more refractory component (i.e. peridotite from lower mantle) during the early shield stages. In contrast, lavas from some of the late-stages of the Hawaiian shields are Loa-like with isotopically enriched characteristics, implying that the proportion of the subducted oceanic crust (eclogite) component contributing to the melt may be higher relative to the peridotitic matrix. This is because during the generation of the later-stage lavas, their mantle source is located significantly away from the mantle plume axis where the temperature is lower than in the center of plume, and therefore the lower melting point component (eclogite) is preferentially sampled by the melt.