WHAT'S DIFFERENT ABOUT HUALALAI, KOHALA AND LOIHI?

New high precision isotope data on Hawaiian basalts indicate the relationship between the structure of the Hawaiian plume and the geographically separated and geochemically distinct Loa and Kea trends is not as simple as the models (e.g. concentric vs. bilaterally zoned plume) would suggest. Both Loa and Kea compositional signatures occur at some volcanoes, regardless of their location on the Kea or Loa side^1,2. We have analyzed samples from three such volcanoes: Hualalai, (6 shield, 2 post-shield), Kohala (18 shield), and Loihi (11 preshield) for Pb-Hf-Nd-Sr-Li isotopes and trace elements. Loihi is the only Hawaiian volcano currently in the preshield stage and is the only Loa trend volcano with Loa-like Pb and Li isotopes but transitional between Kea and Loa types in other isotopes and trace elements³. Kohala, with the most unradiogenic ²⁰⁸Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd of any Kea trend volcano, is the most Loa-like Kea trend volcano. Hualalai has the lowest ²⁰⁶Pb/²⁰⁴Pb at a given ²⁰⁸Pb/²⁰⁴Pb observed in Hawaiian post-shield lavas⁴, and the highest Li/Y ratio and lowest δ⁷Li of any oceanic island basalt⁵.

Hualalai and Kohala were the first subaerial volcanoes to form after a gap in volcanic activity between Maui and the Island of Hawaii. They also erupted during the largest upsurge in magmatic flux of the Hawaiian plume^6,7. Higher magma flux does not necessarily translate into higher degrees of partial melting, as indicated by higher Ba/Y, Nb/Y, and La/Yb for Kohala lavas relative to other Hawaiian shields. In general, Hualalai, Kohala, and Loihi all have intermediate isotope and trace elemental compositions overlapping the Kea-Loa boundary. It is interesting to note that all Island of Hawaii volcanoes are characterized by a juxtaposition of both Kea and Loa trend characteristics in the same volcano, and the most extreme crossover volcanoes occur after a gap in volcanism between islands, i.e. Kauai, West Molokai, Waianae, Mahukona, and Kohala. Approximately 16% of Hawaiian shield samples “crossover” the Loa-Kea divide based on an analysis of radiogenic Pb (N=542).

¹Xu et al. (2014) GCA, 132, 214-327. ²Marske et al. (2007) EPSL, 259, 34-50. ³Jackson et al. (2012) G3, 13(9). ⁴Hanano et al. (2010) G3, 11(1). ⁵Harrison et al. (in press) AGU Mono. ⁶Vidal & Bonneville (2004) JGR 109, B03104. ⁷Robinson & Eakins (2006) JVGR, 151, 309-317.