HF ISOTOPE SYSTEMATICS OF REJUVENATED HAWAIIAN MAGMAS

Rejuvenated stage lavas represent the final stage of Hawaiian volcanism, following the shield, and post-shield stages after a hiatus of 1-2 Ma. These alkalic lavas have more depleted isotopic compositions compared to previous stages [1] (⁸⁷Sr/⁸⁶Sr = 0.7032 instead of 0.7037) and plot outside of the mixing hyperbola described by the shield tholeiites in Pb-Hf or Nd-Pb isotopic space [2], implying a change in the magma source between shield and rejuvenated stage, as the volcano moves away from the center of the plume.

We present new Hf isotope data on 34 previously characterized [3] submarine and subaerial lavas from the islands of Kauai and Niihau (all stages), and Hf-Nd-Sr-Pb measurements on twelve submarine rejuvenated lavas from the North Arch chain, NE of Kauai.

In εNd-εHf space, all rejuvenated lavas from Niihau, Kauai, Kaula, Oahu, Maui, Molokai, North Arch have more radiogenic composition than shield stage lavas and converge around εHf ~14 +/- 1 and εNd ~ 6 to 9, with North Arch showing the most radiogenic values at εNd ~9. Kauai lavas from the shield, post-shield and rejuvenated stages form a continuous array with a slope of ~1.3, between KEA type compositions and a depleted endmember similar to North Arch sources. Conversely, Niihau rejuvenated lavas have more radiogenic εHf (~14 vs. 11) for a given εNd (6-9) than the shield building stages (shield, late-shield, post-shield) which overlap KEA type shield lavas [2]. This offset points to a shift in the source composition late in the volcanism. This relatively radiogenic Hf composition has been recognized previously in pyroxenites from Oahu and Kaula, and peridotites xenoliths from Oahu, and has been attributed to a recycled lithosphere component integral to the Hawaiian hot spot [1]. There is no systematic shift in Hf isotopes of rejuvenated magmas across the Hawaiian swell from Kaula Island to the North Arch field for over 350 km, implying a relatively homogenous rejuvenated source over a large distance, and with distance from the plume center.

[1] Bizimis et al. (2013) G³ 14, 4458-4478. [2] Blichert-Toft et al. (1999) Science 285, 879-882. [3] Cousens & Clague (2015) J. of Pet., egv045.