WHAT DO HAWAII AND KERGUELEN HAVE IN COMMON?

Hawaii and Kerguelen have both been studied extensively by Fred Frey and, together with Iceland, they are the three largest oceanic islands. Kerguelen has evolved from a ridge-centered position at ~35 Ma to an intraplate position today, in a setting comparable to Hawaii. Both Hawaii and Kerguelen have enriched mantle “EM-1” signatures. When projected to the core-mantle boundary (CMB), Hawaii and Pitcairn overlie the edges of the tall steep-sided Pacific large-low-shear-velocity-province (LLSVP), whereas Kerguelen and Tristan are located on the eastern and western edges, respectively, of the African LLSVP. At Hawaii, the ultra-low velocity zone at the CMB is proposed to be the repository for enriched components in the mantle that are responsible for the Loa-side geochemical enrichment [1]. For the Kerguelen Archipelago, the enriched component dominates the chemistry of the alkalic basalts (25-24 Ma), whereas the older (28-26 Ma) tholeiitic-transitional basalts contain also a depleted Southeast Indian Ridge component when the plume was close to the ridge. In isotope plots, Kerguelen compositions form sub-parallel trends that are distinctly more enriched than those from Hawaii. Kerguelen and Tristan carry the strongest enriched signature (DUPAL anomaly), whereas Pitcairn (although very different in Pb isotopes) and Hawaii-Loa have a distinct, slightly less pronounced enriched signature. We infer that some of the material constituting the LLSVP at the base of the mantle is distinct in the African and Pacific anomalies. The ambient deep mantle is also different beneath the Indian Ocean than the Pacific Ocean where the latter is sampled by Kea trend volcanoes in Hawaii [2]. In Hawaii, the EM-1 signature is traced back for at least 5 Ma and an ongoing study along the Hawaiian Ridge indicates that it may be present as early as 45 Ma [3]. In Kerguelen, it can be traced back until 34 Ma on the archipelago and Northern Kerguelen Plateau, and until 82 Ma along the Ninetyeast Ridge [4]. These isotopic signals indicate that LLSVP are long-lived features of the deep mantle and play a significant role in the geochemical signature of strong mantle plumes.

[1] Weis et al (2011) Nature Geosci 4, 831-8; [2] Nobre Silva et al (2013) G-Cubed 14, 659-76; [3] Harrison et al (2014) Goldschmidt Conf #922. [4] Nobre Silva et al (2013) J Pet 54, 1177-1210.