THE HAWAIIAN MANTLE PLUME OVER >81 MILLION YEARS: EVIDENCE FOR MULTIPLE DEPLETED SOURCES

Large igneous provinces that erupt in oceanic settings provide the opportunity to study the deep mantle source removed from continental sources of contamination and, for long-lived systems, the evolution of mantle sources with time. In the case of the Hawaiian-Emperor (HE) chain, the sourcing of magmatic material by a long-lived (~81 Myr), deep-sourced mantle plume allows for insight into how the geochemical components sampled by a plume change with time. The HE chain consists of 3 segments: the Emperor seamounts (>50 Ma), the Northwest Hawaiian Ridge (NWHR; 47-7 Ma), and the Hawaiian Islands (<6 Ma). This research presents Pb-Hf-Nd-Sr isotope compositions and trace element concentrations of 4 rejuvenated and 19 post shield NWHR basalts. Study of the spatial distribution and composition of post shield and rejuvenated basalts rather than the shield stage turns the focus to depleted components that contribute to Hawaiian volcanism. The samples identified as rejuvenated here (i.e., alkalic, silica undersaturated, depleted isotope signature and enriched in incompatible trace elements) are the oldest “rejuvenated” lavas observed on the Hawaiian-Emperor chain. No variation was found in the geochemistry of these rocks along the NWHR, which indicates this source component was apparently homogeneous for ~13 Myr. The high Hf isotope ratios of NWHR rejuvenated basalts suggest a source component such as ultra-depleted oceanic lithosphere that has undergone an ancient partial melt event that separated Sm from residual Lu. The composition of Meiji, Detroit, and Suiko Emperor Seamount basalts are more depleted in both isotopes and trace elements than all other Hawaiian magmas younger than the bend in the HE chain, including NWHR rejuvenated-type samples analyzed here and those from the Hawaiian Islands. This suggests that there are two different sources of depleted compositions in Hawaiian magmas, one resulting from old, ultra-depleted oceanic lithosphere (NWHR and Hawaiian Island rejuvenated lavas) and the other from interaction with a mid-ocean ridge (the oldest Emperor Seamounts). Increasing the sampling resolution from the NWHR will further constrain the formation mechanism and geochemical source of the enigmatic rejuvenated volcanic stage.