Hawaiian postshield volcanism over the past 55 million years

Abstract

The Hawaiian-Emperor chain, the surface expression of the Hawaiian mantle plume which has been active for at least 81 Ma, is divided into the Emperor Seamounts (81–47 Ma), Northwest Hawaiian Ridge (NWHR, 47–6 Ma), and Hawaiian Islands (< 6 Ma). Hawaiian volcanism evolves through four volcanic stages: alkalic preshield, tholeiitic shield (80–90% volcano volume), alkalic postshield (∼1%), and silica undersaturated rejuvenated (< 0.1%). Here, the elemental and isotopic compositions (Pb–Hf–Nd–Sr) of postshield basalts from 13 NWHR and one young Emperor Seamount volcanoes (~ 8.5 to 55 Ma) are investigated. NWHR postshield basalts tend to have more depleted isotopic signatures than shield basalts at the same volcano and exhibit Loa and Kea geochemical affinity identified in the shield stage. The seamounts Unnamed (33 Ma), Academician Berg (32 Ma), and Townsend Cromwell (~ 34 Ma) have the lowest ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf of the NWHR. This indicates a larger proportion of an enriched component in the source of these NWHR volcanoes compared to the younger volcanoes, likely tied to lower degrees of partial melting. The Loa-trend Daikakuji seamount (47.5 Ma) is distinctive and exhibits the most radiogenic Pb and Sr isotopic compositions observed in NWHR postshield basalts. The least radiogenic ⁸⁷Sr/⁸⁶Sr is from Jingū, a young (~ 55 Ma) Emperor Seamount volcano, and plots between the fields defined by Hawaiian Island postshield basalts and those erupted at the oldest Emperor Seamount (Detroit, ~ 76 Ma). These multi-isotopic trends highlight the different mantle source compositions required between the oldest Emperor Seamounts and the younger Hawaiiian basalts and support a direct compositional link between Hawaiian postshield lavas and their respective shield stages regardless of significant changes in magma flux, mantle potential temperature, absolute plate motion, and volcanic propagation rate over time.