Tracking the Geochemical Transition Between the Kea-Dominated Northwest Hawaiian Ridge and the Bilateral Loa-Kea Trends of the Hawaiian Islands

For similar to 82 million years, the Hawaiian-Emperor chain volcanoes have sampled the Pacific mantle via the Hawaiian mantle plume, providing evidence that its composition varies on a range of temporal and spatial scales. Hawaiian volcanoes from 2 to 0 Ma are divided into southwestern (Loa) and northeastern (Kea) geographic and geochemical trends that are interpreted to reflect the bilateral chemical structure of the underlying plume and its corresponding deep mantle sources. Hawaiian volcanoes that formed between 8 and 3 Ma record a geochemical transition between the Kea-dominated Northwest Hawaiian Ridge (49 to 8 Ma) and the bilateral trends of the younger Hawaiian Islands. High-precision Pb isotopic analyses conducted on 55 new shield-stage samples from two of these key volcanoes, Kaua'i and Wai'anae, show that Loa-like Pb isotopic ratios (e.g., elevated Pb-208*/Pb-206*) gradually increase with decreasing age among the northern Hawaiian volcanoes and dominate for over 2 million years prior to the onset of the bilateral Loa and Kea geochemical trends. Distinct isotopic groups are observed across Kaua'i and the distribution of Loa and Kea isotopic compositions is rotated relative to that observed on the younger Hawaiian Islands. Protracted Loa compositions and the atypical Loa-Kea trend on Kaua'i are accounted for by (1) the arrival of a voluminous, Loa mantle heterogeneity possibly associated with anchoring of the Hawaiian plume to the Pacific Large Low Shear Velocity Province and (2) a different orientation of the Pacific plate relative to the Loa-Kea compositional boundary prior to 2 Ma.