Time-Progressive Changes in Hawaiian Mantle Plume Composition Between the Northwest Hawaiian Ridge and Molokaʽi from 7 to 2 Ma

The Hawaiian Island of Kauá i (4-5 Ma) marks the transition between the Kea-dominated Northwest Hawaiian Ridge and the Loa-Kea bilateral trends of the younger Hawaiian Islands, making it key for further constraining the ~82 million-year chemical evolution of the Hawaiian mantle plume. The isotopically distinct Loa and Kea compositions that are erupted in parallel on the younger Hawaiian Islands of Moloká i to Hawaí i (2 to 0 Ma) are interpreted to reflect a compositionally bilateral mantle plume. Studies of seamount volcanoes along the older Northwest Hawaiian Ridge (NWHR, 49 to 7 Ma) show dominant Kea isotopic compositions, as well as a gradual increase in Loa compositions through time that has been linked to the progressive anchoring of the Hawaiian plume to the edge of the Pacific Large Low Shear Velocity Province (LLSVP), the proposed source of the more radiogenic (e.g., elevated ²⁰⁸Pb*/²⁰⁶Pb*), Loa-like compositions. New, high-precision Pb isotopic analyses conducted on 55 shield-stage samples from Kauái and Waíanae support this model and show that radiogenic Pb values in shield basalts continued to gradually increase between the youngest NWHR volcanoes (~7 Ma) and Molokái (2 Ma) . Notably, beginning in eastern Kauá i and continuing to Makapú u on Ó ahu, Loa isotopic compositions dominated for over two million years prior to the development of the bilateral Loa-Kea geochemical trends. The protracted duration of dominantly Loa compositions is accounted for by the arrival of a large Loa heterogeneity that diluted Kea compositions in the plume. On Kauá i, the west and east parts of the island are isotopically distinct, with a rotated geographic distribution of Loa and Kea compositions relative to that observed on the younger Hawaiian Islands. This can be explained by a model where the direction of Pacific plate motion was oblique to the Loa-Kea compositional boundary prior to 2 Ma. Our results constrain a geochemical transition occurring over approximately 5 million years and 550 km of the Hawaiian hotspot track and highlight the importance of mantle chemical heterogeneities in generating time-progressive isotopic trends along volcanic island chains.