Emergence of the Loa Mantle Component in the Hawaiian Islands Based on the Geochemistry of Kauaʻi Shield‐Stage Basalts.

Kauaʻi shield‐stage lavas are central to understanding the origin of the distinct Kea and Loa Hawaiian geochemical trends in Hawaiian basalts. These trends reflect two geochemically distinct sides in the Hawaiian plume, with Loa to the southwest and Kea to the northeast. The geochemistry and Sr‐Nd‐Hf isotopic compositions of shield‐stage lavas from Kauaʻi show a transition from Kea to Loa across the island with the Loa mantle source becoming dominant as the volcano grew. This geochemical transition is gradual from west to east Kauaʻi and supports the hypothesis that the Kauaʻi volcano sampled both sides of the bilateral Hawaiian plume, a phenomenon that is unusual for a Hawaiian volcano. Notably, Kauaʻi marks the arrival of progressively larger volumes of Loa compositions within the Hawaiian mantle plume. The new data from Kauaʻi, combined with an updated and comprehensive database of Hawaiian shield‐stage major element oxides, trace element concentrations, and isotopic compositions normalized to the same standard values, allows for the Pb‐Sr‐Nd‐Hf isotopic compositions of the Average Loa ('ALOA') common geochemical component to be estimated. Despite the bilateral Loa‐Kea geochemical trend beginning at Molokaʻi, Loa compositions dominate the erupted volume of Hawaiian volcanoes younger than 3 Ma, validating the volumetric importance of the Loa source in the lower mantle portion of the Hawaiian plume. Plain Language Summary: Hawaiian volcanoes are arranged along two parallel geographic trends named Loa and Kea. Volcanoes belonging to either trend have distinct geochemical compositions that are linked to their deep mantle sources as sampled by the Hawaiian mantle plume. The Kea composition has been present in shield‐stage basalts for ∼81 Ma, however the Loa composition is relatively new and has mainly been measured in volcanoes formed since 3–4 Ma. We used the geochemistry and isotopic compositions of shield‐stage basalts from the island of Kauaʻi to show that Loa compositions began to appear in larger amounts in the Hawaiian plume around 5 Ma. These new data, combined with a large and carefully curated geochemical data set of Hawaiian samples, has allowed us to estimate the average composition of Loa and its associated isotopic end‐member compositions. This work demonstrates that Loa was an important mantle source for the older Hawaiian volcanoes such as Kauaʻi and dominates shield lavas along the Hawaiian chain. Notably, the geochemistry of Kauaʻi's volcanic rocks represents the long‐term establishment of Loa compositions in the Hawaiian plume. Key Points: Radiogenic (Sr‐Nd‐Hf, and Pb) isotopic compositions change from west to east across Kauaʻi and broadly correlate with ageKauaʻi records the first large‐scale and long‐lasting occurrence of Loa‐trend Hawaiian compositionsThe average Loa composition is constrained in Pb‐Sr‐Nd‐Hf isotopes and dominates compositions along the Hawaiian chain [ABSTRACT FROM AUTHOR]

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