The Size and Emergence of Geochemical Heterogeneities in the Hawaiian Mantle Plume Constrained by Sr‐Nd‐Hf Isotopic Variation Over ∼47 Million Years

The Hawaiian‐Emperor chain is the ∼6,000 km long surface expression of the deeply sourced Hawaiian mantle plume active over the past ∼81 Myr. The Hawaiian Islands (81–47 Ma) show only Kea compositions. New Sr‐Nd‐Hf isotope, trace and major element data of 23 Northwest Hawaiian Ridge (∼47–6.5 Ma) shield‐stage tholeiitic basalts analyzed in this study fill a critical gap and show both Kea and Loa compositions. A logistic regression model fit to a high‐quality isotopic database of Hawaiian Island basalts is used to predict Loa‐type or Kea‐type affinity of new NWHR isotope analyses. Daikakuji, Mokumanamana, West Nīhoa, Nīhoa, and Middle Bank erupt Loa‐type compositions, a finding corroborated by their geochemical characteristics (e.g., low Th/La, CaO/Al2O3, and high Sr/Nb, Zr/Nb, SiO2). Participation of the Loa composition gradually increases toward the Hawaiian Islands with time and there is no evidence for the presence of the Lō‘ihi component along the NWHR or before ∼1 Myr. A new Hf‐Nd Hawaiian array is calculated based on an up‐to‐date extended Hawaiian Island basalt database (n = 403). The NWHR array is slightly steeper than the Hawaiian array, suggesting minimal participation of the high Hf isotopic source component present in Hawaiian Island volcanoes before ∼6.5 Ma. This study fills a significant geochemical data gap in the Hawaiian‐Emperor seamount chain, and shows that Hawaiian plume chemistry evolves significantly with time as the plume samples different deep mantle reservoirs. Plain Language Summary Mantle plumes transport deep mantle material to the surface, where it melts to create volcanoes such as the Hawaiian‐Emperor seamount chain. Analyzing the elemental and isotopic compositions, and how they vary between volcanoes of different ages, allows for reconstruction of the composition and structure of the deep mantle over time. In this study, the chemistry of Northwest Hawaiian Ridge volcanoes (∼47–6.5 Ma) is analyzed and compared to volcanoes from the Hawaiian Islands (6.5 Ma–present). We find that the previously defined geochemical components that explain the observed isotopic variation on the Hawaiian Islands actually change significantly back in time. A new scale of mantle heterogeneity is resolved, showing that mantle components are finite and can reoccur periodically over time. The potential reservoir for these compositional com