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

Abstract

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 (<∼6.5 Ma) present two geographically and geochemically distinct trends, Kea and Loa, while the Emperor Seamounts (>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.

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

“…The range of isotopic compositions in Hawaiian shield‐stage lavas is commonly described using four geochemical end‐members mixed in variable proportions: Depleted Makapu‘u (DMK), Enriched Makapu‘u or “enriched Loa” (EMK), KEA, and Lō‘ihi (Table ; e.g., Eisele et al, ; Harrison & Weis, ; Tanaka et al, , ; Weis et al, ). In addition, an “average Loa” composition is now recognized as a volumetrically important component and is defined by the intersection of regression lines from Loa‐type volcanoes, yielding values of 206 Pb/ 204 Pb = ~18.15 and 208 Pb/ 204 Pb = ~37.95 (Table and Figure ; Weis et al, ).…”

“…Many oceanic islands have paired volcanic chains that erupt magmas of variable isotopic compositions (e.g., Galápagos, Hawai‘i, Marquesas, Samoa, Tristan‐Gough; Chauvel et al, ; Harpp & White, ; Huang et al, ; Hoernle et al, ; Weis et al, ), providing evidence for the presence of mantle heterogeneity even on island‐wide scales (e.g., ~50 km in the case of the Hawaiian Islands). The temporal and spatial evolution of Pacific mantle chemistry over tens of millions of years can be established by studying basalts from the >5,800‐km‐long Hawaiian‐Emperor seamount chain, which formed during the past ~82 million years due to the movement of the Pacific plate over the Hawaiian mantle plume (e.g., Harrison et al, ; Harrison & Weis, ; Regelous et al, ; Tatsumoto, ; Figure a). Seismic anomalies that extend to the base of the mantle beneath the present‐day location of the Hawaiian plume support a lower mantle origin (e.g., French & Romanowicz, ; Montelli et al, ), and geophysical models suggest that the plume is anchored at the northern margin of the Pacific Large Low Shear Velocity Province (LLSVP), a region with different thermochemical properties than the surrounding mantle (French & Romanowicz, ; Garnero et al, ; Lau et al, ).…”

“…The Loa end‐member is thought to include recycled subducted material sourced from the Pacific LLSVP (Harrison et al, ; Weis et al, ) originally enriched in large ion lithophile elements and Th/U, leading to high present‐day radiogenic isotope ratios (i.e., 87 Sr/ 86 Sr and 208 Pb*/ 206 Pb*; e.g., Hofmann, ; White, ; Zindler & Hart, ). Isotopic heterogeneities in the Loa and Kea geochemical groups have been attributed to the sampling of smaller, finite compositional filaments in their corresponding mantle domains (e.g., Farnetani et al, ; Greene et al, ; Hanano et al, ; Harrison & Weis, ; Marske et al, ).…”

“…The Emperor Seamounts (~82 to 50 Ma) are dominated by Kea compositions (Figure a; e.g., Frey et al, ; Tanaka et al, ), as are all but three Northwest Hawaiian Ridge (NWHR; 47 to 8 Ma) volcanoes (Diakakuji at 47 Ma, Nīhoa and Middle Bank at ~7–8 Ma; Figure a; Garcia et al, ; Harrison et al, ; Harrison & Weis, ). Isotopically, basalts from the NWHR volcanoes become more Loa‐like toward the Hawaiian Islands, which has been linked to the anchoring of the Hawaiian plume base to the edge of the LLSVP and an increase of magmatic flux, mantle potential temperature, and volcanic propagation rate toward the youngest end of the chain (Harrison et al, ).…”

2.0 to 1.9 Ga large igneous province magmatism in northern Quebec — U-Pb geochronology and geochemistry

“…They attribute the Kea compositional component to the plume and the Loa compositional component to the LLSVP (Harrison et al, 2017;Harrison and Weis, 2018). These components ascended on opposite sides of the plume and reflect the structure of the underlying deep mantle.…”