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Advances in multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) have led to the widespread use of iron (Fe) isotopes to elucidate the (bio)geochemical history of a range of environments. To generate Fe isotope ratio measurements, standard‐sample bracketing (SSB) is commonly used to correct for instrumental mass bias inherent to MC‐ICP‐MS. However, SSB is only accurate when sample and isotope standard Fe concentrations match, in addition to the bulk solution matrix. When the Fe concentrations differ, Fe isotope ratio measurement results may be inaccurate, a phenomenon known as the "self‐induced matrix effect." This study systematically characterised the self‐induced matrix effect for dry plasma Fe isotope ratio measurements on three MC‐ICP‐MS instruments and three introduction systems. Our extensive dataset indicates that: (1) the degree of mass bias is consistent regardless of MC‐ICP‐MS front‐end design, (2) the degree of mass bias becomes less reproducible as the concentration difference between the sample and bracketing standard increases, and (3) this applies to both pure Fe solutions and solutions from geological materials. This study reinforces the requirement to match bracketing standard and sample concentrations within 10% and provides a correction method for that fall beyond the recommended concentration range to subsequently allow for proper concentration matching during SSB. [ABSTRACT FROM AUTHOR]

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]

Fourteen whole rock samples were analyzed for 40Ar/39Ar geochronology to determine the timing of an important transition in mantle source geochemistry that occurred on the island of Kaua'i, located at the junction between the Northwest Hawaiian Ridge and the Hawaiian Islands. Kaua'i's shield-stage lavas have lead (Pb) isotopic compositions that change from west to east across the island. Given previous age constraints, it was unclear whether western and eastern portions of the shield were coeval. The new dates from this study range from 4.95 ± 0.19 Ma to 4.02 ± 0.04 Ma, decrease broadly from west to east Kaua'i, and correlate with Pb isotopic ratios. The results indicate that the transition from Kea to Loa isotopic compositions across Kaua'i occurred between ~4.7 and 4.4 Ma. The new 40Ar/39Ar geochronological results require a modification of the order of shield-building events for Kaua'i. The easternmost lavas of the Nāpali Member formed after the westernmost lavas and are unlikely their stratigraphic equivalents, as previously inferred from mapping and structural observations. The easternmost lavas of the Nāpali Member likely represent the final period of shield-stage volcanism on Kaua'i, which is supported by the location of the residual gravity anomaly beneath the Līhu'e Basin in eastern Kaua'i. This work highlights the importance of combining field-based observations with geochemical, isotopic, and geochronological data when assessing the shield-stage evolution of Hawaiian volcanoes. [ABSTRACT FROM AUTHOR]