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Armstrong, Catherine

University of British Columbia

Among the Hawaiian Islands, the youngest shield-stage tholeiitic basalts from Koʻolau volcano on Oʻahu preserve the earliest evidence of an extremely enriched geochemical signature from the underlying Hawaiian mantle plume. Basalts with high concentrations of incompatible trace elements, with strongly radiogenic ⁸⁷Sr/⁸⁶Sr-²⁰⁸Pb*/²⁰⁶Pb* and unradiogenic εHf-εNd, establish this signature as the most enriched endmember of the LOA part of the bimodal ‘Loa–Kea’ geographic-compositional range observed along the Hawaiian Islands. These basalts have a mantle source that is isotopically distinct (with higher radiogenic Pb or ²⁰⁸Pb*/²⁰⁶Pb*) from the more voluminous components in the heterogeneous Hawaiian mantle plume. At the Koʻolau Range, the enriched signature of the ‘Makapuʻu’ component is only present in basalts from the late, ‘Makapuʻu’ stage of shield building activity after a transition from a more common, average Mauna Loa-like, or LOA composition. The geochemistry of Makapuʻu-stage basalts, with new samples from three locations on Koʻolau including their type locality at Makapuʻu Head, is used to constrain the mean isotopic composition of the Makapuʻu component. High-precision Sr-Nd-Hf-Pb isotopic geochemistry confirms the existence of a small amount of anomalously enriched material, increasing the observed range of ²⁰⁸Pb*/²⁰⁶Pb* on Koʻolau, and reveals heterogeneity within the Makapuʻu component. As the enriched endmember for the entire Hawaiian basalt compositional array, the refined composition of the Makapuʻu component is important for models of the geodynamics of discrete components and their mixtures in the heterogeneous Hawaiian mantle plume.

Text

2024-01-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83608

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/