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Geochemistry of shield basalts from Kaua‘i, Hawai‘i : exploring the emergence of the Hawaiian geochemical trends Williamson, Nicole


Ocean island basalts are indirect records of Earth’s mantle chemistry and are fundamental for constraining its evolution through geological time. Volcanoes of the ~6000 km-long Hawaiian-Emperor chain represent an extraordinary record of Pacific mantle chemistry over the past ~80 Ma, formed as the Pacific tectonic plate passed over the deeply sourced Hawaiian mantle plume. Hawaiian volcanoes are divided into southwestern (Loa) and northeastern (Kea) geographic and geochemical trends that reflect the chemical structure of the underlying plume and its deep mantle sources. This dissertation addresses a change in the chemistry of erupted lavas between the Northwest Hawaiian Ridge and the younger Hawaiian volcanoes using a new database from the northernmost island of Kaua‘i that includes high-precision radiogenic isotopes (Pb, Sr, Nd, Hf), major element oxides, trace element concentrations, and ⁴⁰Ar/³⁹Ar geochronology. Western and eastern Kaua‘i erupted at different times and from distinct mantle sources. The distribution of Loa and Kea compositions on Kaua‘i is opposite to the younger Hawaiian Islands and is explained by a more westward trend of plate motion relative to the Loa- Kea compositional boundary in the plume. Loa compositions dominated the geochemistry of erupted lavas for over 2 Ma before the onset of the bilateral Loa and Kea geochemical trends. Loa compositions are isotopically enriched and may be influenced by subducted materials recycled in the deep mantle source of the Hawaiian plume that were stored within the thermally and chemically distinct Pacific large low shear wave velocity province (LLSVP). Thallium isotopes were used to test whether these materials originally consisted of pelagic sediment and were measured in 33 samples from the entire geochemical range of Hawaiian lavas. Results show evidence of recycled materials on the Kea side of the plume, implying that the lower mantle is heterogeneous both within and outside of the LLSVP. This dissertation provides important insight into the timing and source of two major geochemical components along the most active hotspot system on Earth and expands our knowledge on the chemical structure of the Hawaiian plume, its evolution in space and time, and the nature of its deep mantle sources.

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