Thallium Isotopic Compositions in Hawaiian Lavas: Evidence for Recycled Materials on the Kea Side of the Hawaiian Mantle Plume

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 22; no. 9
• Main Authors: Williamson, Nicole M. B., Weis, Dominique, Prytulak, Julie
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.09.2021; Wiley
• Subjects: Basalt; Earth mantle; Geochemistry; Isotope composition; Isotopes; Isotopic enrichment; Leaching; Lower mantle; Mantle plumes; Oxygen; Oxygen isotopes; Pelagic sediments; Recycled materials; Sediment samples; Sediments; Subduction; Thallium; Tholeiitic basalt; Trace elements; Trends; Volcanic ash; Volcanic eruptions; Volcanoes
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2021GC009765

Hawaiian volcanoes record 6 Ma of potentially deep mantle chemistry and form two parallel volcanic chains that are geochemically unique, named Loa and Kea. Loa volcanoes erupt lavas with isotopically enriched compositions thought to reflect the presence of recycled material in the deep mantle source of the Hawaiian plume. Variations in stable thallium (Tl) isotopes have been used to trace recycled pelagic ocean sediment from subduction to eruption in arc and intraplate lavas. Previous work attributed heavy Tl isotopic compositions in eight Loa samples to recycled sediments in their source. We reexamined this hypothesis using a large sample set (n = 34) of shield‐stage, tholeiitic basalt from 13 Hawaiian volcanoes representing the entire range of isotopically enriched and depleted compositions along the Hawaiian chain. Samples were acid‐leached prior to isotopic analysis to remove post‐eruption alteration and resulting ε205Tl values show statistical differences between Loa and Kea volcanoes. Corresponding isotopic data and re‐analyzed trace element concentrations suggest that the ε205Tl values are primary magmatic signatures. Possible co‐variations between heavy ε205Tl and oxygen isotopes in samples from Kea‐trend volcanoes could reflect the presence of ancient, recycled pelagic sediment on the Kea side of the Hawaiian plume, which samples the average deep Pacific mantle. As such, the deep mantle source of both Loa and Kea Hawaiian volcanoes may contain recycled materials of different natures and recycling histories, which supports work from both geophysical and geochemical studies suggesting that the Earth's lower mantle is chemically heterogeneous on multiple spatial scales. Plain Language Summary The Hawaiian volcanoes form two parallel geographic and geochemical trends, named Loa and Kea, that are produced by a deep mantle plume originating at the core‐mantle boundary. Volcanoes from the Loa trend have more “enriched” isotopic compositions, indicative of recycled surface materials in their source, whereas volcanoes from the Kea trend tend to have average Pacific mantle compositions. Thallium (Tl) isotopes (205Tl and 203Tl) are unequally distributed across Earth's chemical reservoirs and can show large concentration contrasts, for example between pelagic sediments (>>100 ng/g) and the Earth's mantle (<1 ng/g). We measured the Tl isotopic composition in Hawaiian samples and found that, among other indicators, the heavier Tl isotopic compositions measured in some volcanoes of the Kea geochemical trend might co‐vary with oxygen isotopes, suggesting that their Tl compositions could result from recycled surface materials in their source. This shows that the mantle source of both the Loa and Kea geochemical trends likely contains materials recycled through the mantle, which is significant because thus far the Kea volcanoes have shown fairly uniform isotopic compositions representative of the average, deep Pacific mantle. Key Points Thallium isotopic compositions measured in a sample set of Hawaiian shield lavas represent primary magmatic signatures Heavier isotopic values in some Kea‐trend volcanoes suggest the presence of ancient pelagic sediment in the Kea source of the Hawaiian plume The deep mantle source of both Loa and Kea Hawaiian volcanoes contains recycled materials of varying lithologies, histories, and ages