UBC Theses and Dissertations
Deciphering mantle source components in basalts from hotspot tracks and oceanic islands Nobre Silva, Inês G.
The isotopic compositions of oceanic island basalts erupted at the Earth’s surface can be used to directly constrain the isotopic signatures of their deep mantle sources. Basaltic rocks are highly susceptible to seawater alteration, which can significantly modify elemental contents and potentially hinder the use of radiogenic isotopes as geochemical tracers. In this study, multi-isotopic (e.g., Pb-Hf-Sr-Nd) analyses on the same acid-leached sample aliquot are shown to produce reliable results for use in the discrimination of mantle source components of oceanic basalts. Application to basalts from the Ninetyeast Ridge in the Indian Ocean and from Mauna Kea volcano on Hawaii in the Pacific Ocean allows for an enhanced resolution of their source components and distribution in the deep mantle. Isotopic geochemistry of the Ninetyeast Ridge sampled during Ocean Drilling Project Leg 121 reveals a Kerguelen and Amsterdam-St. Paul mantle plume origin with the presence of at least three source components and no contribution from an Indian mid-ocean ridge basalt source. The isotopic characteristics of the Ninetyeast Ridge basalts are typical of the Dupal isotopic domain and consistent with recycling of altered oceanic crust and a mixture of pelagic sediments and lower continental crust into their mantle source. This supports a deep origin for the enriched mantle “EM-1”-like Dupal signatures encountered in Indian Ocean island basalts. In the Pacific Ocean, isotopic heterogeneity in basalts from the deepest parts of the Hawai’i Scientific Drilling Project (HSDP2) core on Mauna Kea, combined with prior results from overlying flows, indicate that shield basalts can be explained by mixing of variable proportions of four isotopically distinct components intrinsic to the Hawaiian mantle plume. The “Kea” component is the prevailing composition in Mauna Kea basalts and throughout volcanic activity of the Hawaiian hotspot. The relatively depleted isotopic compositions of this “Kea” component are shared by other Pacific oceanic island basalt groups and are very similar to those of the common mantle component “C”. This suggests that “Kea” may be a common and widespread composition within the deep mantle beneath the Pacific Ocean basin.
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