SULFUR ISOTOPE CHARACTERIZATION OF MANTLE RESERVOIRS SAMPLED BY OCEAN ISLAND BASALTS
Dottin, James Wosley
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Ocean islands are volcanic islands in the middle of oceans that are formed when magma deep inside the Earth buoyantly rises to the surface and erupts as basaltic lava (termed Ocean Island Basalt, OIB). Geochemical characterization of OIB have identified multiple chemical reservoirs that reside in the deep Earth, including primordial, recycled, and convective mantle components that contribute material to OIB systems. Stable sulfur isotope compositions of these reservoirs provide an opportunity to constrain the types of material that contribute to this heterogeneity and processes involved in the production of the respective reservoirs. In this dissertation, I present sulfur isotope compositions of basalts from two OIB localities: the Samoan islands and Mangaia (an island in the Cook-Austral suite). Samoa is unique in that it preserves evidence for sampling all identified mantle reservoirs and Mangaia is unique in that it preserves evidence for hosting Archean/early-Proterozoic related sulfur as a result of mantle recycling. I use sulfur isotopes to constrain how the mantle plume plumbing system beneath Mangaia and Samoa affect the S-isotope compositions erupted at individual islands and identify the prevalence of anomalous S-isotope compositions in different mantle components that reflect a common link to surface material recycled throughout Earth’s history. I constrain the primordial sulfur isotope composition of the mantle (Δ33S= 0‰) through analyses of basalts from Ofu, Samoa and show that it is similar to estimates from meteoritic materials. I also test whether Samoa shows geographical zonation for geochemical components through the eruption of multiple isotopically distinct recycled sulfur components that are delivered to specific groups of islands within the island chain. I find that Sulfur isotope compositions of Samoan basalts are unique to each grouping of islands, where HIMU-related and rejuvenated lavas show the most distinct compositions, revealing positive and negative Δ33S compositions respectively. At Mangaia (extreme HIMU), I test whether isotopically distinct sulfur reservoirs exist within the Mangaia mantle plume. The sulfur isotope compositions measured at Mangaia demonstrate an increased level of S-isotope heterogeneity in the Mangaia mantle plume from previous measurements and point to an additional recycled compositional endmember that is potentially younger than the Archean/early-Proterozoic recycled component previously identified.