Sulfur isotope evidence of geochemical zonation of the Samoan mantle plume

The radiogenic Pb isotope compositions of basalts from the Samoan hotspot suggest various mantle endmembers contribute compositionally distinct material to lavas erupted at different islands [1]. Basalts from the Samoan islands sample contributions from all of the classical mantle endmembers, including extreme EM II and high ³He/⁴He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks associated with several Samoan volcanoes—Malumalu, Malutut, Upolu, Savaii, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and allow for an assessment of the S-isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10-1000 ppm) and δ³⁴S values (-0.29‰ to +4.84‰ ± 0.3, 2σ). The variable S concentrations likely reflect weathering, sulfide segregation and degassing processes. The range in δ³⁴S reflects mixing between the primitive mantle and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ³³S within uncertainty of Δ³³S=0 ‰ ± 0.008, suggesting Δ³³S is relatively well mixed within the Samoan mantle plume. Important exceptions to this observation include: (1) a negative Δ³³S (-0.018‰ ±0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (2) a previously documented small (but resolvable) Δ³³S values (up to +0.027±0.016) associated with the Vai Trend (associated with a diluted HIMU component) [2]. The variability we observed in Δ³³S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ³⁶S vs. Δ³³S relationships suggest all recycled S is of post-Archean origin. The heterogeneous S isotope values and distinct isotopic compositions associated with the various compositional trends confirms a prior hypothesis; unique crustal materials are heterogeneously delivered to the Samoan mantle plume and compositionally influence the individual groups of islands.

[1] Jackson et al. (2014), Nature; [2] Dottin et al. (2020), EPSL