Sulfur Isotope Evidence for a Geochemical Zonation of the Samoan Mantle Plume

James W. Dottin , Jabrane Labidi (1) , Matthew G. Jackson , James Farquhar

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

Basalts from the Samoan volcanoes 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 (N = 16) associated with several Samoan volcanoes—Vailulu`u, Malumalu, Malutut, Upolu, Savai`i, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and upon exhaustive compilation of S-isotope data for Samoan lavas, 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 (10-1,000 ppm) and δ³⁴S values (−0.29‰ ± 0.30 to +4.84‰ ± 0.30, 2σ). The observed variable S concentrations are likely due to sulfide segregation and degassing processes. The range in δ³⁴S reflects mixing between the mantle origin and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ³³S within uncertainty of Δ³³S = 0‰ ± 0.008. Important exceptions to this observation include: (a) a negative Δ³³S (−0.018‰ ± 0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (b) previously documented small (but resolvable) Δ³³S values (up to +0.027 ± 0.016) associated with the Vai Trend (associated with a diluted HIMU component). The variability we observed in Δ³³S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ³⁶S versus Δ³³S relationships suggest all recycled S is of post-Archean origin.

Mots clés: Ocean island basalt; stable isotopes; sulfur isotopes; recycling

Domaines: Planète et Univers [physics]

James W. Dottin, Jabrane Labidi, Matthew G. Jackson, James Farquhar. Sulfur Isotope Evidence for a Geochemical Zonation of the Samoan Mantle Plume. Geochemistry, Geophysics, Geosystems, 2021, 22, pp.1510-1517. ⟨10.1029/2021GC009816⟩. ⟨insu-03590046⟩