Near-zero 33 S and 36 S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume

Jabrane Labidi (1) , James W. Dottin Ill (2) , M Clog , C Hemond (3) , P Cartigny (1)

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris
2 University of Maryland [College Park]
3 LGO - Laboratoire Géosciences Océan

Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32 S, 33 S, 34 S, 36 S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ~2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ~ 2,500 mbsl) and 600 ppm S (~2,000 mbsl). The d 34 S values can be accounted for under equilibrium isotope fractionation during degassing, with agas-melt between 1.0020 and 1.0001 and starting d 34 S values between-0.9‰ and +0.6‰. The d 34 S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ~ 1‰ 34 S enrichment compared to pacific upper mantle. The D 33 S and D 36 S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1s). Only D 33 S is statistically different from MORBs, by +0.02‰. The D 33 S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33 S-36 S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn.

Mots clés: sulfur isotopes; Pitcairn; recycling; global geodynamics

Domaines: Géochimie

Jabrane Labidi, James W. Dottin Ill, M Clog, C Hemond, P Cartigny. Near-zero 33 S and 36 S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume. Earth and Planetary Science Letters, 2022, 584, pp.117422. ⟨10.1016/j.epsl.2022.117422⟩. ⟨insu-03629791⟩