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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 (32S, 33S, 34S, 36S) 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 δ34S values can be accounted for under equilibrium isotope fractionation during degassing, with αgas-melt between 1.0020 and 1.0001 and starting δ34S values between −0.9‰ and +0.6‰. The δ34S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ∼ 1‰ 34S enrichment compared to the Pacific upper mantle. The Δ33S and Δ36S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ33S is statistically different from MORB, by +0.02‰. The Δ33S 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 33S-36S 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.
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This content will become publicly available on May 1, 2027
Secular Evolution of Mantle Sulfur Isotopes Since the Archean
Abstract Sulfur isotopes in mantle plume‐derived magmas show heterogeneities attributed to recycling of material from the Earth's surface. The sedimentary sulfur isotope record exhibits dramatic temporal variations over Earth history, raising the question of whether its secular evolution is echoed in mantle plume‐derived magmas. We present new secondary ion mass spectrometry and X‐ray absorption near‐edge structure measurements of δ34S and Fe3+/ΣFe in naturally glassy melt inclusions from the 2.7 Ga Belingwe komatiites. We find that δ34S of Belingwe melt inclusions is relatively homogeneous (+3.2 ± 0.9) and elevated relative to the depleted upper mantle. We evaluate explanations for elevated magmatic δ34S including degassing, sulfide fractionation, assimilation, subduction‐like processes, and mantle recycling. We find no evidence for significant sulfur isotope fractionation via degassing or sulfide saturation. The elevated δ34S in the Belingwe komatiites could reflect late‐stage assimilation of sediments or seawater. Alternatively, Belingwe komatiites may have formed in a subduction‐like setting, as modern arcs display a bias toward positive δ34S. However, we find these scenarios less favorable. Instead, our preferred interpretation is that elevated δ34S was supplied to the komatiite mantle source via recycled lithologies such as sediments, altered oceanic crust, and/or pyroxenite. Combined with δ34S data from mantle plume‐derived magmas spanning a wide age range, our results resemble the secular evolution of δ34S in surface reservoirs. We suggest that the δ34S record in mantle plume‐derived magmas may echo secular evolution in the surficial sulfur reservoir with a delay of several hundred million years, linking Earth's surface and interior sulfur cycles.
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- Award ID(s):
- 2426925
- PAR ID:
- 10692314
- Publisher / Repository:
- American Association for the Advancement of Science
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 27
- Issue:
- 5
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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