Missing Archean sulfur returned from the mantle

Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava

“…Since Mangaia is the representative end-member of the ancient ‘HIMU’ (high μ = 238 U/ 204 Pb) mantle component 10 , 11 , the potential positive covariation of Δ 33 S and δ 34 S values in Mangaia sulphides was first used to suggest a specific Archaean protolith to the HIMU source characterised by negative Δ 33 S and δ 34 S values 4 . The subsequent finding of S-MIF at Pitcairn, representative of the enriched mantle I end-member 10 , 11 (EM-I; characterised by unradiogenic Pb isotope signatures), in association with negative δ 34 S values, lends support to this hypothesis, potentially resolving the positively-skewed imbalance of Δ 33 S values observed in Archaean surface reservoirs 5 , 13 . These studies thus imply that a missing Archaean sulphur pool is stored in the deep mantle and occasionally resurfaces at hotspots.…”

“…Moreover, while degassing cannot explain the negative MIF Δ 33 S values observed at Mangaia 4 and Pitcairn 5 , our results clearly show that it can impart negative δ 34 S signatures to volcanic rocks and their S-rich inclusions, potentially overprinting the isotopic composition of the source. Degassing thus offers an alternative mechanism to produce the highly negative δ 34 S values of the Mangaia and Pitcairn sulphides, which have been interpreted to reflect an intrinsic feature of the presumed Archaean protolith within these plumes 4 , 5 , 13 . Disentangling the effects of degassing from source heterogeneity is thus critical for understanding the geodynamic implications of S isotopes at volcanic hotspots.…”

“…However, negative, non-zero Δ 33 S values have now been reported twice for young volcanic rocks from hotspot settings: in olivine-hosted sulphides from Mangaia, Cook Islands 4 , and in sulphides from the Pitcairn hotspot 5 . These anomalous signatures are thought to reflect the cycling of Archaean sulphur from Earth’s surface to the mantle by subduction, and back to the surface via mantle plumes 4 , 5 , 13 . Since Mangaia is the representative end-member of the ancient ‘HIMU’ (high μ = 238 U/ 204 Pb) mantle component 10 , 11 , the potential positive covariation of Δ 33 S and δ 34 S values in Mangaia sulphides was first used to suggest a specific Archaean protolith to the HIMU source characterised by negative Δ 33 S and δ 34 S values 4 .…”

Broader Impacts of the Metasomatic Underplating Hypothesis

“…Deeply subducted crust that lies above the underplated layer chemically resembles the ancient carbonated-eclogite mantle source inferred for HIMU end-member OIBs (Jackson & Dasgupta, 2008). Given that the EM1 and HIMU mantle reservoirs are suggested by mass-independent fractionation of sulfur isotopes to have originated in the late Archean prior to the Great Oxidation Event (Farquhar & Jackson, 2016), we speculate that metasomatic underplating became a major Earth-system process at that time, adding weight to the view that modern plate tectonics dates from the late Archean, and not before.…”

Deep Carbon

“…One of the most exciting hypotheses coming along with the presence of negative Δ 33 S in some OIB is that part of the missing surficial sulfur could be stored in the deep mantle. 95 The data obtained so far on mantle samples tend to indicate that the Archean surficial components recycled in the SCLM differ from those found in some OIB. Additional data are required to confirm this view.…”