Abstract
Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ34S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ33S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ34S ~ + 3‰), which supports the presence of younger 34S-rich recycled oceanic material in the Canary Island mantle plume.
Highlights
Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material
The Canary Island hotspot in the eastern Atlantic Ocean is characterised by an exotic geochemistry, producing ocean island basalts (OIB) that is mostly alkaline in composition and displays isotopic affinities with the HIMU, EM and depleted MORB mantle (DMM) end-members[20,21], suggesting that the mantle source constitutes a mix of different reservoirs[21]
This context offers the potential to confirm or contrast S isotope signals from the Canary Islands to those observed at hotspots in the South Pacific (i.e. Mangaia/HIMU; Pitcairn/EM-I; Samoa/EM-II)
Summary
Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Negative, non-zero Δ33S values have 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]. 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 δ34S values, lends support to this hypothesis, potentially resolving the positively-skewed imbalance of Δ33S values observed in Archaean surface reservoirs[5,13] These studies imply that a missing Archaean sulphur pool is stored in the deep mantle and occasionally resurfaces at hotspots. We find that the S isotope signals at El Hierro reflect a post-Archaean origin for recycled S in the Canary Island hotspot, contrasting with the recent findings of SMIF at other OIB localities
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