Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust

Mass-independent fractionation of sulphur isotopes in basalts from the oceanic island of Mangaia (Cook Islands) indicates ancient subducted Archaean (>2.45 Gyr) oceanic crust and lithosphere survives in the mantle to be sampled beneath hotspot volcanoes. A timescale for cycling mantle materials Some of the oceanic crust injected into the mantle at subduction zones is thought to return to the surface to be erupted at hotspot volcanoes, but the timescale for this process has been difficult to constrain. Rita Cabral and colleagues report mass-independently fractionated sulphur isotope signatures in ocean island basalts from the Cook Islands. Given that such sulphur isotope signatures were presumably generated exclusively through atmospheric photochemical reactions before there was significant oxygen in the atmosphere, from about 2.45 billion years ago, the authors conclude that their observations provide a constraint on the length of time that subducted crustal material can survive in the mantle and perhaps on the timescales of mantle convection from subduction to upwelling beneath hotspots. Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials 1 , 2 . However, the residence time of these subducted materials in the mantle is uncertain and model-dependent 3 , and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust 3 , 4 . Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago 5 , 6 , 7 . Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur—probably derived from hydrothermally altered oceanic crust—was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Δ 33 S values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sediment