Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials. However, the residence time of these subducted materials in the mantle is uncertain and model-dependent, 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. 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. 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 sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions. This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H 2 O loss, degassing, and assimilation. H 2 O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H 2 O/Ce ratios vary up to 200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO 2 concentrations range up to 2346 ppm CO 2 in the inclusions. Relatively high CO 2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO 2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 6 9; 2r standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 6 0.028) fall in the range of pristine mantle (0.02-0.08).

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International audienceThe Ontong Java Plateau (OJP) represents the largest large igneous province (LIP) preserved in the geologic record. The most voluminous volcanic types of the OJP—the Kroenke and Kwaimbaita groups, which dominate the accessible portions of the plateau—have relatively flat primitive mantle normalized rare earth element (REE) patterns. With the exception of relatively small volumes of late-stage melts—referred to as the Singgalo group—that are characterized by slightly enriched REE patterns relative to a chondritic pattern, the volcanic groups that dominated the eruptive history of the OJP exhibit remarkably homogeneous, flat REE patterns. Here we isolate, for the first time, olivine-hosted melt inclusions from the OJP. We show that the melt inclusions have two clear populations defined by distinct trace element characteristics. The first population has relatively flat trace element patterns that are similar to those observed in whole rock lavas from the most voluminous volcanic groups (Kroenke and Kwaimbaita) recorded in the OJP. In contrast, a second group of melt inclusions, referred to as UDM (ultra-depleted melt) inclusions, exhibit (light-REE) LREE-depleted patterns relative to a chondritic pattern; these trace element patterns are far more depleted than any previously reported lava from OJP. The UDM have unique trace element signatures that preclude an origin by re-melting the depleted mantle source left over after melt extraction during construction of the OJP. We interpret the new UDM compositions to be the result of melting a previously unrecognized (in lavas) ultra-depleted component hosted in the OJP mantle source