Collaborative Research: Using Sulfur Isotopes to Identify Subducted Archean Crust in Modern Oceanic Hotspot Lavas

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A consequence of modern plate tectonics is that subducting ocean plates transport oceanic crust and sediment into the mantle. However, the fate of the subducted package--oceanic crust and sediment--in the mantle is poorly understood. A long-standing hypothesis maintains that subducted materials residue in the mantle for an extended, but unknown, period of time and are then recycled back to the Earth's surface in regions of buoyantly upwelling mantle and melted beneath hotspots. If this hypothesis is correct, ocean island basalts (OIB) erupted at hotspots should exhibit geochemical signatures associated with the crustal protoliths that were injected into the mantle at a subduction zone in the geologic past. However, it has been difficult to unequivocally detect geochemical signatures of ancient subducted materials in hotspot lavas. This proposal will use measurements of mass independently fractionated sulfur (MIF-S) isotope signatures--made using two complementary techniques--in key hotspot lavas to trace crustal cycling from the surface, through the mantle and back again.MIF-S were recently discovered in olivine phenocrysts and olivine-hosted sulfides in 20 million-year-old lavas from Mangaia, the archetypal HIMU (high 238U/204Pb) locality. Widespread terrestrial MIF-S isotope signatures were generated exclusively through atmospheric photochemical reactions until ~2.45 Ga, and the discovery of MIF-S isotope signatures in young hotspot lavas therefore appears to provide a "timestamp" and "signature" for preservation of subducted Archean surface materials in the mantle sourcing OIB. This proposal is a 1-year pilot study that aims to perform a S-isotope study on olivine-hosted sulfides and phenocrysts in lavas from the other two mantle endmembers found in OIB, EM1 (in Pitcairn lavas) and EM2 (Samoa), to determine whether these lavas also host an Archean recycled component. We will also target large sulfides globules hosted in volcanic glass from Loihi and Samoa for analysis by both ion probe and standard sulfur extraction and isotope ratio mass spectrometry (IRMS) techniques, to further establish a direct comparison between the two analytical techniques. A survey of abyssal peridotite sulfides will be undertaken to identify primary mantle sulfides of sufficient size for sulfur isotopic analyses, and if found, the sulfides will be measured by ion probe and (if large enough) by sulfur extraction and IRMS. The analytical campaign will allow us address whether oceanic hotspots in general exhibit evidence for recycling of Archean protoliths. This proposal offers an unprecedented test of the recycling hypothesis.