Volatiles in Earth’s mantle: primordial or subducted? Insights from new generation nitrogen and sulfur isotope systematics

Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen
Nitrogen (N) is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle is uncertain. In this presentation, we discuss nitrogen enrichments in multiple mantle reservoirs. We show that subduction may not be as important as previously thought to account for mantle nitrogen. We use the rare 15N15N isotopologue of N2 as a novel tracer of air contamination in volcanic gas effusions. By correcting for air contributions in the gases using this tracer, we derive new estimates for mantle 15N and N2/3He ratios from multiple volcanic regions. We focus on Yellowstone, a primitive hotspot, and the central American subduction zone. We show that subduction may cause elevated 15N and N2/3He values in a mantle source, as the result of the accumulation of surface-derived components. However, our 15N15N-based analysis requires the Yellowstone plume to have some of the lowest N2/3He ratio. This is inconsistent with subducted volatiles in this mantle source, and allows plume nitrogen to be a primordial component. This result opens the possibility that the budget of mantle volatiles was at least partly established during planetary formation, rather than exclusively reflecting subduction and tectonic plate activity.Isotopic evidence of multiple sulfur sources delivered to the Samoan islandsLike nitrogen, sulfur isotopes can be used as a tool in placing constraints on crustal recycling and the nature of volatiles in primordial mantle reservoirs. Basalts from the Samoan islands sample contributions from all classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. The geochemical heterogeneity of the Samoan islands provides an opportunity to test whether distinct S-isotope compositions are delivered to the Samoan mantle plume and whether they are linked to the various observed mantle components. Through high precision, quadruple S-isotope analyses of Samoan Basalts we observe unique S-isotope compositions linked to the HIMU, EM II, and EM I components at Samoa. We also use relationships between sulfur and tungsten isotopes to show that the primordial S-isotope composition of the mantle is within uncertainty of the convective mantle, suggesting S isotope compositions were well mixed within 60 Ma of Earth’s accretion.