Primordial neon in high-³He/⁴He Baffin Island olivines

Abstract

Paleocene basaltic lavas exposed on Baffin Island have the highest ³He/⁴He found in any terrestrial igneous rocks and potentially contain the most pristine primordial mantle material exposed on Earth's surface. By vacuum-crushing large (1–3 g) olivine mineral separates, we extracted enough magmatic gas to obtain the first coupled helium, neon, and argon isotopic compositions of Baffin Island lavas. The five Baffin Island olivine samples have ³He/⁴He ranging from 36.2 ± 0.6 to 48.6 ± 1.3 (1σ) times the atmospheric ratio (Ra), overlapping with the highest known mantle values. Neon isotopic results fall on a mixing line between atmosphere and a high ²⁰Ne/²²Ne mantle endmember (with a maximum ²⁰Ne/²²Ne of 12.2). The slope of this mixing line is indistinguishable from that in subglacial Holocene glass from Iceland, but distinct from other hotspots and mid-ocean ridge basalt trends. This result supports the hypothesis that Baffin Island and Iceland lavas share a common high-³He/⁴He mantle component, despite the fact that recent paleogeographic reconstructions place the Iceland hotspot far from Baffin Island at the time of eruption (61 Ma). Our results also demonstrate that high-³He/⁴He mantle reservoirs have ³He/²²Ne variability that either reflects ancient mantle heterogeneity or helium addition in the upper or lower mantle.