Tungsten isotopes in Baffin Island lavas: Evidence of Iceland plume evolution

Abstract

Tungsten and helium isotope ratios in lavas derived from deeply rooted mantle plumes are tracers of lower mantle compositional heterogeneity or core–mantle exchange. We measured the tungsten isotopic compositions of lavas with exceptionally high-3He/4He ratios that erupted above the head of the Iceland plume on Baffin Island. These lavas have 182W/184W ratios that are indistinguishable from the convecting upper mantle, unlike younger lavas in Iceland that have lower 182W/184W ratios. This implies that only the Iceland plume tail was infused with low-182W/184W material, likely from the core. If high-3He/4He helium also comes from the core, then diffusion across the core–mantle boundary may stratify plume-source mantle domains, with elevated 3He/4He travelling farther into the lower mantle than 182W/184W anomalies. Over Earth history, tungsten diffusion from the core can explain the decline of 182W/184W in the convecting mantle. We speculate that the uneven pace of this decline corresponds with evolving lower mantle dynamics.