Thallium isotope data suggest minimal recycled material within the proto-Iceland mantle plume source

Mantle plume isotopic compositions are useful for understanding the processes that produce heterogeneity within the mantle. Thallium (Tl) isotopic compositions are uniquely well suited for tracing pelagic sediments and low-temperature altered oceanic crust in the mantle. We measured the Tl isotopic composition of Paleocene lavas on Baffin Island, the earliest lavas associated with the Iceland mantle plume, to constrain the geochemical evolution of the mantle plume. After correcting for assimilation and degassing, these lavas have 205 Tl/ 203 Tl ratios indistinguishable from the inferred ratio of the upper mantle and from modern Iceland lavas. Therefore, the amount of subducted pelagic sediments or low temperature altered oceanic crust potentially within the head of the Iceland mantle plume is vanishingly small. These components cannot account for the well-documented evolution in other isotopic systems over time in the Iceland mantle plume. Thus, we evaluate other processes that could account for this evolution, including other subducted components and core-mantle interactions.