The geochemical and isotopic composition of the Miocene Vestfirðir basalts, Northwest Iceland: Tracking the evolution of the Iceland mantle plume

The Miocene basalts of Vestfirðir, northwest Iceland, provide a record of the Iceland mantle plume shortly after its arrival beneath the mid-Atlantic ridge. We present new trace element and Sr-Nd-Pb-He isotope data from a comprehensive collection of 16–8 Ma basalts to investigate the temporal and compositional evolution of the plume. Basalts erupted before a volcanic hiatus at ∼14.9 Ma include both depleted, negative ΔNb lavas that resemble N-MORB and enriched, plume-derived basalts with positive ΔNb that resemble E -MORB. Basalts erupted after the hiatus all have positive ΔNb, they are manifestly more enriched than the pre-hiatus lavas and have high 3 He/ 4 He (up to 42 R a ), consistent with the presence of plume mantle beneath the relocated rift. The plume-derived basalts record contributions from two distinct enriched components (NWE1 and NWE2) and a depleted component (NWD1), which show geochemical signatures similar to those recorded in modern Icelandic basalts. Both the enriched mantle components have high 3 He/ 4 He ratios, and the depleted high- 3 He/ 4 He mantle component that dominates the proto-Iceland plume sampled by Baffin Island and West Greenland lavas is not evident in the Miocene sequence. The absence of 3 He/ 4 He correlations with trace elements or radiogenic isotopes suggests the high- 3 He/ 4 He source does not have a unique geochemical composition.