Heat and Helium in the early Iceland plume

The North Atlantic igneous province preserves a complete magmatic record from its inception at 61 Ma through to present activity in Iceland and provides a unique natural laboratory for the study of LIPs. The earliest magmatism was synchronous across a pre-drift area extending almost 2000 km from Baffin Island (BI), through West Greenland (WG), to Scotland. This start-up phase resulted in the eruption of voluminous picrite lava flows in BI and WG, and these provide information on the temperature and composition of the early Iceland plume. Major-element composition of BI and WG picrites suggests eruption- and mantle potential temperatures of at least 1400°C and 1500°C, respectively. These are comparable to estimates for the Ontong Java Plateau, and at least 100°C higher than those for mid-ocean ridge basalt (MORB). The picrites, however, are very similar to MORB in their ¹⁴³Nd/¹⁴⁴Nd and incompatible trace-element abundances and ratios. Normal (NMORB) and relatively enriched (EMORB) types are present in both areas. Crustal contamination is negligible in most of the picrites, and it is not possible to derive the EMORB type by contamination of NMORB-type magma with any plausible composition of continental crust. BI and WG picrites have the highest ³He/⁴He (up to 50Ra) yet measured in terrestrial basalts. These high values were previously thought to be restricted to the more depleted (NMORB) types but we now report data showing high ³He/⁴He also in EMORB types from both BI and WG. This observation appears to contradict models of He-isotope evolution in which primitive ³He is stored in ancient, highly depleted mantle domains. The early Iceland plume appears to have had a composition very similar to the mantle beneath mid-ocean ridges. It differed only in its high temperature and extreme ³He/⁴He, suggesting that primitive ³He was introduced from a hot reservoir with high ³He/(U Th). If the Earth's core is the source of the heat and ³He, then the deep mantle must have a similar bulk composition and degree of heterogeneity to the upper mantle.