Helium and oxygen isotopic variations in the Iceland plume source controlled by entrainment of recycled oceanic lithosphere

Icelandic basalts have low oxygen isotope (delta O-18) values compared to other ocean island localities. While this observation is often ascribed to the assimilation of low -delta O-18 crust, a low -delta O-18 mantle beneath Iceland has also been suggested. To discern crustal from mantle-derived signals, high-quality in-situ and bulk crystal delta O-18 measurements have been obtained from olivine crystals covering 16 Ma of activity at the Iceland hotspot. The results are combined with olivine (ol) major, minor and trace element chemistry. Relationships between delta O-18(ol) and indicators of melt evolution do not support a singular process responsible for lowering of delta O-18 values. However, correlations are observed between delta O-18(ol) values and indicators of crustal processes. Such patterns are used to filter out data that are likely to reflect effects from crustal assimilation to highlight delta(18)O(ol)values indicative of source-derived variability only. Although filtered, the dataset reveals, that delta O-18(ol) values, significantly lower than the canonical depleted upper mantle value, are derived from the Iceland mantle. Coupled delta O-18(ol) and He-3/He-4(ol) measurements done on olivine crystals from the same samples demonstrate that low -delta O-18 components (down to delta O-18(olivine) = 4.2%o) are a trait of the modern Iceland plume and that low -delta O-18 and low-He-3/He-4 components have become more apparent in the hotspot products since 60 Ma. Olivine chemistry characteristics suggest that this low -delta O-18 component is best sampled in melts that reflect contributions from pyroxenitic mantle lithologies, likely related to the recycling of oceanic lithosphere within the plume. An increase in plume flux, as traced by increasing plume temperatures and plume buoyancy after 35 Ma, led to enhanced entrainment of lower mantle material carrying recycled low -delta O-18 oceanic lithosphere. Such material has become more apparent with time as is reflected in source-derived low -delta O-18 and high He-3/He-4 values in olivine from the modern Iceland plume. Moreover, the coincidence of the Iceland plume-head and the North Atlantic Rift at from similar to 25 Ma likely assisted and further promoted enhanced plume-melting. Thus, the combination of changes in mantle upwelling and tectonic reorganisation of the North Atlantic led to the introduction of recycled oceanic lithosphere into the Iceland plume and the formation of the Iceland Plateau similar to 25 Ma. (c) 2022 Elsevier B.V. All rights reserved.