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

Abstract

Icelandic basalts have low oxygen isotope (δ18O) values compared to other ocean island localities. While this observation is often ascribed to the assimilation of low-δ18O crust, a low-δ18O mantle beneath Iceland has also been suggested. To discern crustal from mantle-derived signals, high-quality in-situ and bulk crystal δ18O 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 δ18Ool and indicators of melt evolution do not support a singular process responsible for lowering of δ18O values. However, correlations are observed between δ18Ool 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 δ18Ool values indicative of source-derived variability only. Although filtered, the dataset reveals, that δ18Ool values, significantly lower than the canonical depleted upper mantle value, are derived from the Iceland mantle. Coupled δ18Ool and 3He/4Heol measurements done on olivine crystals from the same samples demonstrate that low-δ18O components (down to δ18Oolivine = 4.2‰) are a trait of the modern Iceland plume and that low-δ18O and low-3He/4He components have become more apparent in the hotspot products since 60 Ma. Olivine chemistry characteristics suggest that this low-δ18O 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-δ18O oceanic lithosphere. Such material has become more apparent with time as is reflected in source-derived low-δ18O and high 3He/4He values in olivine from the modern Iceland plume. Moreover, the coincidence of the Iceland plume-head and the North Atlantic Rift at from ∼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 ∼25 Ma.