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

•We present high-precision oxygen isotope values for high-forsterite olivine.•We show that values down to 4.2±0.3‰ are a trait of the modern Iceland plume.•Lowering of He and O isotope values is attributed recycling of oceanic lithosphere.•Evidence of recycled lithosphere coincides with an increase in plume activity.•Tectonic changes enabled incorporation of recycled material into the Iceland plume.