Isotopic evidence for interaction between Öræfajökull mantle and the Eastern Rift Zone, Iceland

Abstract

The off-rift central volcano of Oraefajokull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Oraefajokull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-207Pb/204Pb, low-143Nd/144Nd end-member that resembles Oraefajokull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Oraefajokull exhibit the most Oraefajokull-like isotopic compositions, implying that a homogenised Oraefajokull source with positive ∆207Pb is focused under the Oraefajokull centre and its associated flank zone. This then mixes laterally with the dominant negative-∆207Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ207Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in 206Pb/204Pb in mantle sources with negative Δ207Pb was probably generated by heterogeneity in 206Pb/204Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.