Abstract
The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ(18)O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ(18)O values are invariable (mean δ(18)O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from "normal" upper mantle, in contrast to significantly lower δ(18)O values reported for erupted materials elsewhere in Iceland (e.g., the 2014-2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for (18)O-depleted mantle or interaction with low-δ(18)O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.