Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport, important characteristics of mid-ocean ridge magmatism. A consequence of such shallow crustal processing of magmas is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal...