Geochemically enriched plume sources interacting with depleted, adiabatically upwelling mid-ocean ridge mantle allow for the tracing of processes of melting and melt transport through the mantle and crust. New major, trace element and Sr-Nd-Pb isotope data from glasses along the Southern Kolbeinsey Ridge and Tjörnes Fracture Zone, along with published data from the northern Iceland Theistareykir, Krafla and Askja volcanoes. We combine these with new whole rock and glass data from the early-Holocene Sveinar-Randarhólar fissure, which is located 60 km North from Askja volcano and 30 km east off-axis of the Northern Volcanic Zone rift. In the case of the recent plume-ridge interaction between the Kolbeinsey Ridge and the Northern Volcanic Zone, the preservation of the geochemical mantle heterogeneity is the result of a preferential extraction along existing structural weaknesses. The mixing of plume and mid-ocean ridge sources prior to melting is preserved because of limited crustal interaction of the melts. In contrasting, the Sveinar-Randarhólar fissure displays a remarkable major, trace element and petrological homogeneity over a total length of ~75 km indicating limited fractionation, and interaction with the neighbouring country rocks. We can show that the magmas were transported from a reservoir underneath Askja volcano at sub-surface levels for ~60 kms likely due to remnant ice coverage in the region before reaching surface levels. We conclude that the efficient transport of off-axis magmas is the result of a strong, pre-existing structural domain opening extensive pathways for magma transport over lateral distances of 10-100’s of kilometres.