Abstract
Magmatism in Iceland is explained by the interaction of the mid-Atlantic ridge with the Iceland plume. Holocene volcanism is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snaefellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The distribution of volcanic activity between the RZ and the SVB is not continuous as there is a gap of 60-80 km where no Holocene volcanoes are observed. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than MORB. Based on geochemical modelling and numerical simulations for melt extraction at mid-ocean ridge1, we suggest that the SVB alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing metasomatic hydrous cumulates. Incongruent melting of these cumulates can reproduce the alkaline compositions observed in the SVB. In contrast, for rift magmas, melt extraction models (super 1) suggest that lowdegree melts produced as far as approximately 65km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels and acquire specific chemical signature. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of the RZ lavas. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.