Off-axis volcanism Snaefellsnes Peninsula, Iceland – constraining source lithology, melt generation, and magma plumbing

Iceland is a volcanically active island that straddles the boundary between the North American and European tectonic plates in the North Atlantic. There are three primary mechanisms that facilitate melt generation in the mantle, decompression, additional heat (i.e., mantle plume), and the addition of volatiles/water, effectively lowering the melting temperature. Two of these processes, decompression and additional heat (from a mantle plume), are active in Iceland and explain the extensive volcanic activity focused the central and eastern areas of Iceland. However, the Snaefellsnes Peninsula in western Iceland, more than 60 km away from the central volcanic areas, has also experienced continuous volcanism throughout the last 12,000 years and why melt is generated away from the plate boundary is not clear. This goal of this study is to answer the broad question of why there are volcanic eruptions on Snaefellsnes? Previous hypotheses argued that the mantle under Snaefellsnes must be cooler and different in mineralogy to that beneath the central volcanic areas. This study found, relative to the central volcanic areas, this mantle under Snaefellsnes is slightly cooler (~100 °C) but still hotter than ambient mantle. This study also showed that the mantle mineralogy is similar throughout Snaefellsnes and the central volcanic zones. Detailed compositional analysis of lava flows from Ljosufjoll in the eastern half of the Snaefellsnes peninsula showed that the easternmost part shares a common melt source with the western portion of the central volcanic zones (Western Volcanic Zone). Lavas further to the west that make up the bulk of Snaefellsnes magmatism have different compositions that require a different mechanism of formation, but the details are obscured by mixing with crustal materials during melt storage and transport in the shallow crust.