The Fagradalsfjall volcano in Iceland is located on the Reykjanes Peninsular which forms the onshore portion of the Mid-Atlantic Ridge system and erupted several times in 2021.
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There is on average an eruption every 3–5 years in Iceland and prior to the 2021 Fagradalsfjall event, the last eruption occurred in 2014–2015. It was however not expected that a volcanic system on the Reykjanes Peninsula would erupt, as no eruptions had occurred in the region for almost 800 years. The Peninsula comprises four main volcanic systems, from west to east: the Reykjanes, Svartsengi, Krýsuvík, and Brennisteinsfjöll. Each of these systems is characterized by numerous strike-slip and normal faults, eruptive fissures and post-glacial lava flows.
While earthquake activity is high on the Peninsula eruptive periods occur infrequently; at intervals of 800–1000 years. These eruptions are generally effusive, although minor ash has been produced. On the Peninsula, the Fagradalsfjall volcanic system appears to have been the least active during postglacial times. The last prior eruption occurred over 6000 years ago and the system also has fewer faults and eruptions than the other volcanic systems in Iceland.
During the August 2021 eruption lava erupted mainly from a central cone, containing a lava pond, and flowed SE. An estimated 12 million cubic meters of lava was erupted. The lava near the vent was 20-40 m thick, but flows were 5-15 m thick in the Meradalir valley, outside the crater area.
The eruption comprises olivine tholeiite lava with whole rock MgO between 8.7 and 10.1 wt%. The macrocryst cargo comprises olivine up to Fo90, plagioclase up to An89 , and Cr-rich clinopyroxene up to Mg# 89. Gabbro and anorthosite xenoliths are rare. Olivine-plagioclase-augite-melt (OPAM) barometry of the groundmass glass from tephra yield high equilibration pressures and suggest that this eruption is originally sourced from a deep (0.48±0.06 GPa) storage zone at the crust-mantle boundary.
Over the course of the eruption, Fagradalsfjall lavas changed significantly in source signature. The first erupted lavas were more depleted (K2O/TiO2 = 0.14, La/Sm = 2.1, 87Sr/86Sr = 0.703108, 143Nd/144Nd = 0.513017, 206Pb/204Pb = 18.730) and similar in composition to basalts previously erupted on the Reykjanes Peninsula. As the eruption progressed, the lavas became increasingly enriched (K2O/TiO2 = 0.27, La/Sm = 3.1, 87Sr/86Sr = 0.703183, 143Nd/144Nd = 0.512949, 206Pb/204Pb = 18.839), having unusual compositions for Reykjanes Peninsula lavas and similar only to enriched Reykjanes melt inclusions.
The major, trace, and radiogenic isotope compositions indicate that binary mixing controls the erupted basalt compositions. The mixing endmembers appear to be depleted Reykjanes melts, and enriched melts with compositions similar to enriched Reykjanes melt inclusions or Snaefellsnes alkali basalts.Edward Marshall et al An overview of the geochemistry and petrology of the mantle-sourced Fagradalsfjall eruption, Iceland, EGU General Assembly 2022