EGU22-8304, updated on 30 Nov 2023
EGU General Assembly 2022
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

An overview of the geochemistry and petrology of the mantle-sourced Fagradalsfjall eruption, Iceland

Edward Marshall1, Maja Rasmussen1, Saemundur Halldorsson1, Simon Matthews1, Eemu Ranta1, Olgeir Sigmarsson1,2, Jóhann Robin1, Jaime Barnes3, Enikö Bali1, Alberto Caracciolo1, Guðmundur Guðfinnsson1, and Geoffrey Mibei1
Edward Marshall et al.
  • 1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, 101 Reykjavik, Iceland
  • 2Laboratoire Magmas et Volcans, Université Clermont Auvergne - CNRS, Campus Universitaire des Cézeaux, 6 Avenue Blaise Pascal, 63178 Aubière, France
  • 3Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA

The recent eruption of the Fagradalsfjall complex in the Reykjanes Peninsula of Iceland represents incompletely mixed basaltic magma directly erupted from a sub-crustal storage region. 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 collected from 28th April to 6th May 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 have changed significantly in source signature. The first erupted lavas (mid-March) 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 continued, the lavas became increasingly enriched and were most enriched in early May (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. From early May until the end of the eruption (18th September), the lava K2O/TiO2 and La/Sm compositions displayed a sinuous wobble through time at lower amplitude than observed in the early part of the eruption. The enriched lavas produced later in the eruption are more enriched than lavas from Stapafell, a Reykjanes eruption thought to represent the enriched endmember on the Reykjanes. The full range of compositional variation observed in the eruption is large – about 2.5 times the combined variation of all other historic Reykjanes lavas.


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. The physical mechanism of mixing and the structure of the crust-mantle boundary magmatic system is a task for future study.


In contrast to the geochemical variations described above, the oxygen isotope composition (δ18O) of the groundmass glass (5.1±0.1‰) has little variation and is lower than MORB (~5.5‰). Olivine phenocrysts δ18O  values range from typical mantle peridotite values (5.1‰) to lower values (4.6‰), with the lower values in close equilibrium with the host melt. Given the crust-mantle boundary source of the eruption, these low δ18O values are unlikely to represent crustal contamination, and are more likely to represent an intrinsically low δ18O mantle beneath the Reykjanes Peninsula.

How to cite: Marshall, E., Rasmussen, M., Halldorsson, S., Matthews, S., Ranta, E., Sigmarsson, O., Robin, J., Barnes, J., Bali, E., Caracciolo, A., Guðfinnsson, G., and Mibei, G.: An overview of the geochemistry and petrology of the mantle-sourced Fagradalsfjall eruption, Iceland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8304,, 2022.