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The nature of crustal xenoliths brought to the surface with the 2021 eruption in Fagradalsfjall

Bryndís Ýr Gísladóttir1, Edward Wayne Marshall1,2, William Charles Wenrich1,3, Enikő Bali1,4, Sæmundur Ari Halldórsson1, John Maclennan5, Halldór Geirsson1,4

  • Affiliations: 1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland; 2GeoZentrum Nordbayern, Friedrich‐Alexander‐Universität Erlangen‐Nürnberg, Erlangen, Germany; 3HS Orka, Svartsengi, 241 Grindavík, Iceland; 4Faculty of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland; 5Department of Earth Sciences, University of Cambridge, Cambridge, UK;

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 204

  • Programme No: 1.9.11

  • Theme 1 > Session 9

Abstract

We studied 23 xenoliths from the 2021 eruption in Fagradalsfjall, Reykjanes Peninsula, Iceland. Based on modal composition we distinguished gabbro, olivine-gabbro, leuco-gabbro and anorthosite. Plagioclase is the most abundant mineral in every xenolith. In gabbros, plagioclase features bimodal size distribution, with smaller euhedral grains and larger subhedral grains. Clinopyroxenes are sub to anhedral and interstitial among plagioclase±olivine. Olivines, if present are generally smaller than the other two minerals, are mostly subhedral. Spinels are inclusions in olivines and plagioclases. Gabbros have ophitic and poikilitic texture. In anorthosites the plagioclases are anhedral to subhedral with porphyroclastic texture and sometimes with bent albite twinning. Plagioclases in all xenoliths are Anorthite (An)-rich with An66 to An95, in gabbros we get a narrower range from An75 to An90. Olivine has forsterite contents of 82-95. Mg# and Cr2O3 in clinopyroxene ranges from 82-88 and 0.2 to 1.46 wt%, respectively. Pressure and temperature of crystallization was calculated by clinopyroxene-melt thermobarometry and plagioclase-melt thermometry (Neave and Putirka, 2017, Putirka 2008). Median temperature from clinopyroxene-melt thermometry is 1208±4.1 °C (SEE of 27°C). The median pressure is 3.36±0.32 (SEE of 1.4 kbar) kbars, corresponding to crystallization depth of 9.4 ± 0.9 km (assuming the crustal density is 2.8 g/cm3) for gabbros. Plagioclase-melt thermometry did not show differences between anorthosites and gabbros showing median temperature of 1232±8.1 °C (SEE of 36°C) for both rock types. These suggest that these xenoliths are sourced from the lower crust or perhaps the moho-transition zone as has been described for e.g.: the Oman ophiolite.