Diverse mantle components with invariant oxygen isotopes in the 2021 Fagradalsfjall eruption, Iceland

Abstract

The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ18O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable (mean δ18O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from “normal” upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland (e.g., the 2014–2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.

Evolving shallow conduit revealed by tremor and vent activity observations during episodic lava fountaining of the 2021 Geldingadalir eruption, Iceland

“…The time-averaged magma discharge is steady at about 7±2.5 m 3 /s (DRE, range, 2 to 10 m 3 /s, Bindeman et al (2022). The total lava field covers about 5 km 2 and has an approximate DRE rock volume of 0.1 km 3 (Bindeman et al 2022).…”

“…By the end of September 2021, the eruption has formed a volcanic cone rising about 120 m above the pre-eruption surface (Pedersen et al 2022). The time-averaged magma discharge is steady at about 7±2.5 m 3 /s (DRE, range, 2 to 10 m 3 /s, Bindeman et al (2022). The total lava field covers about 5 km 2 and has an approximate DRE rock volume of 0.1 km 3 (Bindeman et al 2022).…”

“…The red and dark red colour indicates undegassed and degassed magma, respectively, accumulating in a shallow-conduit container evolving from 2 to 11 May. Water content and storage depth as in Bindeman et al (2022).…”

“…The magma erupted at Geldingadalir is basaltic, with a whole rock composition containing 47.1 to 49.8 wt% SiO 2 and 8.6 to 9.7 wt% MgO (the latter is most likely due to slight variations in abundance of olivine macrocrysts). Its initial water content ranges from 0.38 to 0.53 wt% (Bindeman et al 2022). The estimated magma source depth ranges from 10 to 17 km or from a magma reservoir within the lower crust (Halldórsson et al 2022).…”

Supplemental Material: Deep magma mobilization years before the 2021 CE Fagradalsfjall eruption, Iceland

“…The samples contain a primitive (i.e., early crystallising) crystal cargo of olivine (Fo84-89), plagioclase (An78-90), clinopyroxene (Mg#82-88), and Cr spinel (Cr#39-50). Erupted melts (i.e., tephra glass) are of tholeiitic composition and have 7.8-8.7 wt% MgO, 1.0-1.3 wt% TiO2 and 1.8-2.0 wt% Na2O (Halldórsson et al, 2022;Bindeman et al, 2022). Macrocryst clusters contain direct internal planar boundaries between primitive olivine, clinopyroxene, and plagioclase cores that are not in chemical equilibrium with the external carrier melt.…”