Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland

Abstract

Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport1–4, important characteristics of mid-ocean ridge magmatism1,5. A consequence of such shallow crustal processing of magmas4,5 is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust6,7. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 107–108 m3 of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems.

The eruption in Fagradalsfjall (2021, Iceland): how the operational monitoring and the volcanic hazard assessment contributed to its safe access

“…5a and 5b. Different gas compositions were measured by FTIR during and between lava fountaining events (Halldórsson et al 2022;Pfeffer et al, in preparation (2021a)) and SO 2 concentrations directly above the vent were much higher during fountaining (Halldórsson et al 2022;Pfeffer et al in preparation (2021a)). The following phase (III-c) saw the alternation of lava fountains and intra-crater activity accompanied by sustained lava outpouring.…”

The complex construction of a glaciovolcanic ridge with insights from the 2021 Fagradalsfjall Eruption (Iceland)

“…Reykjanes Peninsula lavas show La N /Sm N ranging from 1.20 to 1.67 (Kokfelt et al, 2006;Peate et al, 2009;Koornneef et al, 2012;Eason and Sinton, 2015;Halldórsson et al, 2016). Fagradalsfjall lavas show a wider range in La N /Sm N (1.24-2.74) and extend to lower REE abundances (Bindeman et al, 2022;Halldórsson et al, 2022). Using incompatible trace element ratios, Gee et al (1998) divided Reykjanes Peninsula samples into "depleted" (Nb/Zr < 0.08) and "enriched" (Nb/Zr > 0.08) groups.…”

“…During this first eruptive phase, effusion rates were low to moderate (Pedersen et al, 2022) and periodic fire fountaining from multiple vents built spatter cones and fed lava flows (Bindeman et al, 2022;Halldórsson et al, 2022). After about 3 months of activity, the eruption focused to one vent and effusion rates increased with more intense fire fountaining (Halldórsson et al, 2022;Pedersen et al, 2022). The expansion of the lava field was constrained by the topography of the surrounding ridges.…”

“…Intense earthquake swarms and changes in ground deformation patterns indicated the intrusion of a NE-SW-trending vertical segmented dyke with sustained magma inflow (Sigmundsson et al, 2022). On 19 March 2021, a fissure eruption began as an effusive outpouring of basaltic lava from closely-spaced vents (Bindeman et al, 2022;Halldórsson et al, 2022). After about a month of activity, five additional vents opened along a 1 km-long segment extending to the northeast (Bindeman et al, 2022;Pedersen et al, 2022).…”

“…After about a month of activity, five additional vents opened along a 1 km-long segment extending to the northeast (Bindeman et al, 2022;Pedersen et al, 2022). During this first eruptive phase, effusion rates were low to moderate (Pedersen et al, 2022) and periodic fire fountaining from multiple vents built spatter cones and fed lava flows (Bindeman et al, 2022;Halldórsson et al, 2022). After about 3 months of activity, the eruption focused to one vent and effusion rates increased with more intense fire fountaining (Halldórsson et al, 2022;Pedersen et al, 2022).…”