Melt Transport Directly from the Mantle During the Fagradalsfjall Eruption, Iceland

The Reykjanes Peninsula in Iceland represents a subaerial leaky transform fault within the Mid-Atlantic Ridge system. Following over a year of tectonic unrest on the peninsula, an eruption began on 19 March 2021 in the Fagradalsfjall complex, which is one of five intra-transform spreading centers of the area. From March to mid-April the eruption was characterized by effusive activity with the emplacement of low-viscosity lava flows and sporadic tephra fall. Fire fountaining activity was prevalent in late April through May, followed by episodic activity with the flow of lava only. The eruption has produced an olivine tholeiite lava with whole rock MgO content of 8.8-9.7 wt%, carrying macrocrysts with primitive cores of Fo89.1 olivine, An89.2 plagioclase, and Cr-rich clinopyroxene with Mg# up to 88.8. During the first six weeks the minor and trace element composition of the lava changed remarkably with an increase in the incompatible element and radiogenic isotope ratios, suggesting change in the degree and depth of melting, as well as the source composition (Marshall et al. this session). The olivine-augite-plagioclase-melt (OPAM) and clinopyroxene-melt thermobarometers reveal that, in the early phase of the eruption, the groundmass glass equilibrated with the evolved rims of the crystal cargo at a low pressure of 0.160.05 GPa (~5-6 km) at a temperature of 11957 . This is consistent with the intrusion of a shallow dyke shortly before the eruption, as indicated by geophysical measurements. In contrast, primitive macrocryst cores equilibrated with whole-rock-like liquid and primitive melt inclusions at sub-Moho depth (>15 km), with the most probable storage conditions of 0.55 to 0.65 GPa (~18 km) and 124810 . The start of fire fountaining activity in late April occurred when the compositional shift towards a more enriched magma composition was complete (Marshall et al. this session). OPAM barometry based on tephra glass composition from 28th of April to 6th of May suggests that the magma erupted during this phase was directly sourced from the deep (0.48±0.06 kbar) sub-Moho storage zone. Thus, by this time the shallow dike that developed before the eruption was drained and a fracture system reaching down into the mantle enabled direct melt transport to the surface. References: Marshall et al. 2021, AGU FM abstract.