Basaltic magma dynamics during the 2021-2023 eruptions at Reykjanes Peninsula, Iceland, constrained by Sr, Nd and Th isotope ratios

The composition of MgO-rich basalt produced during the six-months long 2021 eruption at Fagradalsfjall, Reykjanes Peninsula, varied from those with low K₂O/TiO₂ and incompatible element contents to basalt enriched in incompatible elements with high K₂O/TiO₂ over a timespan of a month, explained by binary mixing of different mantle melts (Halldórsson et al., 2022). For the rest of the eruption, the basalt composition varied in a periodic manner as illustrated by long-lived radiogenic isotope ratios. For example, the periodicity of ⁸⁷Sr/⁸⁶Sr from early May to middle of September 2021 was 52 days, likely representing the time elapsed from the final melt mixing at depth (the moment of acquiring the measured isotope composition) until eruption at surface. Preliminary results on Th isotope ratios in the 2021 Fagradalshraun yield relatively high Th isotope ratios [(²³⁰Th/²³²Th) = 1.30] in the first lava emitted (the low K₂O/TiO₂ basalt) relative to basalt erupted at the end of the eruption [(²³⁰Th/²³²Th) = 1.18]. While this observation could be explained by slower mantle melting, excellent correlation between Th and Nd isotope ratios confirms the importance of binary mixing of melts from a depleted mantle source (with high Th and Nd, and low Sr isotope ratios) with melt(s) from enriched mantle lithology (having lower Th and Nd, and higher Sr isotope ratios) before magma ascent and eruption. Consequently, mantle heterogeneity, melt aggregations and mixing dominate the composition of the erupted basalt, rather than the duration of mantle melting. Correlation between Sr isotope ratios and time-averaged discharge rate indicate mantle source fertility control on the discharge rate during the 2021 eruption.  Both the 2022 and the summer 2023 eruptions started with a stronger discharge rate than that of 2021 and produced basalt of similar composition as that emitted in September 2021, interpreted as a result of an establishment of crustal magma accumulation zone since at the end of the 2021 eruption. Moreover, whether the exhaustion of the low K₂O/TiO₂ and ⁸⁷Sr/⁸⁶Sr magma reservoir led to the end of the 2021 eruption remains to be explored.