Temporal Fe-Zn isotopic variations in the chemically heterogeneous Fagradalsfjall eruption, 2021

Lavas from the 2021 Fagradalsfjall eruption, Iceland, show remarkable, day to month scale temporal variations in trace element and radiogenic isotopic compositions. Changes have been attributed to variation in the depth and degree of melting and/or source lithology, with progressive melting of a deeper, more enriched source as the eruption proceeded [1]. Distinguishing melting processes from source composition can be difficult to untangle using trace elements alone. Radiogenic isotopes are unaffected by the melting processes, but pinpointing lithological variations requires that the radiogenic isotopic compositions of the (unknown) endmembers are distinct and fairly restricted to be able to calculate relative contribution(s) to a lava.

Stable isotopic composition may provide another perspective on the cause of the clear temporal chemical trends in the eruption. For example, it has been proposed that Fe stable isotopes may detect the contribution of distinct mantle lithologies to a lava, due to the contrasting bonding environment of Fe in mantle minerals. Both empirical and theoretical studies show that at equilibrium, pyroxenite should be enriched in heavy Fe isotopes compared to typical mantle peridotite [e.g. 2]. Due to limited (<0.1‰) isotopic fractionation during mantle melting, unevolved basalts should capture this lithological variation. However, more recent theoretical work has argued that unrealistically high proportions of pyroxenite are needed to cause resolvable variations in basalt Fe isotopic composition [3]. Zinc stable isotopes provide a complementary system, with variation in Zn isotopic composition detected between garnet and spinel bearing lithologies [4], and without the added complexities of redox-driven fractionation that may affect Fe isotopes. The basaltic Fagradalsfjall eruption thus provides a unique time series to test whether the changes in trace element chemistry of the erupted lavas is mirrored by Fe-Zn isotopic variation. Variation in degree of melting alone is not expected to cause significant Fe-Zn isotopic fractionation, whereas a change in contribution to the lavas from pyroxene and/or garnet bearing lithologies may be reflected in the Fe-Zn isotopic composition. By combining redox sensitive (Fe) and redox insensitive (Zn) isotope systems we can potentially investigate magmatic processes in terms of the redox evolution of the source. We will present the Fe and Zn isotopic compositions of 15 fresh, glassy basaltic lavas collected during the first 4 months of the eruption. We will discuss the possible cause(s) of isotopic variations and how this adds to our understanding of the Fagradalsfjall eruption, specifically. Finally, this timeseries allows us to re-visit and evaluate the efficacy of using Fe-Zn isotopes to determine variations in mantle lithology.

[1] Marshall et al. (2021), AGU FM Abstract [2] Williams and Bizimis (2014), EPSL, 404, 396-407 [3] Soderman et al. (2021), GCA, 318, 388-414 [4] Wang et al. (2017), GCA, 198, 151-167