Basalt production controlled by mantle source fertility at Fagradalsfjall, Iceland

Mantle melting processes and the characteristics of the source lithologies are mostly derived from basalt compositions of the mid-ocean ridge system and from oceanic islands. However, these basalts are in most cases the products of crustal processes resulting from magma storage, mixing, differentiation and crustal interaction. In Iceland, magma mixing and homogenization in thoroughly stirred magma reservoirs appear to be the norm, leading to restricted variations of Sr and Nd isotope ratio for a given volcanic system. In contrast, more primitive basalts were erupted during the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula with a large spread in isotope ratios. A strong negative correlation between Sr and Nd isotopes is observed from ratios that span a range from a depleted mantle composition to values akin to the Icelandic mantle such as that of the basalts of the Grímsvötn volcanic system. The isotope ratios are also correlated with the measured discharge rate during the eruption, with a depleted Sr isotope ratio appearing during the period of low discharge (around 5 m³/s) for the first month and a half of the eruption. In early May, the magma flux doubled and basalts with more radiogenic Sr isotope composition were produced. During the summer 2021, the Sr isotope ratios declined, due to lower proportions of melts from undepleted mantle source in the basalt mixture erupted. Whether the eruption ended when melts from the enriched mantle was exhausted or not remains to be elucidated, but clearly the highest eruption discharge rate resulted from melts of a more fertile mantle source.

The variable proportions of depleted versus enriched melts in the eruption products demonstrate the absence of a magma reservoir in which homogenization could take place, and from which decreasing discharge rate with time would be expected.  Instead, the initially low and steady and then increasing magma extrusion rate measured, strongly indicate direct mantle melt ascent to surface, which is also supported by the primitive mineralogy of the high-MgO basalt produced. Leaky-transform faults on the mid-ocean ridge system are characterized by eruptions of primitive basalts on intra-transform spreading centres (e.g. Garrett and Siqueiros fracture zones in the East Pacific). The Fagradalsfjall complex appears to be of similar nature, and the primitive magma and the important compositional and temporal variations demonstrate the effect of mantle source composition and associated processes on the eruption behaviour, as reflected in the magma discharge rate.