A mineralogical approach to the 2021 Geldingadalir eruption of the Fagradalsfjall Fires,SW-Iceland – the study of water in clinopyroxene phenocrystals

The magmatic water content plays an important role in the evolution and explosivity of volcanic eruptions, due to the influence of water on magma density, viscosity, crystallization and melting temperatures. A reliable method for determining the pre-eruptive magmatic water content is to use nominally anhydrous minerals (NAMs) which can preserve various hydrogen configurations (as water proxy) in structural defects. This method allows to experimentally reconstruct the water lost during magmatic processes such as degassing and analyse it by infrared spectroscopy. Applying this to crystallographically oriented clinopyroxene crystals from lava samples collected in April 2021 from the Geldingadalir eruption, SW-Iceland, we obtain water contents of 0.69 ± 0.07 to 0.86 ± 0.09 wt. % H₂O. Because these values are higher than those expected for typical mid-ocean ridge basalts (MORB: 0.3 – 0.5 wt. % on average) it reveals a significant plume (OIB) contribution to the magma source. The implications of such water concentrations are that water saturation was attained only at very shallow levels within the plumbing system of the ascending Geldingadalir magmas. This can further explain the occurring pulsing behaviour of the lava pond and within the upper conduits, as the result of shallow, episodic, vapour exsolution.