| Summary: | The recent Fagradasfjall (Iceland) rifting event lasted for 6 months and brought to the surface primitive magma from an around 20km deep source [1]. We gathered olivine-hosted melt embayments from tephra produced by lava fountain events between de 21 th of March and 4 th of April 2021. We measured H 2 O, CO 2 and S diffusion profiles by nanoSIMS and obtained mean conduit magma ascent rates ranging between 0.006 and 0.26 MPa/s (0.2 to 10 m/s) from diffusion modeling. During magma ascent, melt embayment typically records simple diffusion profiles with volatile species diffusing from the core of the embayment to the outside. In a few cases, however, we found CO 2 and S profiles indicative of a slow volatile re-enrichment step at-depth. We interpret this behavior as downward magma movement prior to final ascent. We modified our EMBER software to model the impact of magma convection on embayment profiles and explored the timescales involved with such dynamic at the 2021 Fagradasfjall eruption.[1] Halldórsson et al., Nature, 602, 2022
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