Crystal-Mush Remobilization Timescales and Magma Storage Depth in the Snæfellsnes Volcanic Zone (W-Iceland): Insights from Olivine Fe-Mg Diffusion Chronometry and Fluid Inclusion Barometry

Abstract

Constraining the conditions and timescales of magma storage, mixing, and transfer - and understanding how deep-seated unrest progresses toward eruption - is key to deciphering early stages of magma system reactivation. While these processes are increasingly well resolved in Iceland’s on-rift zones, magma dynamics in off-rift flank zones remain poorly understood, despite their potential for hazardous explosive eruptions after long periods of dormancy. We investigate magma storage depths and pre-eruptive timescales of magma mobilization and crystal-mush erosion in the Snæfellsnes Volcanic Zone (SNVZ), a relict Tertiary volcanic belt hosting the most extensive off-rift volcanism in Iceland. Using a Combined Diffusion-and-Growth (CDG) olivine diffusion approach together with fluid inclusion and clinopyroxene barometry, we reconstruct storage conditions preceding the Holocene Búðahraun and Berserkjahraun eruptions. Our results identify a key magma storage zone at ~12-14 km depth. The absence of fluid inclusions recording shallow magma storage suggests rapid olivine entrainment and swift magma ascent from mid-crustal depths to the surface. These storage depths broadly align with deep (median: 17 km) seismicity detected in the SNVZ since August 2024, consistent with reactivation of a mid-crustal magma domain by ongoing mantle-derived melt intrusions. Olivine diffusion chronometry indicates mush erosion began ~4.9 and ~1.8 years before the Búðahraun and Berserkjahraun eruptions, respectively, with final mobilisation and ascent occurring within ~1.5 months. Compared to isothermal models, the CDG method - accounting for heating, cooling, and growth - reduces timescale estimates by ~60%, yielding refined durations of 7 days to 1.8 years (median: 22.5 days). These short timescales, similar to those in on-rift systems, suggest off-rift and on-rift volcanoes may mobilize magma in more comparable ways than previously thought. Given ongoing seismicity in the SNVZ, our findings highlight the need for enhanced monitoring and provide a framework for comparing magma dynamics in on- and off-rift settings, improving hazard assessment for systems without historical eruption records