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

Constraining the conditions and timescales of magma storage, mixing, and transfer—and understanding how deep-seated unrest progresses towards 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. Integrating olivine Fe-Mg diffusion chronometry 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 ~ 11–15 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 (15–20 km, median depth: 17 km) seismicity detected in the SNVZ since August 2024, consistent with potential 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 mobilization occurring within ~ 1.5 months. Re-modelling reversely zoned olivines with a Combined Diffusion-and-Growth (CDG) model shortens timescale estimates by ~ 60% compared to isothermal approaches, yielding refined durations of 7 days to 1.8 years (median ~ 22.5 days). Across the full dataset, however, median timescales cluster at 39–47 days. These records are broadly comparable to those in on-rift systems, suggesting off-rift and on-rift volcanoes may mobilize magma in more similar 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.