Clinopyroxene megacrysts from Mount Etna's most destructive historic eruption. Triggers and timescales recorded in trace element zonation.

The 1669 flank eruption of Mount Etna (Sicily, Italy) was one of the most voluminous and destructive in its recorded history. This study focuses on cm-sized megacrysts of clinopyroxene, found in the 1669 scoria deposits. We present a multi-method approach to reconstruct magma dynamics in the plumbing system preceding the eruption. Petrological observations in combination with laser ablation ICP-MS mapping reveal sharp compositional zoning of the megacrysts, not seen in major element transects. Trace element data, including Cr, Ni, Zr and Rare Earth Elements, show that core, mantle and rim zones originated in distinct magmatic environments. Chromium-rich cores (400 - 1315 ppm) are in disequilibrium with the glassy host groundmass and indicate crystal inheritance from a primitive magma source. Oscillatory zoning in the mantle of the crystals suggests a sustained period of magma replenishment and crystallisation. Finally, ubiquitous Cr-rich rims (150-260 ppm) are in equilibrium with the groundmass and rich in melt inclusions, suggesting a final recharge event inducing rapid crystal growth and eruption. We found that Cr- enrichment at the rim is not coupled with increase in MgO, suggesting the eruption was triggered by a cryptic mixing, involving two melts of similar composition. Clinopyroxene-only thermobarometry was applied to approximate the conditions of crystallisation in each magmatic environment, returning temperatures of 1110-1140°C and pressures of 3-6 kbar (considering 2 wt.% H₂O in the melt). Timescales from mixing to eruption were calculated based on the thickness of clinopyroxene rims, where two distinct groups were identified. Average measured thicknesses of 236 μm and 160 μm at a growth rate of 10^-8 cm/s indicate a two-stage ascent, initiated a maximum of 32 days before the eruption onset.