This study demonstrates that crystal size may be used to temporally constrain melt inclusion data. This approach is applied to the 1974 Vulcanian-type eruption of Volcan Fuego in Guatemala to study magma conditions immediately prior to eruption. The 1974 ash deposit shows significant variations in matrix glass composition, crystal size, crystal content, and volatile (H2O and CO2) content in each of four eruptive phases occurring over a 10-day period. Matrix glass and melt inclusions hosted in small (∼0.6 mg) olivine crystals have a particularly wide range in composition (basalt to andesite; 0.52–1.22 wt% K2O) and a wide range in volatile saturation pressures (∼1.0–4.4 kbar). That small (i.e., young) crystals host diverse melt inclusions demonstrates significant compositional variability in the magma system prior to eruption. The volatile saturation pressures indicate that magma was vertically distributed in the crust. During the eruption, different magmas from varying depths were hybridized, giving rise to short-term (hours to days) variation in the observed products.

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