Time scales of crystal residence and magma chamber volume from modelling of diffusion profiles in phenocrysts: Vesuvius 1944

Diffusional smoothing of Fe–Mg compositional gradients in clinopyroxene crystals from the 1944 eruption of Vesuvius is used to determine the preeruption residence times of crystals at magmatic temperatures. The result is a distribution of crystal residence times from one sample that can be used to constrain magma chamber volumes. Diffusional zones between compositionally distinct cores and rims were observed by backscattered electron (BSE) imaging to vary in width from 1.6 to 11.9 μm and represent storage of crystals at magmatic temperatures for periods ranging from 4.5 months up to 9 years prior to eruption. The distribution of residence times is skewed to young ages and is best explained by the open-system behaviour of an 8.0×10 7 m 3 chamber that received an input of between 3.5×10 7 and 7.0×10 7 m 3 in the final 6 months prior to eruption. The calculated inputs, compared with contemporary observations of the emitted magma volumes, implies that some 3.0×10 7–6.5×10 7 m 3 of magma input did not erupt but was accommodated by chamber expansion before the 1944 eruption. This would place the preeruptive chamber volume between 1.15×10 8 and 1.5×10 8 m 3. The eruption was then triggered by a further input of ∼1.9×10 7 m 3 which forced the extrusion of a similar volume of relatively degassed magma as lava flows prior to the fire-fountaining episode. The backscattered electron imaging technique for residence time determination has great potential in unravelling the histories of populations of phenocrysts in many volcanic systems.