Reactive dissolution of plagioclase in a basaltic melt: A chronometer for pre-eruptive volcanic processes

Dissolution and reaction textures of plagioclase phenocrysts in basaltic rocks testify to perturbations of the magmatic system that are frequently associated with pre-eruptive mixing processes. Dissolution-reaction ex- periments performed at 150 MPa and 1150–1300◦C are used to examine and quantify the timescales of reactive dissolution of plagioclase in a basaltic melt. Simple dissolution occurs under high degrees of plagioclase undersaturation, whereas, at conditions near the plagioclase liquidus, reactive dissolution is expressed by a noticeable decrease in crystal size and the formation of An-rich reaction zones. The total amount of crystal dissolution (ddissolution) and the width of the reaction zone (dreaction) increase with time according to an expo- nential law, yet more rapidly than what predicted by assuming diffusion in plagioclase as the rate limiting factor. The remarkably fast dissolution rate (~10–8 m/s) is explained by the formation of planar dissolution interfaces that initially accelerate the dissolution process, whilst the increasing textural maturation of the reaction zone counteracts this effect. A chronometer for retrieving the timescales of reactive dissolution from the width of reaction bands and rims in plagioclase phenocrysts contained in basaltic rocks is derived from the experimental data. The application of this chronometer to Stromboli volcano (Italy), where reaction rims in plagioclase are attributed to the pre-eruptive mixing between a deeper volatile-rich magma (lp-magma) with a degassed magma residing at shallow depths (hp-magma), permits to determine a characteristic timescale of 161±43 min for mixing episodes preceding more energetic eruptions.