Experimental constraints on the interaction between hydrous and anhydrous magmas at Stromboli volcano.

Triggers of eruptions or important changes in eruptive style have been often associated to the arrival of batches of new undegassed magma in the volcanic plumbing system. This process has been proved to occur in basaltic and in more evolved magma. What happen in a magmatic reservoir and how do melts interact when this occur has been theoretical inferred or modeled using silicate analogues, thermodynamics or numerically. But only few laboratory experiments on molten silicates exist. Analogues of magmas, as syrups or waxes allow scaling of some important parameters like density contrast, rheology, and relative proportion between volumes of the interacting melts, but some other important process like diffusion, crystallization and gas exsolution is overlooked. These processes can significantly affect the mobility of magmas and the extent of mixing. This paper reports main results of laboratory experiments carried at high pressure and temperature on natural melts aimed to reproduce interaction between hydrous and dry magmas with same compositions. A grounded basalt from Stromboli is used as starting material in order gain insights on the dynamics of magmas within the plumbing system. In fact the interaction between hydrous crystal-poor and dry crystal-rich magmas is recognized to trigger violent paroxysmal eruptions in this volcano. A dry glassy (or slightly crystallized) cylinder was superimposed to an hydrous grounded glass in a single Au- Pd capsule. Charges were held for between 15 minutes and 13 hours at about 1130°C , 50 Mpa in a EHPV and then rapidly quenched isobarically. Capsules were cut longitudinally and interactions between melts were observed by SEM, EMP and FTIR. Results evidence that in runs with shorter duration (<1 hour) water diffusion dominates at the interface and strongly controls liquidus temperature, crystallization and rheology. Experiments of longer duration show buoyant plumes or blobs of water-rich melts that rise and mix with the upstanding dry crystal-rich portion. Plumes generate convection, enhances mixing and control crystals distribution along the whole capsule. Experimental results demonstrate that interaction between magmas with different water and crystal content can be successfully reproduced using natural materials. Obtained information can be fully generalized to understand dynamics of magmas within a large number of volcanoes.