Decoupled convection cells from mixing experiments with alkaline melts from Phlegrean Fields

Geochemical evidence for the possibility of pre-eruptive magma mixing in the Phlegrean Fields (Campi Flegrei) volcanic system has prompted us to study the efficiency of the physical mixing of magmas of similar compositions. We have performed mixing experiments of trachytic to phonolitic trachytic compositions. To enhance mixing, the initial magmas contained partially dissolved crystals. The experiments took the form of a time series at 1300 °C and stirring at 0.5 rotations per minute. The two starting compositions were loaded as cylinders and convection was forced via rotation of a spindle. After 16 and 25 h, under constant stirring, Al-Ti-Fe spinel crystals were still present and locally traced the flow directions. Two separate convection cells originated: a lower one with a primary flow parallel to the bottom and side walls of the crucible, and an upper one, with a primary flow approximately at right angle to that of the lower cell. Electron microprobe analyses of the products indicate a complex layering of cells bounded by clear gaps in oxide ratios and containing compositional gradients. At the cell interface, all analyzed oxides exhibit a horizon, where oxides contents range over 2% (SiO₂ and Na₂O). The very low Reynolds numbers (10^-7 to 10^-9) in these experiments point towards mixing under laminar flow conditions. In the absence of a significant temperature gradient, convection in our experiments was driven primarily by the applied forced convection combined with the effect of local compositional gradients (diffusion) along the sample leading to a density distribution similar to a double diffusive system. Comparisons with natural systems is challenging due to scale restrictions arising from the small size and low rotation speed. Notwithstanding, our results are in good accordance with the geochemical data from Phlegrean Fields natural glasses from the literature, confirming the possible importance of magma mixing in its evolution.