Kinematic significance of morphological structures generated by mixing of magmas: a case study from Salina Island (southern Italy)

Morphological features of mixing/mingling structures of heterogeneous juveniles from the 13-ka-old Upper Pollara eruption (Salina Island, southern Italy) are studied. These heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma. Concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion. Results show that the Upper Pollara rocks were produced by mixing 24–49% of andesite and 76–51% of rhyolite. Multifractal analysis of the mixing structures is performed and the mixing intensity, i.e. the degree of interaction between the two end-member magmas, has been deduced from the analysis of the singularity spectrum. Results show that narrower is the multifractal spectrum, the higher is the degree of homogenization of magmas. Reynolds number (Re) during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and Re. Results show that Re is between ca. 500 and 7000. There is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and Re, in agreement with the observation that the higher is the percentage of the mafic, lower viscosity magma, the higher is the turbulence of the mixing system. In addition, our analysis reveals an unexpected inverse relationship between the calculated Re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency. Results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit. It is also suggested that mixing occurred in a shear layer-type or pipe-type flow.