  The interdiffusion of six major elements (Si, Ti, Fe, Mg, Ca, K) between
natural shoshonite and a high-K calc-alkaline rhyolite (Vulcano island, Aeolian
archipelago, Italy) has been experimentally measured by the diffusion couple
technique at 1200{\deg}C, pressures from 50 to 500 MPa and water contents from
0.3 (nominally dry) to 2 wt%. The experiments were carried out in an internally
heated pressure vessel, and major element profiles were later acquired by
electron probe microanalysis. The concentration-distance profiles are evaluated
using a concentration-dependent diffusivity approach. Effective binary
diffusion coefficients for four intermediate silica contents are obtained by
the Sauer-Freise modified Boltzmann-Matano method. At the experimental
temperature and pressures, the diffusivity of all studied elements notably
increases with dissolved H2O content. Particularly, diffusion is up to 1.4
orders of magnitude faster in a melt containing 2 wt.% H2O than in nominally
dry melts. This effect is slightly enhanced in the more mafic compositions.
Uphill diffusion was observed for Al, while all other elements can be described
by the concept of effective binary interdiffusion. Ti is the slowest diffusing
element through all experimental conditions and compositions, followed by Si.
Fe, Mg, Ca and K diffuse at similar rates but always more rapidly than Si and
Ti. This trend suggests a strong coupling between melt components. Since
effects of composition (including water content) are dominant, a pressure
effect on diffusion cannot be clearly resolved in the experimental pressure
range.
