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The diffusive exchange of 30 trace elements during the interaction of naturalmafic and silicic alkaline melts was experimentally studied at conditionsrelevant to shallow magmatic systems. In detail, a set of 12 diffusion coupleexperiments have been performed between natural shoshonitic and rhyolitic meltsfrom the Vulcano Island (Aeolian archipelago, Italy) at a temperature of 1200{\deg}C, pressures from 50 to 500 MPa, and water contents ranging fromnominally dry to ca. 2 wt. %. Concentration-distance profiles, measured byLaser Ablation ICP-MS, highlight different behaviours, and trace elements weredivided into two groups: (1) elements with normal diffusion profiles (13elements, mainly low field strength and transition elements), and (2) elementsshowing uphill diffusion (17 elements including Y, Zr, Nb, Pb and rare earthelements, except Eu). For the elements showing normal diffusion profiles,chemical diffusion coefficients were estimated using a concentration-dependentevaluation method, and values are given at four intermediate compositions (SiO2equal to 58, 62, 66 and 70 wt. %, respectively). A general coupling ofdiffusion coefficients to silica diffusivity is observed, and variations insystematics are observed between mafic and silicic compositions. Results showthat water plays a decisive role on diffusive rates in the studied conditions,producing an enhancement between 0.4 and 0.7 log units per 1 wt.% of added H2O.Particularly notable is the behaviour of the trivalent-only REEs (La to Nd andGd to Lu), with strong uphill diffusion minima, diminishing from light to heavyREEs. Modelling of REE profiles by a modified effective binary diffusion modelindicates that activity gradients induced by the SiO2 concentration contrastare responsible for their development, inducing a transient partitioning ofREEs towards the shoshonitic melt.

Diffusive exchange of trace elements between alkaline melts: implications for element fractionation and timescale estimations during magma mixing