Diego González-García , Florian Pohl, Felix Marxer, Stepan Krasheninnikov, Renat Almeev, Francois Holtz

The diffusive exchange of major elements in Na-series tephrite-phonolite diffusion couples with compositions relevant to Canary Islands magmatism were determined at 300 MPa and variable H2O concentrations (0.3 to 3.3 wt.%), temperature (1150 to 1300°C) and fO2 (NNO-1.5 to NNO+1.7). Composition-dependent effective binary diffusion coefficients (D) were determined from concentration-distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na >> Al >> K ≥ Mg = Fe = Ca > Si > Ti, with a mild diffusivity contrast (0.2-0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling ~1.0 log unit above those of Si for any given conditions. An anomalously fast Al diffusion is observed, with DAl falling ~0.4 log units above Si and ~0.6 log units below Na, suggesting a prevalence of Al-alkalis coupling across our range of run conditions. The relationships between log D and H2O content in melt for all cations in an intermediate composition are strongly non-linear and can be fitted using an exponential function, with a convergence in diffusion coefficients for different temperature with increasing H2O contents. Thus, Arrhenius analyses result in a decrease of activation energies from 222-293 at 1.7 wt% H2O to 48-112 kJ/mol at 3.0 wt% H2O. These results provide new data on chemical interdiffusion in Na-rich, highly alkaline melts and suggest that H2O contents play a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignmimbrite, Tenerife, where banded pumices involving basanitic-tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units.