Chemical interdiffusion in Na-rich alkaline melts: insights from tephrite-phonolite couple experiments

Although alkaline melts represent an important fraction of the magmatism in ocean islands, there is a lack of chemical diffusion data in such compositions. We present here the result of a series of interdiffusion experiments using natural tephritic and phonolitic melts from the Canary Islands as endmembers. Experiments were run in two internally heated pressure vessels at a pressure of 300 MPa, temperatures from 1150 to 1300°C, oxygen fugacity from NNO to approx. NNO+3, and water contents between 0.3 and 3.3 wt.%. Major element concentration-distance profiles were measured by electron microprobe and effective binary diffusion coefficients (D) were obtained by a modified Boltzmann-Matano analysis.

Results show a wide range of diffusivities for different cations, following the sequence Na >> Al > K Mg = Fe = Ca > Si > Ti. Na diffusivities are the fastest and are consistently 1 log unit above those of Si for a given experiment. Al diffusion is remarkably fast, falling ca. 0.4 log units above Si, an effect not observed before in diffusion studies. The relationships between log D and H2O content in melt for all cations are strongly non-linear and can be fitted by using an exponential function, with an apparent convergence in diffusion coefficients towards the water-rich end. Our results provide new data on chemical interdiffusion in Na-rich, highly alkaline melts and allow to test chemical controls of magma mixing in Tenerife, where banded pumices are common in several bimodal ignimbritic units.