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3.4 e. Chemical diffusion in granitic melts. II. Diffusivities of Be, B, Mg, Ca, Ti, Ge, Sr, Y, Zr, Nb, Cs, Ba, Nd, Tb, Lu, Hf, Ta, and W at trace concentrations (J.E. Mungall and D.B. Dingwell, in collaboration with M. Chaussidon/Nancy)

The rates and mechanisms of diffusion of minor constituents of natural magmas are relevant to a number of problems in igneous petrology, and in the study of silicate melts in general. Chemical diffusion controls, among other things, the partitioning of trace elements between melt and rapidly growing crystals, the efficiency of contamination of magmas by xenoliths and wall-rocks, the rate of homogenization of comingled magmas, and the partitioning of volatile elements between melt and vapour during rapid vapour exsolution. Comparisons of rates of diffusion of different elements with such parameters as ionic radius and charge can lead to insights into the details of their solvation and transport mechanism, and into the structure and atomic-scale dynamics of the host melts.

We have measured about 200 diffusion coefficients in ten experiments conducted on melts of three different compositions, using an infinite couple configuration. Building on our earlier success at modelling trace element diffusion using the Eyring equation relating viscosity to diffusivity of network-forming cations in one-component systems, we have developed an extended form of the Eyring equation, appropriate for multicomponent melts. For those elements whose solution properties are known well enough to allow us to implement the modified multicomponent Eyring equation, we can reproduce our measured diffusion coefficients within an order of magnitude.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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