COMPARISON OF ELEMENT AND ISOTOPE DIFFUSION OF K AND CA IN MULTICOMPONENT SILICATE MELTS

Recent experimental work has shown that the homogenization of elementa l concentrations can be much slower than that of isotopic ratios when there are strong concentration gradients in SiO2 and Al2O3. The ramifi cations of this result for magma homogenization and other petrological problems related to diffusion are significant. We report here a compa rison of experimental profiles of elemental concentrations and isotopi c fractions of K and Ca in rhyolite-andesite (large concentration grad ients) and rhyolite-rhyolite (small concentration gradients) melt coup les. When the concentration profile and the isotopic profile of the sa me element in a single couple are compared, the former is much shorter than the latter in the rhyolite-andesite couple, consistent with othe r recent studies. However, the lengths of both concentration and isoto pic profiles are similar in the rhyolite-rhyolite couple. Therefore, d iffusion of an element or oxide may be decoupled from or coupled with isotopic 'diffusion', depending on whether large concentration gradien ts of major components are present. When the two couples are compared, the intrinsic effective binary diffusivities obtained from isotopic p rofiles are similar for each element in the two couples, whereas the e ffective binary diffusivity of K obtained from the concentration profi le in the rhyolite-rhyolite couple is 37 times that in the rhyolite-an desite couple. Therefore, isotopic homogenization is roughly independe nt of elemental homogenization and the presence of SiO2, Al2O3, and ot her concentration gradients, whereas elemental homogenization is stron gly affected by concentration gradients. Our experimental data (isotop ic and concentration profiles including uphill diffusion profiles) can be modeled quantitatively to a good approximation using a modified ef fective binary diffusion model in which the flux of a component is ass umed to be proportional to its activity gradient instead of its concen tration gradient. Therefore, the multicomponent diffusion effect in th e silicate systems of our experiments seems to be largely due to contr ibutions of non-ideal mixing to the cross-terms of the diffusivity mat rix.