Tracer versus trace element diffusion: Diffusional decoupling of Sr concentration from Sr isotope composition

Abstract

Chemical diffusivities for non-alkali major, minor, and selected trace elements (Sr, Y, Zr, and Nb) have been measured at 10 kbar, 1300°C during interdiffusion of dacitic and rhyolitic melts. Sr tracer diffusivities have been measured at the same conditions in melts of dacitic and rhyolitic composition and in a diffusion couple of those two melts undergoing simultaneous chemical diffusion. Chemical diffusivities were calculated by Boltzmann-Matano analysis of electron and X-ray microprobe traverses; tracer diffusivities were measured using radioactive 85Sr thin source techniques. Chemical diffusivities of all elements studied are functions of the silica content of the melt, but at any one silica content the chemical diffusivities of all elements studied are within a factor of 5 of the Si and Al diffusivities. Si diffusivity varies from 2.8 × 10 −9 cm 2/sec at 65% SiO 2 to 5.1 × 10 −10 cm 2/sec at 75% SiO 2. Diffusivities of alkalies, Na, K, and possibly Rb, are much greater than those of the non-alkalies, but were not determined in this study. The diffusivities of minor and trace elements are proposed to be controlled by the ability of the local melt structure to accommodate these elements while maintaining local thermodynamic equilibrium; this ability is in turn controlled by the SiO 2 and A1 2O 3 content of the melt at any location along the diffusion profile. Therefore, the diffusivities of Si and Al control the chemical diffusivities of all non-alkali elements. The measured Sr tracer diffusivities (4.1 × 10 −8 cm 2/sec) are an order of magnitude above Sr chemical diffusivities. No significant compositional effects are observed for Sr tracer diffusivities. Because diffusive transport of elements in a chemical potential gradient is rate-limited by chemical diffusivities and isotopic equilibrium of an element is rate-limited by self-diffusivities (presumed equal to measured tracer diffusivities), the differences between these tracer and chemical diffusivities can lead to decoupling between elemental concentrations and isotopic systematics for an element.