Diffusion of Sr in fluorphlogopite determined by Rutherford backscattering spectrometry

Abstract

We have measured the diffusivity of Sr in fluorphlogopite in a direction perpendicular to the basal planes (in the c direction). Annealing experiments were performed on single crystals, and most of them at ambient pressure. The penetration profiles of the diffusant were determined by Rutherford backscattering spectrometry. Sr diffusion parallel to c obeys an Arrhenius law, whose equation is: D Sr=2.7·10 −14 exp − 13 5900±3100 RT where D Sr is given in m 2 s −1 and activation energy in J mol −1. The values measured in fluorphlogopite overlap the range previously determined in other silicates, but the activation energy for fluorphlogopite is approximately half that of the other minerals for which diffusion data exist. A summary of the available data on diffusion in micas of all compositions shows that activation energies for species such as Sr, O, Rb and K fall in the same range, i.e. 100–175 kJ per mol. When plotted together, these data define two groups of parallel Arrhenian lines. One group consists of the measurements performed in the c direction, the other of those performed normal to the c direction. The distinction between the two groups is clear, and diffusion parallel to the c direction is slower by several orders of magnitude. The corresponding closure temperature for fluorphlogopite, when diffusion occurs in the c direction, is of the order of 700°C, relatively close to that of feldspars. On the basis of some simple assumptions about diffusion anisotropy in micas, we also estimated the closure temperature when diffusion takes place in a direction parallel to the basal planes. In that case, the value is noticeably lower, of the order of 400°C. However, this value, if applicable to other micas, is higher than what is currently accepted, which could have some implications for models of thermal histories or uplift rates of rocks.