Diffusion modelling of trace elements like Li, Mg and Sr in zoned plagioclases were often applied to reconstruct time scales of magma mixing, magma residence, cooling rates or ascent rates. These models rely on a model of Costa et al. (2003), which considers the coupling of trace element diffusion with the anorthite content. In this model the coupling is characterized by a single thermodynamic parameter that represents basically how the chemical potential of the trace component varies linearly with the anorthite content. However, neither the diffusion model nor the thermodynamic parameter directly inferred from the partitioning between plagioclases of different anorthite content were experimentally tested.

We therefore performed experiments with two types of diffusion couples composed of plagioclase single crystals with compositions An15 vs. An61 and An71 vs. An96. Experiments were performed in the temperature range 1038 – 1193 °C at 200 MPa in an internally heated pressure vessel. After the diffusion anneals cross sections of the couples were analysed by EMPA and TOF-SIMS to obtain concentration profiles of Mg and K. We were able to experimentally confirm the general behaviour predicted by the diffusion model of Costa et al. (2003). For example, we observe up-hill diffusion and a step in the concentration of Mg and K at the interface of the two plagioclase crystals, which is controlled by the thermodynamic factor of the trace element. The sign of this thermodynamic factor for Mg is different in the C1 and I1 stability field as was suggested by Mutch et al. (2022). However, the values of the thermodynamic factors for Mg and K determined by us are significantly higher than predicted in any of the previous indirect calibrations.

Costa et al. (2003) Geochim. Cosmochim. Acta 67, 2189-2200

Mutch et al. (2022) Geochim. Cosmochim. Acta 339, 173-189