ISOVALENT TRACE-ELEMENT PARTITIONING BETWEEN MINERALS AND MELTS - A COMPUTER-SIMULATION STUDY

We present a new approach for the rationalisation of trace element par titioning between silicate melts and minerals, which is not based on t he empirical, parameterised continuum models in common use. We calcula te the energetics of ion substitution using atomistic simulation techn iques, which include an explicit evaluation of the relaxation energy ( strain energy) contribution to this process. Solution energies are est imated for isovalent impurities in CaO, diopside, orthoenstatite, and forsterite. These show a parabolic dependence on ionic radius, similar to the variation of mineral-melt partition coefficients with ionic ra dius. The success of the empirical models, which often include only th e strain energy, appear to have been due to the partial cancellation o f energy terms, and to the empirical fitting of the parameters include d in these models. Our approach can be readily extended to aliovalent substitution.