The effect of diffusive re-equilibration time on trace element partitioning between alkali feldspar and trachytic melts

Our crystallization experiments reveal a strong influence of disequilibrium crystal growth on the partitioning of Ba and Sr. In particular, short-duration experiments show that rapid alkali feldspar crystal growth after nucleation, promotes disordered growth and less selectivity in the partitioning of compatible trace elements that easily enter the crystal lattice (e.g., Ba and Sr). This produces partition coefficients of compatible elements higher than those obtained through long-duration experiments, in which growth is slower and more selective. On approach to equilibrium, decreasing growth rate and timescales of diffusive re-equilibration facilitates the incorporation of compatible elements within the crystal lattice (through ordered growth), on the basis of the charge and size of the octahedral site, resulting in lower partition coefficients of Ba and Sr. Therefore, our results indicate that besides crystal-chemical effects, i.e. lattice strain, the substitution mechanisms during growth (with different degrees of ordering) of Ba and Sr in alkali feldspar are strongly influenced by diffusive re-equilibration and crystal growth kinetics. This implies that partition coefficients of Ba and Sr can be used to establish timescales of crystallization processes under pre- and syn-eruptive conditions. The application of our results to alkali feldspar in rocks from Campi Flegrei, constrain the magma residence time at subliquidus conditions in a reservoir to a maximum of 6 days under disequilibrium conditions and to a minimum of 9 days upon approaching near-equilibrium conditions.