Modelling the Sr isotopic evolution of melts produced by fractional melting

A model is proposed to calculate the 87Sr/86Sr ratio in melts produced by ideal fractional partial melting of rock systems. In ideal fractional melting, during melt formation, diffusion (among crystals in the residue and between crystals in the residue and the melt) is absent, so that the evolution of the 87Sr/86Sr ratio in the melt is governed by mineral stabilities and by the self-production of radiogenic 87Sr due to 87Rb decay. The contribution of intra-melt production of radiogenic 87Sr has been considered in the calculations because it may be non-negligible in high Rb/Sr melts. In the model, the 87Sr/86Sr ratio in the aggregate melt is calculated considering the contribution of each mineral phase involved in melt formation as ideally separated from the others and then summing the contributions. This approach has two advantages: 1) it avoids serious uncertainties which would be introduced if composition were calculated by mixing an unknown number of unknown melts in unknown proportions; 2) equations designed to describe the behaviour of the 87Sr/86Sr ratio in the aggregate melt in modal melting processes may also be used for non-modal ones (until one of the minerals in the melting assemblages is completely consumed). Calculations indicate that Rb and Sr mineral/melt fractionation coefficients may play an important role in determining the value of the 87Sr/86Sr ratio in the aggregate melt. Different values of the fractionation coefficients may determine very different evolution paths for the isotopic ratio, in both modal and nonmodal processes and, if Sr is chemically fractionated during mineral melting, large Sr isotope differences may be generated between melts and respective source rocks, even if melting is modal.