The influence of melt composition on the partitioning of trace elements between anorthite and silicate melt

The effect of melt composition on the partitioning of trace elements between anorthite and silicate melts has been studied experimentally in five compositions in the system CaO–Al ₂ O ₃ –SiO ₂ (CAS) at ~ 1400 °C and four compositions in the system CaO–MgO–Al ₂ O ₃ –SiO ₂ (CMAS) at 1332 °C. Melt composition has a significant impact on the substitution of trace elements into anorthite, particularly if the trace-element substitution is aliovalent and requires a charge balance for substitution. Melt composition strongly influences the partitioning of the trivalent rare earth element (REE) cations into the large-cation site (M) of anorthite. Due to charge balance requirements, the activity of alumina in the melt is the most important compositional variable for the REE partitioning in anorthite. Scandium, another trivalent cation, is much more compatible than is predicted for trivalent cations partitioning on the M-site. Therefore, scandium is likely partitioning onto the tetrahedral site in place of aluminium, which requires no charge balance and therefore is not affected strongly by melt composition. Similarly, the partitioning of the small divalent cations (Be and Mg) show a stronger relationship with changing melt composition than the large divalent cations (Ca, Sr, and Ba) and therefore are likely to partition on the tetrahedral site (T) of plagioclase rather than the large-cation site (M). Detailed thermodynamic modelling of the effects of melt composition is required for an adequate parameterization of trace-element mineral/melt partition coefficients, in addition to models of the effects of mineral composition.