The effect of melt composition on mineral-melt partition coefficients: The case of beryllium

• Be is incompatible in olivine; DBe varies by a factor of 2 with melt composition.
• Beryllium exhibits unique geochemical behaviour among the divalent cations.
• Activity in melt may be controlled by coordination number.

The divalent cation Be2 + is considerably smaller than other divalent cations (Mg2 +, Fe2 +, Ca2 +, et cetera), leading to a strong preference for tetrahedral coordination in minerals. Its thermodynamic properties in silicate melts may accordingly differ from these other divalent cations, potentially distinguishing its mineral/melt partition coefficients. In order to investigate this possibility, the partitioning of Be between silicate melt and forsterite was examined for 16 melt compositions in the systems CaO–MgO–Al2O3–SiO2 at 1400 °C with additional experiments to investigate the effect of added Na2O and TiO2, and temperature at 1300 °C. The relative activity coefficient of BeO in the melts decreases with increasing CaO and NaO1.5. The results are compared to Mg and Ca partitioning in the same experiments, and to the partitioning of other divalent cations (Ni, Co, Mn) from the literature. While the partition coefficient of the latter correlate positively with the Mg partition coefficient, Be shows only a weak negative correlation. Compared to Ca, Be partitions less strongly into forsterite when the melt has high Na and/or Ca. Partition coefficients for Na, Al and Ti are also reported.