Olivine/melt transition metal partitioning, melt composition, and melt structure—Influence of Al3+ for Si4+ substitution in the tetrahedral network of silicate melts

The influence on olivine/melt transition metal (Mn, Co, Ni) partitioning of Al3+⇔Si4+ substitution in the tetrahedral network of silicate melt structure has been examined at ambient pressure in the 1450–1550 °C temperature range. Experiments were conducted in the systems NaAlSiO4–Mg2SiO4– SiO2 and CaAl2Si2O8–Mg2SiO4–SiO2 with about 1 wt% each of MnO, CoO, and NiO added. These compositions were used to evaluate how, in silicate melts, Al3+⇔Si4+ substitution and ionization potential of charge-balancing cations affect activity–composition relations in silicate melts and mineral/melt partitioning.The exchange equilibrium coefficient, KD(M–Mg)olivine/melt=(XM/XMg)olivine/(XM/XMg)melt(M=Mn2+,Co2+,Ni2+), is a positive and linear function of melt Al/(Al + Si) at constant degree of melt polymerization, NBO/T  . The ∂[KD(M–Mg)olivine/melt]/∂[Al/(Al+Si)] is negatively correlated with the ionic radius, r, of the M-cation and also with the ionization potential (Z/r2, Z = electrical charge) of the cation that serves to charge-balance Al3+ in tetrahedral coordination in the melts. The activity coefficient ratio, (γM/γMg)melt, is therefore similarly correlated.These melt composition KD(M–Mg)olivine/melt relationships are governed by the distribution of Al3+ among coexisting Q-species in the peralkaline (depolymerized) melts coexisting with olivine. This distribution controls Q-speciation abundance, which, in turn, controls (γM/γMg)melt and KD(M–Mg)olivine/melt. The relations between melt structure and olivine/melt partitioning behavior lead to the suggestion that in natural magmatic systems mineral/melt partition coefficients are more dependent on melt composition and, therefore, melt structure the more alkali-rich and the more felsic the melt. Moreover, mineral/melt partition coefficients are more sensitive to melt composition the more highly charged or the smaller the ionic radius of the cation of interest.