Competing effects of crystal chemistry and silicate melt composition on trace element behavior in magmatic systems: insights from crystal/silicate melt partitioning of the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh

Abstract

We present new partition coefficients for the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh between clinopyroxene, olivine and basaltic melt as a function of crystal chemistry and melt composition at temperatures of 1190–1300 °C and 1-bar pressure. Two components, namely $$\mathrm {Al_2O_3}$$ and $$\mathrm {Na_2O}$$ , were chosen to be investigated since they are known to affect the structure of silicate melts and especially clinopyroxene crystal chemistry. The amount of $$^{[4]}\mathrm{Al}$$ in clinopyroxene will result in an increase of $$D_i^\mathrm{{cpx/melt}}$$ even after applying a correction factor to account for the effect of melt polymerization. Moreover, the positive correlation between $$^{[4]}\mathrm{Al}$$ and $$D_i^\mathrm{{cpx/melt}}$$ is not restricted to the REE, but also applies for Sn, Ga, In, and Ba. The addition of up to 2.6 wt% $$\mathrm {Na_2O}$$ to the silicate melt universally increases the $$D_i^\mathrm{{cpx/melt}}$$ without any concomitant change in crystal chemistry or a significant effect in melt polymerization. This compositional effect is likely due to the ability of Na to break REE–Al complexes in the melt. Our results emphasize the importance of considering all variables that affect the behavior of trace elements in magmatic systems before applying the lattice strain model and derive meaningful results for the changes in the parameters of the crystallographic sites.