Effect of fO2 on Eu partitioning between clinopyroxene, orthopyroxene and basaltic melt: Development of a Eu3+/Eu2+ oxybarometer

We present partition coefficients for Cs, Li, Sr, Ba, Zn, Mn, Co, Ni, Sc, Ga, Y, La, Nd, Sm, Eu, Tb, Yb, Lu, Ti, Zr, Hf, Ta, Nb and P for synthetic clinopyroxene and orthopyroxene crystallized from a Fe-free basalt at 1.5 GPa and four different oxygen fugacities (fO2) ranging from 6 log units above the quartz-fayalite-magnetite oxygen buffer (QFM + 6) to QFM-5 at temperatures of 1275–1300 °C. Partition coefficients for the rare earth elements (REE) vary as a function of their ionic radii, in agreement with lattice strain theory. We use the lattice strain model to evaluate partition coefficients for Eu2+ and Eu3+. Our results suggest that fO2 exerts a primary and important control on Eu, Ni and Co partitioning in clinopyroxene whereas fO2 seemingly has no recognizable effect on the partitioning behavior of these elements in orthopyroxene. However Eu anomalies in orthopyroxene, orthopyroxene Onuma diagrams and calculated proportions of Eu2+ in melts all show evidence that fO2 influence Eu partitioning also in orthopyroxene. We combined our results, available experimental data and predictive models for divalent and trivalent Eu clinopyroxene-melt and orthopyroxene-melt partitioning to parameterize fO2-dependent Eu partitioning models. The reported models reproduce measured Eu partition coefficients within a factor of two. Our model can be applied as clinopyroxene-melt and orthopyroxene-melt oxybarometers if Eu equilibrium partitioning between melt and pyroxene can be demonstrated.