Experimental determination of trace element partition coefficients between spinel and silicate melt: the influence of chemical composition and oxygen fugacity

摘要

We present new experimentally determined trace element partition coefficients between spinel and silicate melt. The experiments were performed at atmospheric pressure and at temperatures between 1220 and 1450 degrees C. To study the effect of redox conditions on trace element partitioning, we performed experiments under different redox conditions, with fO(2) ranging from log -12 to log -0.7. The effect of different spinel compositions is also investigated. Our results show that spinel of all compositions readily incorporates the transition metals Ni, Co and Ga and the corresponding partition coefficients are >1. D-Ni,D-Co,D-Ga are not significantly affected by changing melt composition, crystal composition or redox conditions. However, the multivalent trace elements V and Mo show a strong effect of redox conditions on their partitioning behavior with D-V and D-Mo highest at very reducing conditions and considerably lower at more oxidizing conditions. Partition coefficients for the high field strength elements Ti, Zr, Hf, Nb, and Ta and the elements Sc and Lu strongly depend on crystal composition, with D-Ti and D-Sc >1 for very Fe3+-or Cr-rich (and Al-poor) spinels, but one to two orders of magnitude lower in systems with Al-rich spinels. We present some examples on how our data may be used to reconstruct redox conditions of spinel formation. We also present some results on the partitioning of Pt and Rh between spinel and melt. D-Rh depends strongly on redox conditions, while D-Pt is not significantly affected.