Partitioning of the Transition Metals Mn, Fe, Co, Ni and Zn between Orthopyroxene and Silicate Melt : Ionic Size Effect to the Partitioning Behavior

Partition coefficients of Mn2+, Fe2+, CO2+, Ni2+, and Zn2+ between orthopyroxene and silicate melt have been determined in the system Mg2SiO4-SiO2-H2O at pressures of 1 and 3 GPa and at a temperature of 1400℃. The partition coefficient was defined by the ratio of the concentration of an element in orthopyroxene to that of the element in silicate melt. Partition coefficients were plotted on a partition coefficient versus ionic radius (PC-IR) diagram. Partition coefficients were linearly related to ionic radii. However, the partition coefficient of Zn deviated from the linear relationship between partition coefficients and ionic radii. This was called the Zn2+ anomaly. However, a partition coefficient versus metal and oxygen (M-O) distance (PC-MO) diagram was depicted instead of a PC-IR diagram. On a PC-MO diagram, partition coefficients were linearly related to M-O distances. An element with a small M-O distance are concentrated in orthopyroxene. The Zn2+ anomaly disappeared on PC-MO diagrams. The partitioning behavior between a site (an M1 or M2 site) in orthopyroxene and silicate melt was estimated by using the intracrystalline cation distribution data. These partition coefficients were plotted on both a PC-M(1)O diagram and a PC-M(2)O diagram. The element of a small M(1)-O distance was concentrated in orthopyroxene on the PC-M(1)O diagram. The lines were negative gentle slopes for the PC-M(2)O diagram. Partition coefficients between the M2 site in orthopyroxene and silicate melt were not almost dependent on the M(2)-O distance.