THE EFFECT OF OXYGEN FUGACITY ON THE PARTITIONING OF NICKEL AND COBALT BETWEEN OLIVINE, SILICATE MELT, AND METAL

Partitioning of nickel and cobalt between olivine, silicate melt, and metal has been determined in the system olivine-albite-anorthite-CaO at 1350-degrees-C and 1 atm over the oxygen fugacity (f(O2) range 10(-5.5) to 10(-12.6) atm. This range of f(O2), from one log unit above Nickel-Nickel Oxide (NNO + 1) two log units below the Iron-Quartz-Fayalite (IQF-2) buffers, spans the range relevant for crystal/liquid processes in terrestrial planets and meteorite parent bodies. There is no effect of oxygen fugacity on D(Ni)Ol/gls over the range of f(O2) from 10(-5.5) to 10(-9.4). Over the range of f(O2) from 10(-9.4) to 10(-10.3) D(Ni)Ol/gls shows an apparent decrease from a value of 6.8 at log f(O2) = -9.4 to a value of 5.5 at log f(O2) = -10.3, a factor of 1.2. At the lowest value of f(O2), = 10(-12.6) the value for D(Ni)Ol/gls is 3.4, a factor of 2 lower than that measured under oxidizing conditions. Experimentally produced melts are metal saturated over the range of f(O2), from 10(-10.3) to 10(-12.6). partition coefficients for Co are constant (D(Co)Ol/gls = 2.8) over the range of f(O2) from 10(-6.6) to 10(-10.6). The Co partition coefficient decreases to a value of 1.65 at log f(O2) = -12.2. The variations in D(Ni)Ol/gls are examined using equilibrium constants that account for the effects of variable silicate melt composition on Ni partitioning. Measured values of D(Ni)Ol/gls and values estimated from equilibrium constants are identical for the experiments carried out under oxidizing conditions. The predicted values of D(Ni)Ol/gls for the low f(O2) experiments are higher than the measured values. These changes in Ni and Co partitioning behavior at low f(O2) may indicate that Ni and Co are present as Ni2+ and Ni0 and Co2+ and Co0 in the silicate melt. We estimate that approximately 30% of the Ni may be present as Ni0 in the low f(O2) experiments.