Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500 °C to 950 °C

The geochemical implications of mass-dependent fractionation of stable Ni isotopes in natural materials are difficult to assess because the inter-mineral equilibrium fractionation factors for Ni are unknown. We report experimentally determined equilibrium fractionation of Ni isotopes between Ni-metal and Ni-talc from 500 degrees C to 950 degrees C and 8 to 13 kbar using the three-isotope method. While significant isotope exchange was observed between metal and talc, little or no exchange occurred between Ni-metal and Ni-oxide under similar conditions. These results demonstrate the importance of chemical potential differences between the initial and final phase assemblages in facilitating isotope exchange in the experiments. Metal had greater Ni-62/Ni-58 than talc in all experiments and the equilibrium temperature dependence may be described by the relation Delta Ni-62(metal-talc) = 0.25(+/- 0.02) x 10(6)/T-2 (+/- 2se). Kinetic fractionation resulting from diffusion appeared to perturb isotopic equilibration at long time scales and/or high temperatures. These experiments place constraints on Ni isotope variations observed in natural systems. For example, Ni isotope fractionation between alloy and chondrule in the Ausson meteorite is larger than predicted for an equilibrium process at the canonical metamorphic temperatures of L5 chondrites. At 2500 K, the results imply a difference of 0.04 parts per thousand between equilibrated bulk silicate Earth and chondrites, indicating that Ni isotopes are not likely to be useful as tracers of planetary differentiation, unless fractionation occurred by a Rayleigh distillation process involving a well-mixed silicate reservoir. (C) 2012 Elsevier Ltd. All rights reserved.