Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500°C to 950°C
2012
Lazar, Codi | Young, Edward D. | Manning, Craig E.
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°C to 950°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 ⁶²Ni/⁵⁸Ni than talc in all experiments and the equilibrium temperature dependence may be described by the relation Δ⁶²Niₘₑₜₐₗ₋ₜₐₗc=0.25(±0.02)×10⁶/T² (±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 2500K, the results imply a difference of 0.04‰ 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.
Show more [+] Less [-]AGROVOC Keywords
Bibliographic information
This bibliographic record has been provided by National Agricultural Library