Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations
Résumé
Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10−8.2 to 10−9.9 atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO2 dependent and decreased from 17 to 1 hour, as conditions became more reducing. The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to −0.98‰.amu−1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNiMetal-Silicate = 0.02 ± 0.04‰.amu−1. Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically. Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion.
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