The effect of silica activity on the diffusion of Ni and Co in olivine

Irina Zhukova*, Hugh StC O'Neill, Ian H. Cambell, Matt R. Kilburn

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    38 Citations (Scopus)

    Abstract

    The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg2SiO4) from 1,200 to 1,500 °C at the oxygen fugacity of air, along [100], with the activities of SiO2 and MgO defined by either forsterite + periclase (fo + per buffer) or forsterite + protoenstatite (fo + en buffer). Diffusion profiles were measured by three methods: laser-ablation inductively-coupled-plasma mass-spectrometry, nano-scale secondary ion mass spectrometry and electron microprobe, with good agreement between the methods. For both Ni and Co, the diffusion rates in protoenstatite-buffered experiments are an order of magnitude faster than in the periclase-buffered experiments at a given temperature. The diffusion coefficients D M (M = Ni or Co) for the combined data set can be fitted to the equation: (Formula presented.) with Ea(Ni) = - 284.3 kJ mol-1 and Ea(Co) = - 275.9 kJ mol-1, with an uncertainty of ±10.2 kJ mol-1. This equation fits the data (24 experiments) to ±0.1 in log D M. The dependence of diffusion on aSiO2 is in agreement with a point-defect model in which Mg-site vacancies are charge-balanced by Si interstitials. Comparative experiments with San Carlos olivine of composition Mg1.8Fe0.2SiO4 at 1,300 °C give a slightly small dependence on aSiO2, with D ∝ (aSiO20.5), presumably because the Mg-site vacancies increase with incorporation of Fe3+ in the Fe-bearing olivines. However, the dependence on fO2 is small, with D ∝ (fO2)0.12±0.12. These results show the necessity of constraining the chemical potentials of all the stoichiometric components of a phase when designing diffusion experiments. Similarly, the chemical potentials of the major-element components must be taken into account when applying experimental data to natural minerals to constrain the rates of geological processes. For example, the diffusion of divalent elements in olivine from low SiO2 magmas, such as kimberlites or carbonatites, will be an order of magnitude slower than in olivine from high SiO2 magmas, such as tholeiitic basalts, at equal temperatures and fO2.

    Original languageEnglish
    Article number1029
    Pages (from-to)1-15
    Number of pages15
    JournalContributions to Mineralogy and Petrology
    Volume168
    Issue number2
    DOIs
    Publication statusPublished - Aug 2014

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