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

作者：Zhukova Irina^*; O'Neill Hugh StC; Cambell Ian H; Kilburn Matt R

来源：Contributions to Mineralogy and Petrology, 2014, 168(2): 1029.

DOI：10.1007/s00410-014-1029-z

• Diffusion
• Olivine
• Silica activity
• Divalent cations
• Vacancies
• LA-ICP-MS
• NanoSIMS

The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg2SiO4) from 1,200 to 1,500 degrees 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: laserablation 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:
log D-M in m(2) s(-1) = -6.77(+/- 0.33) + Delta E-a(M)/RT + 2/3 log a(SiO2) with E-a(Ni) = -284.3 kJ mol(-1) and E-a(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 a(SiO2) 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 degrees C give a slightly small dependence on aSiO(2), with D proportional to (a(SiO2)(0.5)), presumably because the Mg-site vacancies increase with incorporation of Fe3+ in the Fe-bearing olivines. However, the dependence on fO(2) is small, with D proportional to (fO(2))(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 fO(2).

• 出版日期2014-8