A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

作者：Zhukova Irina^*; O'Neill Hugh; Campbell Ian H

来源：Contributions to Mineralogy and Petrology, 2017, 172(7): 53.

DOI：10.1007/s00410-017-1365-x

• Olivine
• Diffusion
• Trace-element partitioning
• Substitution mechanisms

Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 degrees C in air along [ 100] with activities of SiO2, MgO and Al2O3 (a(SiO2), a(MgO) and a(Al2O3)) buffered. At low a(SiO2), the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a(SiO2) and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100-1350 degrees C. Experiments at high a(SiO2) and 1400 degrees C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 degrees C, fo + melt. The resulting diffusion profiles were analysed by LA-ICPMS in scanning mode. Diffusion profiles in the high a(SiO2) experiments were generally several hundred microns in length, but diffusion at low a(SiO2) was three orders of magnitude slower than in high a(SiO2) experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a(Al2O3))(1/2) and (a(siO2))(3/4), pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al(4/3)(3+)vac(2/3)SiO(4), whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl2O4. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high aSiO(2), even faster than Fe-Mg interdiffusion.

• 出版日期2017-7