RATES OF INTERGRANULAR DIFFUSION OF ALUMINUM FROM CORONAL REACTION TEXTURES
The intergranular diffusivity of Al is a principal determinant of the kinetics of many metamorphic reactions and the mechanisms of some. In terms of chemical equilibration, it occupies a pivotal position, with diffusion and equilibration of most common uni- and divalent cations occurring more rapidly, and diffusion and equilibration of most tri- and higher-valent cations occurring more slowly. But no experimental determinations of this vital quantity exist, and only two published estimates from natural occurrences are available to constrain it, both near 600 °C.
New estimates of Al diffusivity spanning a range of temperature were obtained from coronas of crd + opx, rimmed with a layer of coarse opx, that grew in diffusion-controlled reactions between grt and qtz (± sil) when the anorthositic MLP intruded regional high-grade gneisses. The widths of these coronas, when corrected for the effects of their near-spherical geometry, yield quantitative information on Al diffusivity -- provided that thermal histories during corona development are known. Conductive thermal models in 2D, suitable for this steep-sided and almost completely anhydrous aureole, constrain these thermal histories by closely reproducing the spatial distribution of peak temperatures and heating/cooling episodes recorded by Al-in-opx thermometry.
Twelve determinations of the effective diffusivity of Al were made for rocks in the aureole at distances from 450 to 4560 m from the contact. The results define a strongly linear array (r2=0.80) on an Arrhenius diagram, with an intercept log10 D0 = -9.8(±2.0) m2/sec and an activation energy of 262(±42) kJ/mol. When extrapolated to 600 °C, these results imply diffusivities that are smaller than previous determinations by about 2.5 log10 units. This difference, and the high activation energy, are interpreted as reflecting the nearly anhydrous character of the intergranular medium during MLP reactions, in contrast to the hydrous (though probably fluid-unsaturated) intergranular media inferred for the two prior examples.