Diffusion along interphase boundaries and its effect on retrograde zoning patterns of metamorphic minerals

摘要: In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of transport along interphase boundaries (ib) through grain (s) interiors during retrograde Fe–Mg exchange between garnet biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed garnet–quartz interfaces, deviations from concentric zoning start develop when becomes slow cooling. The capacities for an boundary depend on product D ib K ib/s a, where ib is diffusion coefficient exchangeable components within medium, ib/s equilibrium partitioning cation partners medium width. model applied analyze garnets Mozambique belt (SE-Kenya), cooled 820°C rate ca. 2°C/my. It found non-equilibrated compositions garnet along garnet/quartz started below 700°C due insufficient these boundaries. was estimated have been about 1E-23 cm3/s (575°C) 1E-20 cm3/s (700°C) X Fe pattern close garnet–quartz–biotite junction relying published data diffusivity Fe2+ garnet. Similar obtained; assumed be filled with material has properties composition free melt garnet, quartz respective conditions. contrast, if related diffusivities solubility FeOH+ aqueous solution, would much more efficient, maintained entire cooling history rocks. observation local leads us exclude presence fluid any time