Asymmetrically zoned reaction rims: assessment of grain boundary diffusivities and growth rates related to natural diffusion-controlled mineral reactions

This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite -> garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms similar to 20-mu m-wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low-angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase vertical bar garnet vertical bar hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe2+, Fe3+ and Al across the garnet layer; and (ii) micron-scale compositional variations in the near-grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast-path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe2+ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast-path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast-path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20-mu m-wide garnet rim is estimated at similar to 10(3)-10(4) years at the inferred reaction conditions of 760 +/- 50 degrees C at 7.6 kbar. Using the same parameterization of the growth law, 100-mu m- and 1-mm-thick garnet rims would grow within 10(5)-10(6) and 10(6)-10(7) years respectively.