HR: 1340h
AN: MR13C-1406    [Abstracts]
TI: Grain boundary diffusion in garnet and fast-path diffusion along diffusion channels in plagioclase: Growth of garnet reaction rims at granulite facies conditions
AU: * Keller, L
EM: lkeller@zedat.fu-berlin.de
AF: Earth and Planetary Sciences, University of California, Berkeley, 371 McCone Hall, Berkeley, Berkeley, CA 94720, United States
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14482, Germany
AU: Rhede, D
EM: rhede@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, 14482, Germany
AB: We explored corona structures with the succession plagioclase/garnet/pyroxene, which were formed by the reaction plagioclase + pyroxene = garnet + quartz, in order to derive information about diffusion paths that allowed for material redistribution during reaction progress. While quartz formed disconnected single grains along the garnet/hedenbergite boundaries, garnet formed about twenty μm wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low angle grain boundaries, which often form a direct link between the reaction interfaces of the plagioclase/garnet/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 and evaluation of the corona structure using diffusion modeling suggests that the material transport rates between sites of local reaction are regulated by local equilibrium with the minerals surrounding the reaction sites and by effective transport rates across the growing reaction rim. Transport in the garnet polycrystal occurred partly by fast-path diffusion along grain boundaries and slow volume diffusion, which in combination determine effective bulk diffusion in garnet. In the studied example effective diffusion in garnet was too slow to allow for chemical equilibration of the two reaction fronts. This resulted in the formation of a characteristic asymmetric growth zoning. Because the characteristic of such an asymmetric growth zoning involves information of the relative contribution of fast-path and slow volume diffusion the evaluation of the observed growth zoning by diffusion modeling allows derivation of a set D_gb/D_vol ratios for Ca, Fe²⁺ and Al in garnet. Based on the observation of nano-scale diffusion channels in plagioclase and on the base of our diffusion modeling effective material transport in plagioclase towards reaction sites is suggested to be determined by a combination of fast-path diffusion along diffusion channels and slow volume diffusion.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3660 Metamorphic petrology
DE: 5139 Transport properties
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting