THE OXYGEN-ISOTOPE ANATOMY OF A SLOWLY COOLED METAMORPHIC ROCK

The distribution of O isotope ratios in a slowly cooled metamorphic ro ck is investigated by a combination of ion microprobe and laser probe techniques. The sample has unusually simple geometry: centimeter-size domains of feldspar and garnet separated by a one-millimeter to one-ce ntimeter thick layer of millimeter-size magnetite grains. Isotopic zon ation at two different scales is documented. A sharp decrease in delta (18)O in the outer 10-100 mu m of individual magnetite grains is in ag reement with modeling of isotopic reequilibration during cooling by in terdiffusion between feldspar and magnetite. The presence of this zona tion in the outer portions of magnetite grains that are adjacent to ga rnet indicates extensive transport along grain boundaries during diffu sive exchange. Millimeter-scale zonation in the polygranular layer of magnetite is caused by the interaction of volume diffusion and grain b oundary transport, the effects of which depend upon local textures. Gr ain boundaries enhance bulk diffusion in polygranular aggregates and a llow exchange between nontouching grains. The results document a compl ex textural control to millimeter-scale O isotope zonation and geother mometry in slowly cooled rocks and document the importance of fast gra in boundary transport in controlling diffusive exchange.