The oxygen isotope anatomy of a slowly cooled metamorphic rock

The distribution of O isotope ratios in a slowly cooled metamorphic rock 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-centimeter thick layer of millimeter-size magnetite grains. Isotopic zonation at two different scales is documented. A sharp decrease in δ^(18)O in the outer 10-100 μm of individual magnetite grains is in agreement with modeling of isotopic reequilibration during cooling by interdiffusion between feldspar and magnetite. The presence of this zonation in the outer portions of magnetite grains that are adjacent to garnet indicates extensive transport along grain boundaries during diffusive exchange. Millimeter-scale zonation in the polygranular layer of magnetite is caused by the interaction of volume diffusion and grain boundary transport, the effects of which depend upon local textures. Grain boundaries enhance bulk diffusion in polygranular aggregates and allow exchange between nontouching grains. The results document a complex textural control to millimeter-scale O isotope zonation and geothermometry in slowly cooled rocks and document the importance of fast grain boundary transport in controlling diffusive exchange