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
delta(18)O in the outer 10-100 mu m 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.