The oxygen isotope anatomy of a slowly cooled metamorphic rock
Laser-probe and ion microprobe analyses of δ^(18)O in magnetite from a granulite facies rock (Adirondack Mts., NY) document gradients over two scales: 10-100 µm within grains and 5-10 mm across a polygranular matrix. Analyses focus on layer of magnetite grains (5-10 mm wide) bounded by alkali feldspar on one side and garnet on the other. Ion microprobe analyses of magnetite adjacent to alkali feldspar document a depletion in δ^(18)O of up to 3‰, in the outer 100 µm of individual grains, steepening toward magnetite-feldspar grain boundaries. The magnitude, direction and shape of this zonation are in agreement with model predictions for diffusive exchange between magnetite and adjacent alkali feldspar during cooling. The same pattern of zonation is found in magnetite grains adjacent to garnet. However, slow rates of oxygen self diffusion in garnet prohibit such gradients forming by interdiffusion between magnetite and garnet. The presence of gradients requires that magnetite underwent exchange with another phase through grain boundaries.
© 1994 Geological Society of America.