The degree to which oxygen-isotope equilibrium has been attained between the minerals of different, contiguous rock layers during metamorphism is correlated with evidence of enhanced permeability. Quartz separated from various rock layers of naturally occurring 'diffusion couples' has uniform values of δl8O where rocks have either (1) experienced intense devolatilization reactions, or (2) been severely fractured. Quartz from interlayered rock types lacking evidence of enhanced permeability differs by 1–2%o over distances as small as 1 cm. These results suggest that intergranular diffusion through static pore fluid is not an efficacious process for exchanging oxygen isotopes between different rock types. Infiltrating pore fluid, however, can serve as a medium of isotope exchange over distances greater than bedding thickness where rocks are sufficiently permeable for fluid flow to occur

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