Constraints on effective diffusivity during oxygen isotope exchange at a marble-schist contact, Sifnos (Cyclades), Greece

A smooth sigmoidal δ¹⁸O profile is observed at a marble-schist contact within rocks affected by the Miocene greenschist overprint in the Alpine metamorphic complex of the Cyclades on Sifnos. The point of inflection of the profile occurs at the lithological contact, and indicates that advective movement of fluids into the marble was negligible despite previous evidence showing that ¹⁸O-enriched fluids infiltrated the schist sequence. Least squares model fits of the diffusion equation to the data give Dt products of 0.10 m² and 0.13 m² for marbles and schists, respectively (D = effective diffusion coefficient, t = time). To reconcile these figures with quoted free fluid diffusivities one has to propose that either the effective porosities were extremely low and/or the diffusional exchange time was as low as 10²-10³ years. Low effective porosities in schists are consistent with a model of greenschist evolution in which hydration reactions induced by (¹⁸O-enriched) fluids substantially decreased porosity and reduced exchange to fluid diffusional control.