Marble-schist contacts are frequently characterized by fine-scale interlaying which prevents the application of semi-infinite layer width assumptions to advection/diffusion models of isotope profiles. Using a finite layer width boundary condition, we show that the minimum width (w) of a layer required in order that diffusional exchange at both boundaries will not mutually interfere is given by the relation w<&kgr;(Dt)1/2, where D is the effective diffusion coefficient, t is the duration of the diffusion and &kgr; is a constant determined by the initial compositional step and the noise (i.e., natural homogeneity and/or analytical accuracy). When this condition is not satisfied, a solution of the diffusion equation for a multilayer transition zone containing several schist and marble layers of different, finite, widths is applicable. Application of this refinement of the diffusion model to an oxygen isotope profile from Tinos (Cyclades) Greece gives 10-4≤Dt≤10-2 m2.

This Dt range, and a Dt value of 10-1 m2 previously determined for Sifnos, indicates that rocks undergoing the Oligocene-Miocene greenschist facies overprint of Eocene high-pressure metamorphic rocks in the Cyclades were characterized by extremely low porosities. Permeability perpendicular to marble-schist contacts was sufficiently low to prevent advection into the marbles; correspondingly, it is suggested that the layer-parallel schistosity permitted infiltration of the fluids required for the hydration recations of the metamorphic overprint. ¿American Geophysical Union 1991