Advective-diffusive transport of isotopic fronts: An example from Naxos, Greece

Abstract

Oxygen isotopic compositions on the margins of marble bands from kyanite-staurolite grade Miocene metamorphic rocks on the island of Naxos in the Aegean have been modified by infiltration of a fluid from underlying pelitic rocks. Detailed isotopic profiles across 1 m wide basal boundary layers fit, within error, the curves predicted for combined advective-diffusive modification of an initial step function in composition. This justifies the twin assumptions of constant porosity within the marbles and local (grain-scale) fluid-solid oxygen isotopic equilibrium on which the transport theory is based. The advective displacements give time integrated fluid fluxes of only 0.2 and 1.0 m3/m2 at the two higher grade localities sampled. Such volumes represent only a fraction of the fluid lost by devolatilisation of the underlying pelitic sequence. Flow regimes for two possible porosity structures can be derived from analysis of boundary layer shapes in terms of diffusive broadening and a gravitationally driven fluid flux. These are: (1) flow along a small (≈ 10−6) interconnected porosity along grain edges over a duration of ≈ 0.3Ma to 1Ma, or (2) flow in cracks with a larger porosity (≈ 10−4) but a much shorter time duration of ≈ 103 years. The time scale calculated for crack flow is that during which cracks were open to longitudinal diffusion. This may reflect the cumulative active time for intermittent deformation events.