Timescales and mechanisms of fluid infiltration in a marble: an ion microprobe study

Abstract

Using a recently developed ion microprobe technique, a detailed oxygen isotope map of calcite grains in a coarse-grained marble has been constructed, supported by trace element (Mn, Sr, Fe) analysis and cathodoluminescence (CL) imaging, in order to constrain scales of oxygen isotope equilibrium, timescales and mechanisms of metamorphic fluid infiltration, and fluid sources and pathways. Results are compared with a previous study of this sample (Wada 1988) carried out using a cryo-microtome technique and conventional oxygen isotope analysis. The marble, from the high temperature/low pressure Hida metamorphic belt in north-central Japan, underwent granulite facies followed by amphibolite facies metamorphic events, the latter associated with regional granite intrusion. The CL imaging indicates two types of calcite, a yellow luminescing (YLC) and a purple luminescing (PLC) variety. The YLC, which occupies grain boundaries, fractures, replacement patches, and most of the abundant deformation twin lamellae, post-dates the dominant PLC calcite and maps out fluid pathways. Systematic relationships were established between oxygen isotope and trace element composition, calcite type and texture, based on 74 ^(18)O/^(16)O and 17 trace element analyses with 20-30 µm spatial resolution. The YLC is enriched in Mn and Fe, and depleted in ^(18)O and Sr compared to PLC, and is much more ^(18)O depleted than is indicated from conventional analyses. Results are interpreted to indicate infiltration of ^(18)O-depleted (metamorphic or magmatic) fluid (initial δ^(18)O = 9‰-10.5‰) along grain boundaries, fractures and deformation twin lamellae, depleting calcite grains in Sr and enriching them in Mn and Fe. The sample is characterised by gross isotopic and elemental disequilibrium, with important implications for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks. Areas of PLC unaffected by "short-circuiting" fluid pathways contain oxygen diffusion profiles of ~10‰/~200µm in grain boundary regions or adjacent to fractures/patches. When correction is made for estimated grain boundary/fracture and profile orientation in 3D, profiles are indistinguishable within error. Model- ling of these profiles gives consistent estimates of Dt (where D is the diffusion coeffcient and t is time) of ~0.8 x 10^(-8) m^2, from which, using experimental data for oxygen diffusion in calcite, timescales of fluid transport along grain boundaries at amphibolite facies temperatures of ~10^3 to ~10^4 years are obtained. These short timescales, which are much shorter than plausible durations of metamorphism, imply that rock permeabilities may be transiently much higher during fluid flow than those calculated from time integrated fluid fluxes or predicted from laboratory measurements. The preservation of ^(18)O/^(16)O profiles requires either rapid cooling rates (~100-600°C/million years), or, more plausibly, loss of grain boundary fluid such that a dry cooling history followed the transient passage of fluid. The δ^(18)O/ trace element correlations are also consistent with volume diffusion-controlled transport in the PLC. Fluid transport and element exchange occurred by two interrelated mechanisms on short timescales and on different lengthscales - long-distance flow along cracks, grain boundaries and twin lamellae coupled to ~200 µm-scale volume diffusion of oxygen.

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