TIMESCALES AND MECHANISMS OF FLUID INFILTRATION IN A MARBLE - AN ION MICROPROBE STUDY

Using a recently developed ion microprobe technique, a detailed oxygen isotope map of calcite grains in a coarse-grained marble has been con structed, supported by trace element (Mn, Sr, Fe) analysis and cathodo luminescence (CL) imaging, in order to constrain scales of oxygen isot ope equilibrium, timescales and mechanisms of metamorphic fluid infilt ration, and fluid sources and pathways. Results are compared with a pr evious study of this sample (Wada 1988) carried out using a cryo-micro tome technique and conventional oxygen isotope analysis. The marble, f rom the high temperature/low pressure Hida metamorphic belt in north-c entral Japan, underwent granulite facies followed by amphibolite facie s metamorphic events, the latter associated with regional granite intr usion. The CL imaging indicates two types of calcite, a yellow lumines cing (YLC) and a purple luminescing (PLC) variety. The YLC, which occu pies grain boundaries, fractures, replacement patches, and most of the abundant deformation twin lamellae, post-dates the dominant PLC calci te and maps out fluid pathways. Systematic relationships were establis hed between oxygen isotope and trace element composition, calcite type and texture, based on 74 O-18/O-16 and 17 trace element analyses with 20-30 mu m spatial resolution. The YLC is enriched in Mn and Fe, and depleted in O-18 and Sr compared to PLC, and is much more O-18 deplete d than is indicated from conventional analyses. Results are interprete d to indicate infiltration of O-18-depleted (metamorphic or magmatic) fluid (initial delta(18)O = 9 parts per thousand-10.5 parts per thousa nd) along grain boundaries, fractures and deformation twin lamellae, d epleting calcite grains in Sr and enriching them in Mn and Fe. The sam ple is characterised by gross isotopic and elemental disequilibrium, w ith important implications for the application of chromatographic theo ry to constrain fluid fluxes in metacarbonate rocks. Areas of PLC unaf fected by ''short-circuiting'' fluid pathways contain oxygen diffusion profiles of similar to 10 parts per thousand/similar to 200 mu m in g rain boundary regions or adjacent to fractures/patches. When correctio n is made for estimated grain boundary/fracture and profile orientatio n in 3D, profiles are indistinguishable within error. Modelling of the se profiles gives consistent estimates of Dt (where D is the diffusion coefficient and t is time) of similar to 0.8 x 10(-8) m(2), from whic h, using experimental data for oxygen diffusion in calcite, timescales of fluid transport along grain boundaries at amphibolite facies tempe ratures of similar to 10(3) to similar to 10(4) years are obtained. Th ese short timescales, which are much shorter than plausible durations of metamorphism, imply that rock permeabilities may be transiently muc h higher during fluid flow than those calculated from time integrated fluid fluxes or predicted from laboratory measurements. The preservati on of O-18/O-16 profiles requires either rapid cooling rates (similar to 100-600 degrees C/million years), or, more plausibly, loss of grain boundary fluid such that a dry cooling history followed the transient passage of fluid. The delta(18)O/trace element correlations are also consistent with volume diffusion-controlled transport in the PLC. Flui d transport and element exchange occurred by two interrelated mechanis ms on short timescales and on different lengthscales - long-distance f low along cracks, grain boundaries and twin lamellae coupled to simila r to 200 mu m-scale volume diffusion of oxygen.