Millimetre-scale variation in metamorphic permeability of marbles during transient fluid flow: an example from the Reynolds Range, central Australia

The Upper Gale-Silicate Unit of the Reynolds Range, central Australia, cont ains marbles with centimetre-wide layers of metamorphosed marls. Wollastoni te-bearing marbles record fluid infiltration by water-rich fluids (XCO2 = 0 .1-0.3) at 650-700 degreesC and 300-400 MPa during cooling from granulite-f acies metamorphism at similar to1.6 Ga. The pattern of fluid flow recorded by stable isotope ratios of calcite, the distribution of wollastonite, and calcite luminescence is heterogeneous on a millimetre-scale. Calcite from d ecimetre-size hand specimens has delta O-18 values and delta C-13 values th at vary up to 10 and 12 parts per thousand, respectively. The lowest delta O-18 and delta C-13 values are centred on millimetre-wide grandite-rich ska rn zones, whereas higher values characterise relict calcite that co-exists with quartz. The variation in delta O-18 values together with related delta C-13 Variations and the abundance of wollastonite may be used to determine the distribution of zones of higher and lower permeability. Skarn zones an d regions of low delta O-18 values at marl-marble contacts mark channels of fluid flow. Fluid flow at the contacts was promoted by the theological con trast and by the juxtaposition of calcite tin the marble) and quartz tin th e marl) that could form abundant wollastonite via the reaction: calcite + q uartz wollastonite + CO2. This reaction results in a large negative volume change, and fluid how may have been concentrated in a small number of chann els once they were established, provided that fluid pressures were sufficie nt to overcome compaction. Some isolated skarn zones and low delta O-18 reg ions within the marbles may be located at the intersections of fractures th at would have also been zones of high permeability. The formation of large volumes (>15-20%) of wollastonite within the marble layers requires that si lica metasomatism via the reaction calcite + SiO2(aq) = wollastonite + CO2 has occurred, and the grandite skarns record metasomatic addition of Fe. Th ese metasomatic reactions have positive volume changes that would have seal ed existing fluid flow channels, allowing new channels to develop elsewhere in the rock. Preservation of steep stable isotope gradients suggests that fluid flow occurred over only a few thousand years after which the rocks we re fluid absent.