IRREGULAR ISOGRADS, REACTION INSTABILITIES, AND THE EVOLUTION OF PERMEABILITY DURING METAMORPHISM

Numerical models of two-dimensional heterogeneous reactive transport o f H2O-CO2 fluid and oxygen isotopes were developed to investigate the heterogeneity of fluid now and permeability attending contact metamorp hism of siliceous dolomites. Comparison of results with observations f rom the Alta (Utah) contact aureole indicates that both mineral reacti ons and isotopic alteration record fluid now through highly heterogene ous permeability. We interpret this extreme heterogeneity to be the re sult of positive feedback between reaction enhancement of permeability and flow-focusing. Similar distributions of mineral assemblages and s table isotopic alteration in other aureoles suggest that this is a com mon mechanism of permeability evolution during contact metamorphism of carbonate rocks. Flow-focusing requires fast reaction rates and slow compaction relative to fluid flow. More homogeneous permeability recor ded in regional metamorphic rocks may reflect fundamental differences in the relative rates of fluid now and compaction in regional and cont act metamorphic environments.