Metacarbonate rocks, consisting of ankerite and dolomite together with combinations of amphibole, biotite, muscovite, chlorite, quartz, and clinopyroxene, are an important component of the roughly 3.8 Ga Isua supracrustal belt. Along with layers of banded iron formation, felsic schists, basic amphibolites, and variegated amphibolitic schists, they have previously been described as a single supracrustal suite intruded by ultramafic and Mg-rich basic sills. One of the formal stratigraphic units recognized in this interpretation, the Calc-Silicate Formation, contains abundant metacarbonates and calc-silicates that were regarded as the earliest known examples of metamorphosed calcareous chemical sediments. Field relations and geochemical models, however, suggest that the Isua metacarbonate units are metasomatic in origin and formed where fluids flowed across the contacts between ultramafic bodies and felsic or metabasaltic country rocks at deep crustal levels. Field evidence in support of this includes the common occurrence of metacarbonates at margins of ultramafic rocks, formation of metacarbonate assemblages In veins near ultramafic contact zones, and the replacive nature of the contacts between metacarbonates and felsic or basaltic units. In order to explore possible mechanisms for the origin of the metacarbonates, metasomatic processes accompanying the advection of fluids through ultramafic rocks were simulated numerically using a model configuration in which a column of dunite was sandwiched between layers of host rock. This allowed mineral zonation, bulk composition, and porosity changes to be studied at both upstream and downstream contacts. Fluids entering the ultramafic layer at the upstream contact react with olivine to form talc and magnesite +/- chlorite +/- phlogopite. The same fluids, modified by passage through the dunite, react with country rock at the downstream contact, forming assemblages analogous to those found in the Calc-Silicate Formation. This process is due primarily to the effect of reduced a(SiO2) in changing the saturation state of all host rock minerals of all at the point where fluids exit the ultramafic rock. Metasomatism of the Isua supracrustals probably took place under amphibolite facies conditions between 500 degrees and 600 degrees C in the presence of water-rich fluids where time integrated fluid flux was between 10(3) to 10(4) moles of water cm(-2). The recognition that the Gale-Silicate Formation is metasomatic in origin (and hence of no stratigraphic importance) considerably weakens previous ideas on the origin of the Isua belt as a well-defined stratigraphic package currently exposed as a syncline. A corollary of this study is that metacarbonate-ultramafic associations should be common In supracrustal rocks that originally contained peridotites that have reacted with fluids at depth.