Substantial mobility of high-field-strength (HFS) elements (Ti, Y, P, Zr, and Nb) during greenschist facies metamorphism of two metabasaltic suites from the Appalachians, the Ascot Formation of southeastern Quebec and the Arbuckle Brook Formation of Nova Scotia, is attributed to the presence of CO2 in the fluid phase during metamorphism. The style of the mobility exhibited by these two suites is different. In the Ascot Formation, metasomatism affected rocks rich and poor in carbonates; it led to enrichment in some HFS elements and depletion in others. In contrast, in the Arbuckle Brook Formation there was depletion in CO2-rich rocks of all HFS elements and consequently much less fractionation between HFS elements.These differences may be explained by the relative effectiveness of buffering in the two suites. In the Ascot Formation, buffering was ineffective, and several different mineral assemblages were developed, reflecting a range in . Mobility of HFS elements was controlled by chemical potential differences between these mineral assemblages. In the Arbuckle Brook Formation, , was externally buffered, and all rocks have the same mineral assemblage, varying only in modal calcite. Mobility of the HFS elements was controlled by the partitioning of these elements between the mineral assemblage and the fluid.Depletion of HFS elements in the Arbuckle Brook Formation is well correlated with the amount of calcite in the rocks. The systematics of the depletion allow an estimate of the relative proportions of HFS elements removed in the fluid. They are different from those in the rocks and lead to changes in the interelement ratios with progressive carbonation. However, low-carbonate rocks provide reasonably reliable estimates of primary concentrations. In certain circumstances, low-carbonate rocks (i.e., rocks that appear to be relatively fresh) do not provide reliable estimates of primary concentrations. A careful examination of the systematics of carbonation is required in metabasaltic suites for which HFS-element concentrations are the major indicator of magmatic and (or) tectonic affinity.