The Magnet Cove Complex is a highly differentiated ring dike sequence of alkaline igneous rocks and carbonatite. Analyses of silicate rocks show increasing -values (vs. SMOW) from early to late differentiates: trachyte (+8.6%;), phonolite (+7.9%), nepheline syenites (+8.8 to +9.7%) , jacupriangite (+9.4%), melteigite (+11.%) , ijolite (+11.1%) and late dikes (+12.6 to +13.8%). 6-values of calcites from silicate rocks display a similar trend. Carbonatite, a supposed late-stage differentiate, has a 6 -value of +8.6%. Unexpectedly, this value corresponds with the -values shown by early silicate rocks and associated calcite. OC -values of calcites from the silicate rocks show a decrease with differentiation ranging from -4.8 to -6.7% vs. PDB. Similarly the OC -value of the carbonatite, -5.4, coincides with early rather than late differentiates. Early crystallization and removal of A-enriched mafic minerals shifts the late residuals to higher 6 -values. The 6C -trend is explained by loss of isotopically heavy CO during differentiation. The discrepancy between 6 and 6C values of the late stage silicate rocks and the carbonatite may result from early separation of an immiscible carbonate liquid. The narrow range of 6C values displayed by the complex indicates that the carbonatite carbon source (mantle C?) was uniform in isotopic composition and isotopically similar to supposed sources of carbonatites world wide.