We investigated the isotopic composition of calcium in the metastable calcium carbonate polymorphs vaterite and ikaite. The synthetic vaterite precipitates show a smaller degree of Ca isotope fractionation than calcite or aragonite at comparable temperatures and revealed no significant temperature sensitivity between 10 and 50°C. Ikaite, the water-bearing Ca carbonate modification (CaCO3·6H2O) was precipitated at 2°C in controlled laboratory experiments and in addition recovered from marine sediments and sea-ice. Synthetic and natural ikaites show similar values of Ca isotope fractionation. In general, ikaite is less depleted in heavy isotopes compared to calcite or aragonite at low temperatures. The observed offset of vaterite and ikaite to calcite or aragonite precipitated at similar temperatures is presumably mainly related to differences in precipitation rates and regimes, which seem however to be typical for the growth of these carbonate minerals. Small differences in Ca isotope ratios of naturally formed ikaites from different locations seem to be related to different geochemical conditions in the respective sediment settings. © 2011 Elsevier B.V.