We report the carbon isotope compositions of a set of diamond crystals recovered from an investigation of the experimental interaction of metal iron with Mg–Ca carbonate at high temperature and high pressure. Despite using single carbon source with δ 13 C equal to +0.2‰ VPDB, the diamond crystals show a range of δ 13 C values from –0.5 to –17.1‰ VPDB. Diamonds grown in the metal-rich part of the system are relatively constant in their carbon isotope compositions (from –0.5 to –6.2‰), whereas those diamonds recovered from the carbonate dominated part of the capsule show a much wider range of δ 13 C (from –0.5 to –17.1‰). The experimentally observed distribution of diamond’ δ 13 C using a single carbon source with carbon isotope ratio of marine carbonate is similar to that found in certain classes of natural diamonds. Our data indicate that the δ 13 C distribution in diamonds that resulted from a redox reaction of marine carbonate with reduced mantle material is hardly distinguishable from the δ 13 C distribution of mantle diamonds.