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Abstract: 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 δ13C equal to +0.2‰ VPDB, the diamond crystals show a range of δ13C 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 δ13C (from -0.5 to -17.1‰). The experimentally observed distribution of diamond' δ13C 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 δ13C distribution in diamonds that resulted from a redox reaction of marine carbonate with reduced mantle material is hardly distinguishable from the δ13C distribution of mantle diamonds. [ABSTRACT FROM AUTHOR]