Fig. 3. Carbon isotopic compositions of both starting materials and newly synthesized diamonds.

The starting materials are as follows: diamond seeds (gray triangles), graphite (dark triangle), and carbonate powders MELD and MELD + SIDERITE (white triangle); diamond samples are as follows: HBD01 (dark squares), H3913 (gray squares with black rims for the three fragments from map 2), and H3908 (white squares and white diamond are diamond inclusions). The areas delimited by dotted dark lines represent the two potential starting carbon sources: carbonates (MELD and MELD SID) and graphite. The gray triangles represent the composition of the starting diamond seeds. The isotopic composition of the bulk system (all carbon from graphite plus all carbon from carbonates) is calculated for each sample by mass balance and is presented as white circles: Bulk = xδ¹³C_carbonate + yδ¹³C_graphite, with x and y being initial mass proportions of carbonates and graphite, respectively. For this calculation, no fractionation coefficient is used because this is total mixing. The isotopic compositions of the samples range from the compositions of the starting seeds to those of the carbonate powders, reflecting mixing between the composition of the starting diamond seeds and the newly grown diamond areas. The diamond inclusions from sample H3908 exhibit a δ¹³C of −14.2‰, corresponding to a maximum fractionation Δ¹³C of −2.74‰ (within error bars) with the carbonate source MELD (carbonates).