ABSTRACT

Carbon isotopic ratios and rare-earth element (REE) contents were measured for six Central African carbonados and two Brazilian ones. The δ¹³C values of all the Central African samples and one of the Brazilian samples are from –29.7‰ to –24.4‰, which are within the range of organic matters such as petroleum and coal, and are much lower than the typical values for ordinary diamonds of around -5‰. Another Brazilian sample gives diamond-like δ¹³C value of –8.8‰, suggesting that this is actually not a carbonado but an aggregate of ordinary diamond such as bort and ballas. Samples with a larger amount of impurities show higher REE contents and higher light-REE/heavy-REE ratios, implying the existence of a light-REE-enriched mineral, such as florencite, along grain boundaries between diamond crystals. Chondrite-normalized REE abundance patterns of the samples are similar to crustal materials such as shales rather than to kimberlites and ordinary diamonds, which are much more light-REE enriched than most of the studied samples. The Brazilian sample with a higher δ¹³C value, however, shows a kimberlite-like REE pattern which is clearly different from that of the other Brazilian sample. From our data, the crustal origin of carbonado is preferable to its genesis in the mantle.