Unusual mineral inclusions and carbon isotopes of alluvial diamonds from Bingara, eastern Australia

A suite of diamonds from eastern Australia, “Group B”, are essentially eclogitic. However, mineral inclusion compositions are unlike eclogitic diamonds from other occurrences. Garnets are very Ca-rich (Gr61–84) and clinopyroxenes are mostly diopsidic (16±10 mol% Jd). Other inclusions in these diamonds are SiO2, titanite, olivine, molybdenite and melilite—a truly unique suite! Garnet and clinopyroxene are not accompanied by indications of fertile mantle conditions. Rather, we find depletions in Fe, Mn, Na, K, coupled with high Ni and Cr in some clinopyroxenes. The chemistry of the garnets and clinopyroxenes suggests affinities with rodingites—metasomatised mafic rocks that occur in ophiolites. “Group B” diamonds are 13C-enriched (δ13C=+1.1±1.3‰ (1 S.D.)). Internal structures include evidence of brittle failure and of rapid growth. The diamonds have high nitrogen contents and positive nitrogen isotopes (δ15N=+10.1±4.7‰). This may indicate that the diamonds formed from 13C-enriched carbon (+3‰), such as that derived from marine carbonates within the mafic rocks. The many unusual features of these diamonds can be interpreted to indicate that the diamonds formed in a subduction environment. However, resorption on the surfaces of the diamonds indicates that they were brought to the upper crust by magmatic activity.