Carbon and oxygen isotope variations in diamond and graphite eclogites from Orapa, Botswana, and the nitrogen content of their diamonds

In eclogite xenoliths from Orapa, Botswana, δ 13C of graphite varies between −4.6 and −7.8%. vs. PDB, while that of diamonds ranges from −4.0 to −22.3%.. In graphite eclogites, δ 18O values of garnets fall between 5.2 and 7%. vs. SMOW and those of clinopyroxenes between 4.6 and 6.5%.. For diamond eclogites the respective ranges are wider, i.e., 3.9 to 9.2%. and 5.2 to 8.3%.. The 18O enrichment above “normal” mantle oxygen isotopic compositions (5 to 6.5%.) and the wide · 13C range appear to occur in parts of the mantle in which diamond is stable. Eclogites with · 13C values in the range −4 to −8%. and · 13O values in the “normal” mantle range tend to have positive garnet-clinopyroxene 18O fractionations and lower Fe/Ca and Fe/Mg ratios than eclogites which have simultaneously · 13C values lower than − 8%. and · 18O values higher than 6.5%.. In these eclogites the garnet-clinopyroxene fractionations tend to be negative. Positive garnet-clinopyroxene 18O fractionations increase slightly in the sequence: diamond eclogites (0.36 ± 0.15, n = 6), graphite/diamond eclogites (0.46 ± 0.36, n = 3), and graphite eclogites (0.53 ± 0.15, n = 3). Neither igneous fractionation processes, subduction, nor metasomatic alteration can satisfactorily explain all observed chemical and isotopic trends.