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, delta-C-13 of graphite varies between -4.6 and -7.8 parts per thousand vs. PDB, while that of diamonds ranges from -4.0 to -22.3 parts per thousand. In graphite eclogites, delta-O-18 values of garnets fall between 5.2 and 7 parts per thousand vs. SMOW and those of clinopyroxenes between 4.6 and 6.5 parts per thousand. For diamond eclogites the respective ranges are wider, i.e., 3.9 to 9.2 parts per thousand and 5.2 to 8.3 parts per thousand. The O-18 enrichment above "normal" mantle oxygen isotopic compositions (5 to 6.5 parts per thousand) and the wide delta-C-13 range appear to occur in parts of the mantle in which diamond is stable. Eclogites with delta-C-13 values in the range -4 to -8 parts per thousand and delta-O-18 values in the "normal" mantle range tend to have positive garnet-clinopyroxene O-18 fractionations and lower Fe/Ca and Fe/Mg ratios than eclogites which have simultaneously delta-C-13 values lower than -8 parts per thousand and delta-O-18 values higher than 6.5 parts per thousand. In these eclogites the garnet-clinopyroxene fractionations tend to be negative. Positive garnet-clinopyroxene O-18 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.