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

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

Feb 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..515d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 55, Issue 2, pp.515-524

Computer Science

5

Scientific paper

In eclogite xenoliths from Orapa, Botswana, 13 C 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, 18 O 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 18 O enrichment above "normal" mantle oxygen isotopic compositions (5 to 6.5%.) and the wide · 13 C range appear to occur in parts of the mantle in which diamond is stable. Eclogites with · 13 C values in the range -4 to -8%. and · 13 O values in the "normal" mantle range tend to have positive garnet-clinopyroxene 18 O fractionations and lower Fe/Ca and Fe/Mg ratios than eclogites which have simultaneously · 13 C values lower than - 8%. and · 18 O values higher than 6.5%.. In these eclogites the garnet-clinopyroxene fractionations tend to be negative. Positive garnet-clinopyroxene 18 O 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.

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