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

Peter Deines*, J. W. Harris, D. N. Robinson, J. J. Gurney, S. R. Shee

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

58 Citations (Scopus)

Abstract

In eclogite xenoliths from Orapa, Botswana, δ13C 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, δ18O 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.

Original languageEnglish
Pages (from-to)515-524
Number of pages10
JournalGeochimica et Cosmochimica Acta
Volume55
Issue number2
DOIs
Publication statusPublished - 1991
Externally publishedYes

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