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

Journal Article · Fri Feb 01 04:00:00 EST 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90009-T· OSTI ID:5551398

Deines, P ^[1]; Harris, J W ^[2]; Robinson, D N ^[3]; Gurney, J J ^[4]; Shee, S R ^[5]

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1. Pennsylvania State Univ., University Park, (USA)
2. Univ. of Strathclyde, Glasgow (Scotland)
3. Anglo American Research Labs., Crown Mines (South Africa)
4. Univ. of Capetown, Rondebosh (South Africa)
5. Stockdale Prospecting Ltd., South Yarra, (Australia)

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

• OSTI ID: 5551398
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:2; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ABUNDANCE
AFRICA
BOTSWANA
CARBON
CARBON 13
CARBON ISOTOPES
CHEMICAL COMPOSITION
CHEMISTRY
CONCENTRATION RATIO
DEVELOPING COUNTRIES
DIAMONDS
ELEMENT ABUNDANCE
ELEMENTAL MINERALS
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GARNETS
GEOCHEMISTRY
GRAPHITE
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
KIMBERLITES
LAMPROPHYRES
LIGHT NUCLEI
MINERALIZATION
MINERALOGY
MINERALS
NONMETALS
NUCLEI
OXYGEN 18
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
PETROCHEMISTRY
PETROGENESIS
ROCKS
SILICATES
SILICON COMPOUNDS
STABLE ISOTOPES
VOLCANIC ROCKS