Section 20
Chapter 19,505

Neodymium and strontium isotope systematics of eclogite and websterite paragenesis inclusions from single diamonds, Finsch and Kimberley Pool, RSA

C.B Smith; J.J Gurney; J.W Harris; M.L Otter; M.B Kirkley; E Jagoutz

Geochimica et Cosmochimica Acta 55(9): 2579-2590


ISSN/ISBN: 0016-7037
DOI: 10.1016/0016-7037(91)90374-e
Accession: 019504677

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Garnet inclusions of the eclogite paragenesis from large single diamonds (1.8 to 4.2 carats) from the 120 Ma Finsch kimberlite have Nd model ages ranging from 1443 to 2408 Ma. Ages of 1443 ± 166 and 1657 ± 77 Ma in two samples with 87Sr86Sr = 0.7042 may represent the age of diamond formation and are within error of a 1580 ± 50 Ma Sm-Nd isochron age on clinopyroxene and garnet reported by Richardson et al. (1990). Older model ages in three other samples (1824, 2183, and 2408 Ma) correspond with unsupported radiogenic Sr (87Sr86Sr = 0.7095 to 0.7158) and anomalously high MnO (0.7 to 1.2 wt%.) suggestive of multistage evolution. Carbon isotopic compositions of the diamonds range from 3.6 to 7.7%., and do not correlate with isotopic or chemical features of the inclusions. Compared to world-wide compositional ranges of most eclogite paragenesis garnet inclusions in diamonds, four of five analyzed here are anomalously enriched in Fe and Mn, possibly a characteristic feature of the Finsch eclogitic diamond paragenesis suite.

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