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

Abstract

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.A large diamond with a websterite inclusion assemblage (clinopyroxene, orthopyroxene, garnet) from the Kimberley Pool (85 Ma emplacement age) has clinopyroxene with a Nd model age of 2111 ± 120 Ma. In contrast to the high Sm/Nd and radiogenic Nd of the eclogite garnets, the websterite cpx has low Sm/Nd and unradiogenic Nd more akin to that in garnet of harzburgite association diamonds from the Finsch kimberlite. However, neither the eclogite nor the websterite group diamonds can be related to the harzburgite suite. Analysis of inclusions from single diamonds eliminates the problems inherent in compositing possibly unrelated inclusions from large numbers of stones, but the large diamond size required to yield large inclusions may represent only a specific and possibly rare component of the general diamond population. The samples analyzed here may have been derived ultimately from subducted oceanic crust.