Subducting oceanic crust

The source of deep diamonds

Ralf Tappert*, Thomas Stachel, Jeff W. Harris, Karlis Muehlenbachs, Thomas Ludwig, Gerhard P. Brey

*Corresponding author for this work

Research output: Contribution to journalArticle

88 Citations (Scopus)

Abstract

Inclusions of majoritic garnet in diamonds from the Jagersfontein kimberlite formed at unusually great depths of ∼250 to >500 km in the asthenosphere and transition zone. The original host rocks were derived from a much shallower, basaltic (eclogitic) source. The presence of negative Eu anomalies in all majoritic garnets requires a crustal origin, thereby linking these very deep diamond sources to subducting oceanic crust. The carbon isotope values (δ13C) of the host diamonds fall within a narrow range at ∼ -20%, which is fundamentally different from the broad range (-24‰ to -2‰) and bimodal distribution of carbon isotopes of Jagersfontein diamonds that formed in the shallower lithosphere. This indicates that majoritic garnet-bearing diamonds at Jagersfontein inherited their light carbon isotopic composition directly from organic matter contained in a subducting slab. These diamonds were likely formed by direct conversion from graphite, well within the diamond stability field.

Original languageEnglish
Pages (from-to)565-568
Number of pages4
JournalGeology
Volume33
Issue number7
DOIs
Publication statusPublished - Jul 2005
Externally publishedYes

Keywords

  • Diamonds
  • Eu anomalies
  • Inclusions
  • Majorite
  • Mantle
  • Organic carbon

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