Subducting oceanic crust: The source of deep diamonds
- Ralf Tappert1,
- Thomas Stachel1,
- Jeff W. Harris2,
- Karlis Muehlenbachs3,
- Thomas Ludwig4 and
- Gerhard P. Brey5
- 1Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
- 2Division of Earth Sciences, Gregory Building, Lilybank Gardens, University of Glasgow, Glasgow G12 8QQ, UK
- 3Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
- 4Institut für Mineralogie, Ruprecht Karls Universität, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
- 5Institut für Mineralogie, J.W. Goethe Universität, Senckenberganlage 28, 60054 Frankfurt, Germany
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.
Footnotes
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- Accepted 10 March 2005.
- Received 15 February 2005.
- Revision received 8 March 2005.
- Geological Society of America
