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Geology; July 2005; v. 33; no. 7; p. 565-568; DOI: 10.1130/G21637.1
© 2005 Geological Society of America
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Subducting oceanic crust: The source of deep diamonds

Ralf Tappert1, Thomas Stachel1, Jeff W. Harris2, Karlis Muehlenbachs3, Thomas Ludwig4 and Gerhard P. Brey5

1 Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
2 Division of Earth Sciences, Gregory Building, Lilybank Gardens, University of Glasgow, Glasgow G12 8QQ, UK
3 Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
4 Institut für Mineralogie, Ruprecht Karls Universität, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
5 Institut für Mineralogie, J.W. Goethe Universität, Senckenberganlage 28, 60054 Frankfurt, Germany

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 ({delta}13C) of the host diamonds fall within a narrow range at ~–20{per thousand}, which is fundamentally different from the broad range (–24{per thousand} to –2{per thousand}) 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.

Key Words: majorite • diamonds • inclusions • mantle • Eu anomalies • organic carbon




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