Subducting oceanic crust: The source of deep diamonds

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

doi:10.1130/G21637.1

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 journal › Article

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 language	English
Pages (from-to)	565-568
Number of pages	4
Journal	Geology
Volume	33
Issue number	7
DOIs	https://doi.org/10.1130/G21637.1
Publication status	Published - Jul 2005
Externally published	Yes

Keywords

Diamonds
Eu anomalies
Inclusions
Majorite
Mantle
Organic carbon

Access to Document

10.1130/G21637.1

Link to publication in Scopus

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title = "Subducting oceanic crust: The source of deep diamonds",

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.",

keywords = "Diamonds, Eu anomalies, Inclusions, Majorite, Mantle, Organic carbon",

author = "Ralf Tappert and Thomas Stachel and Harris, {Jeff W.} and Karlis Muehlenbachs and Thomas Ludwig and Brey, {Gerhard P.}",

year = "2005",

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language = "English",

volume = "33",

pages = "565--568",

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T2 - The source of deep diamonds

AU - Tappert, Ralf

AU - Stachel, Thomas

AU - Harris, Jeff W.

AU - Muehlenbachs, Karlis

AU - Ludwig, Thomas

AU - Brey, Gerhard P.

PY - 2005/7

Y1 - 2005/7

N2 - 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.

AB - 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.

KW - Diamonds

KW - Eu anomalies

KW - Inclusions

KW - Majorite

KW - Mantle

KW - Organic carbon

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