Subducting oceanic crust: The source of deep diamonds

Inclusions of majoritic garnet in diamonds from the Jagersfontein kimberlite formed at unusually great depths of similar to 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(13)C) of the host diamonds fall within a narrow range at similar to -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.