Age, paragenesis and composition of diamonds and evolution of the Precambrian mantle lithosphere of southern Africa

Two decades of diamond research in southern Africa allow the age and average composition (C isotope and N abundance) of diamonds and the dominant paragenesis (peridotitic versus eclogitic) of their syngenetic silicate and sulfide inclusions to be reviewed on a cratonwide scale. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa and Jwaneng have Re-Os isotopic ages that range from ∼2.9 Ga to the Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150 to 225km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm-Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds support a model whereby mantle that became part of the keel of the oldest continental nuclei was created by middle Archean (∼3.3 to ∼3.2 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the ∼2.9 Ga age population links late Archean subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely-distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new Iherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.