|
|||||
|
|
Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa |
|
| Authors: | Steven B Shirey Jeffrey W Harris Stephen H Richardson Matthew Fouch David E James Pierre Cartigny Peter Deines Fanus Viljoen |
| Institution: | a Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015, USA b Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK c Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa d Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USA e Laboratoire de Geochemie des Isotopes Stables, Universite de Paris VII, Institut de Physique du Globe de Paris (IPGP), 4 Place Jussieu, 75251, Paris Cedex 05, France f Department of Mineral Sciences, The Pennsylvania State University, University Park, PA, USA g De Beers GeoScience Centre, PO Box 82232, Southdale, 2135, South Africa |
| Abstract: | The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity 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 whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite. |
| Keywords: | Diamond Eclogite Peridotite Inclusion P-wave Craton Lithosphere |
| 本文献已被 ScienceDirect 等数据库收录！ | |