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Diamondiferous mantle beneath the Lace kimberlite in South Africa : evidence from mineral inclusions in diamonds

University of British Columbia

Diamonds and their inclusions from the Lace kimberlite (288 crystals ≤ 0.2 ct) are studied in order to constrain their origin as well as the thermal state and thickness of the ancient lithospheric mantle beneath the Lace kimberlite (Kaapvaal craton). I present the first study on diamonds from the Lace, a Cretaceous kimberlite that belongs to the Kroonstad Group II Kimberlite kimberlite cluster in South Africa. The study is based on optical microscopy, diamonds polishing, Raman and infrared spectroscopy, electron microscopy, U-Pb and Hf geochronology, analyses of C isotopes and trace elements.The studied Lace diamonds are colorless, yellow, brown and off-white and have dodecahedral (n=139), octahedral (n=125), undetermined (n=22) and hemimorphic (n=2) morphology. Infrared spectroscopy classified the diamonds into Type IIa (13) and Type I (73), with 13 to 1199 ppm N. B-centre N aggregation ranges from 2 to 100 % IaB for 44 diamonds, while the remaining 29 stones do not contain B-aggregated N. Lace diamonds (n=19) formed from mantle carbon (– 9.1 to – 2.5 ‰ δ¹³C), with an exception of one eclogitic diamond (– 19.2 ‰) which originated from subducted organic carbon. The eclogitic diamond paragenesis (n=19, 44%) includes garnet, omphacite, kyanite, coesite, K-feldspar and zircon; the peridotitic paragenesis (n=15, 35%) includes forsterite and enstatite and the websteritic paragenesis (n=4, 9%) comprises orthopyroxene, omphacite, garnet and K-feldspar. The difference between the parent diamondiferous rocks below adjacent pipes Lace and Voorspoed kimberlites suggest that diamond formation below this Kaapvaal block of the Kaapvaal craton of the Kaapvaal craton was a spatially restricted process. Equilibration temperatures of Lace peridotitic diamonds were 940 – 1359 °C at 46 – 80 kbar. Lace eclogitic diamonds formed at 1000-1358 °C at 51-79 kbar and websteritic diamonds at 1000-1190 °C at 51-66 kbar. The thermobarometry constrained the maximum heat flow under the Lace kimberlite at 38 mW/m² and the minimum lithospheric thickness of 240 km. The zircon inclusion ²⁰⁶Pb/²³⁸U geochronology yielded an age of 188±37 Ma and a Hf model age of 2-2.6 Ga which constrains the Archean eclogitic formation age and a potential later metasomatic event in the Mesozoic.

Text

2019-10-21

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/72014

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/