Cretaceous mantle of the Congo craton: Evidence from mineral and fluid inclusions in Kasai alluvial diamonds

• Published in: Lithos 2016-11, Vol.265, p.42-56
• Main Authors: Kosman, Charles W., Kopylova, Maya G., Stern, Richard A., Hagadorn, James W., Hurlbut, James F.
• Format: Article
• Language: English
• Subjects: Alluvial diamonds; Carbon isotopes; Congo Craton; Cretaceous mantle; Mineral and fluid inclusions; Nitrogen content and aggregation state
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2016.07.004

Alluvial diamonds from the Kasai River, Democratic Republic of the Congo (DRC) are sourced from Cretaceous kimberlites of the Lucapa graben in Angola. Analysis of 40 inclusion-bearing diamonds provides new insights into the characteristics and evolution of ancient lithospheric mantle of the Congo craton. Silicate inclusions permitted us to classify diamonds as peridotitic, containing Fo91–95 and En92–94, (23 diamonds, 70% of the suite), and eclogitic, containing Cr-poor pyrope and omphacite with 11–27% jadeite (6 diamonds, 18% of the suite). Fluid inclusion compositions of fibrous diamonds are moderately to highly silicic, matching compositions of diamond-forming fluids from other DRC diamonds. Regional homogeneity of Congo fibrous diamond fluid inclusion compositions suggests spatially extensive homogenization of Cretaceous diamond forming fluids within the Congo lithospheric mantle. In situ cathodoluminescence, secondary ion mass spectrometry and Fourier transform infrared spectroscopy reveal large heterogeneities in N, N aggregation into B-centers (NB), and δ13C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962ppm N, show 0–88% NB, and have δ13C isotopic compositions from −12.5‰ to −1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14–1432ppm N, NB spanning 29%–68%, and wider and lighter δ13C isotopic compositions of −17.8‰ to −3.4‰. Fibrous diamonds on average contain more N (up to 2976ppm) and are restricted in δ13C from −4.1‰ to −9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350–1375°C at 5.8 to 6.3GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280°C, if the assumed mantle residence time is 0.9–3.3Ga. Integrated geothermobaromtery indicates heat fluxes of 41–44mW/m2 during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190–210km. The hotter-than-average cratonic mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes. •Alluvial Northeastern Angolan (Kasai River) diamonds from Lucapa Graben Cretaceous kimberlites were studied.•Diamonds formed within harzburgitic (70%) and eclogitic (18%) parageneses.•Kasai fibrous diamonds grew from silicic fluids similar to those from Mbuji-Mayi.•High (41 - 44 mW/m2) geotherms recorded by Kasai diamonds may be due to rifting, plum