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

DCO ID 11121/4016-9828-1145-4274-CC

is Contribution to the DCO

  • YES

year of publication

  • 2016

abstract

  • 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 δ13 C, indicating that diamonds grew episodically from fluids of distinct sources. Peridotitic diamonds contain up to 2962 ppm N, show 0–88% NB , and have δ13 C isotopic compositions from − 12.5‰ to − 1.9‰ with a mode near mantle-like values. Eclogitic diamonds contain 14–1432 ppm N, NB  spanning 29%–68%, and wider and lighter δ13 C isotopic compositions of − 17.8‰ to − 3.4‰. Fibrous diamonds on average contain more N (up to 2976 ppm) and are restricted in δ13 C from − 4.1‰ to − 9.4‰. Clinopyroxene-garnet thermobarometry suggests diamond formation at 1350–1375 °C at 5.8 to 6.3 GPa, whereas N aggregation thermometry yields diamond residence temperatures between 1000 and 1280 °C, if the assumed mantle residence time is 0.9–3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41–44 mW/m2  during diamond formation and a lithosphere-asthenosphere boundary (LAB) at 190–210 km. 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.

volume

  • 265