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Cretaceous mantle of the Congo craton : evidence from mineral and fluid inclusions in Kasai alluvial diamonds Kosman, Charles Walter

Abstract

Alluvial diamonds from the Kasai River, Katanga Province, Democratic Republic of the Congo (DRC), were studied in order to constrain the composition, thermal state, and diamond forming fluids of the ancient lithospheric mantle of the Congo craton. The diamonds originate from Cretaceous kimberlites of the Lucapa graben in northeastern Angola. We report carbon isotope compositions (δ¹³CVPDB), nitrogen concentrations ([N]), and nitrogen aggregation states of 138 diamonds, as well as compositions of mineral and fluid inclusions in the diamonds. Diamonds emplaced by kimberlites of the northeastern Lucapa graben and eroded into alluvials along the Kasai river contain 25–2900 ppm [N], show 0–88% N aggregation and δ¹³C isotopic compositions spanning -27‰ to -2‰ with a mode near mantle-like values. In situ cathodoluminescence (CL), secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) reveal large heterogeneities in [N], N aggregation and δ¹³C, indicating diamonds grew episodically from fluids of distinct sources. Fluid inclusion compositions of fibrous diamonds analyzed by electron probe microanalysis 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. Electron probe microanalysis (EPMA) of trapped silicate inclusions revealed both peridotitic (Fo₉₁₋₉₅ and En₉₂₋₉₄, 78% of the suite) and eclogitic parageneses (Cr-poor pyrope and omphacite with 11–27% jadeite, 17% of the suite) within diamonds (11% remainder unknown). Clinopyroxene-garnet thermobarometry suggest diamond formation at 1350–1375 °C, whereas [N] aggregation thermometry yields diamond residence temperatures between 1000 and 1275 °C, if the assumed residence time is 0.9–3.3 Ga. Integrated geothermobaromtery indicates heat fluxes of 41–45 mW/m² during diamond formation and a shallow lithosphere-asthenosphere boundary (LAB) 175–189 km. The shallow LAB may result from a higher than average cratonic geotherms and the position of the Kasai block near the Congo cratonic margin. The hotter mantle may be attributable to contemporaneous rifting of the southern Atlantic, multiple post-Archean reactivations of the craton, and/or proximal Cretaceous plumes.

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