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

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 (δ 13 C VPDB ), 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 δ 13 C 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 δ 13 C, 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 Cretacious diamond forming fluids within the Congo lithospheric mantle. Electron probe microanalysis (EPMA) of trapped silicate inclusions revealed both peridotitic (Fo 91-95 and En 92-94 , 78% of the suite) and eclogitic parageneses (Crpoor 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/m 2 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.iii Preface Samples used in this research were acquired from the Denver Museum of Nature and Science with the help of Curator James W. Hagadorn and James F. Hurlbut. I completed all polishing, EPMA, and FTIR of the samples. In situ carbon isotope and nitrogen concentration analyses were undertaken at the Canadian

Fluid Inclusions in Fibrous Diamonds

“…Silicic to low-Mg carbonatitic compositions were reported from the Kaapvaal craton Kempe et al 2021a;Weiss unpublished data), the Congo craton (Navon et al 1988;Kopylova et al 2010;Kosman et al 2016;Timmerman et al 2019), the West African craton (Weiss et al 2009), the Amazonian craton (Shiryaev et al 2005), the Slave craton (Klein-BenDavid et al 2007;Logvinova et al 2008b;Weiss et al 2015), the North Atlantic craton (Elazar and Weiss 2021), India , and the Siberian craton Zedgenizov et al 2009Zedgenizov et al , 2011Skuzovatov et al 2016;Gubanov et al 2019). The Siberian HDFs have the most extreme compositions on both ends of the silicic to low-Mg carbonatitic array, with SiO 2 both >65 and <15 wt% (on a carbonate-and water-free basis; Figs.…”

Thermoelastic Properties of Eclogitic Garnets and Omphacites: Implications for Deep Subduction of Oceanic Crust and Density Anomalies in the Upper Mantle

“…Its general crystal‐chemical formula is commonly represented as X 3 Y 2 Z 3 O 12 , where Z are tetrahedral sites mostly occupied by Si 4+ ; Y are octahedral cation sites, usually filled with Al 3+ , Fe 3+ and Cr 3+ ; X cations are coordinated by eight oxygen atoms and can accommodate Mg 2+ , Fe 2+ , Ca 2+ , Mn 2+ , and so on. Eclogites incorporated as xenoliths in eruptive rocks from global major ultrahigh‐pressure metamorphic (UHPM) belts and kimberlites indicate that eclogitic garnets are mainly composed of pyrope (Prp, Mg 3 Al 2 Si 3 O 12 ), almandine (Alm, Fe 3 Al 2 Si 3 O 12 ) and grossular (Grs, Ca 3 Al 2 Si 3 O 12 ) with very few other end‐member garnets like spessartine (Sps, Mn 3 Al 2 Si 3 O 12 ) and andradite (Adr, Ca 3 Fe 3+ 2 Si 3 O 12 ; e.g., Kosman et al, ; Ren et al, ; Taylor et al, ; Wang, ; Wang et al, ; Weber & Bucher, ). In addition, some eclogitic garnets generally contain ~1 wt.% of TiO 2 (e.g., Stachel & Harris, ; Wang et al, ), which could be neglected in that case, although some studies reported garnets with high Ti content (Wang et al, ), in which case the present model may not be applicable.…”

Thermobarometry of Diamond Inclusions: Evidence Mantle Evolution beneath Siberian Craton and Archean Cratons Worldwide.

“…For the Congo craton, diamond-bearing eclogites were suggested in Catoca pipe [121]. However, the largest data set for the diamond inclusions was published for the alluvial deposits of the Kasai River, NE Angola [120].…”