Diamonds from the Murowa Kimberlites: Formation Within Extremely Depleted and Metasomatized Zimbabwean Peridotitic Subcontinental Mantle

Abstract

Abstract Murowa kimberlites are situated in the 3.5 Ga Tokwe block, the oldest part of the Zimbabwe craton. The diamonds are predominantly white or pale brown octahedra of high gem quality. Mineral inclusions are overwhelmingly peridotitic, dominated by chromite, sulfide, and olivine, with only minor amounts of orthopyroxene and garnet, suggestive of a very depleted lithospheric peridotite mantle host. Carbon isotope values for the diamonds range from δ13C –5.8 to –2.5‰ and are consistent with global peridotitic diamond signatures. Diamonds with an eclogitic paragenesis are virtually absent. Most Murowa diamonds have simple octahedral zonation, sometimes with several growth stages. The diamonds have a wide range of nitrogen concentration (10–1,600 ppm) and aggregation state (20–80%), with internal variations. Murowa diamond mantle residence temperatures of 1,090° to 1,210°C, calculated from nitrogen aggregation based on a 2.9-b.y. mantle residence, are in agreement with mineral inclusion thermobarometry of 1,050° to 1,270°C, suggesting formation on a 40-mW/m2 model geotherm with a lithospheric keel depth of ~200 km. Variable inclusion trace element compositions record diamond formation in geochemical environments changing from depleted to strongly metasomatized peridotites. Three populations of chromite inclusions in the diamonds were identified. The main population of the highest-Cr and lowest-Ti inclusions is enriched in Nb, Rb, and V, and depleted in Zr, similar in composition to worldwide chromite diamond inclusions. Two other minor Murowa groups of chromites, less rich in Cr, depleted in Nb, but having up to 2.7 wt % of TiO2 and 10 to 34 ppm Zr concentrations, are extremely rare as diamond inclusions worldwide but common as kimberlite indicator minerals. The variable amounts of Ni, Mn, Zn, Ca, and Ti identified in the olivine inclusions suggest diamond formation occurred during high-Ti, low-Ca metasomatism. Pyrope garnet inclusions show even stronger light-medium rare earth elements (REE) enrichment compared to worldwide records. Compositions of Murowa orthopyroxene inclusions are also variable: several are rich in light REE and middle REE and, additionally, have high concentrations of Na, Ba, Zr, and Nb, but others are depleted in these elements. The wide range of inclusion trace element compositions shows that Murowa diamond geneses were prolonged, taking place in several stages. A previously published 3.4 ± 0.2 Ga Re-Os model age of a single Murowa sulfide inclusion may correspond the formation time of the main diamond population. Previously reported 40Ar/39Ar 892 ± 21 Ma model ages of yimengite inclusions from a diamond at the nearby Sese kimberlites and the occurrences of yimengite and armalcolite inclusions in Murowa diamonds suggest the possible presence of a younger diamond-forming event. Both Murowa and Sese diamonds likely originated during interaction between asthenosphere-derived carbonate and water-rich melt and extremely depleted peridotitic mantle lithosphere.