Multiple metasomatic diamond-forming events in a cooling lithosphere beneath Voorspoed, South Africa

Abstract

Thermal events and metasomatic processes have influenced the Kaapvaal craton in South Africa. High-density fluids (HDFs) trapped as microinclusions in diamond are main metasomatic agents which provide an insight to these processes in the Earth's mantle. Here we present data for 15 fibrous, HDF-bearing diamonds from the Voorspoed mine, South Africa, reflecting multiple diamond-forming events in a cooling lithosphere. Analyzed by FTIR and EPMA, the Voorspoed fibrous diamonds reveal three populations that differ in their nitrogen aggregation and HDF composition. A silicic‑carbonatitic group containing 11–30% B-centers, a saline group containing 5–16% B-centers, and a single high-Mg carbonatitic diamond with 0% B-centers. The distinct nitrogen aggregation of the fibrous diamond groups in Voorspoed and the lack of clear major element evolutionary trends for each HDF type or intermediate compositions between the different types suggest different time-temperature formation histories. Thermobarometry of mineral inclusions in non-fibrous monocrystalline Voorspoed diamonds (Viljoen et al., 2018) indicates that the Voorspoed lithosphere cooled by 100–200 °C since their host diamonds crystallized at high initial temperatures. High temperatures in Voorspoed lithosphere can be correlated with the eruption of the Ventersdorp flood basalts at the central Kaapvaal (ca. 2.7 Ga) or the Bushveld complex (ca. 2.06 Ga), and cooling rates of the lithosphere provide a time frame for a cooling process that originated ~2–3 Ga. Combining these data with the nitrogen aggregation systematics of fibrous and monocrystalline Voorspoed diamonds, we suggest that most Voorspoed diamonds formed during 4 metasomatic events: the oldest one recorded took place between 2 and 3 Gyr as a result of a major thermal perturbation, whereas the following three occurred between 200 and 600 Myr, 30–90 and < 30 Myr before kimberlite eruption in a cooling lithosphere. An even older (or deeper) event is hinted by a few diamonds where all nitrogen is in B-centers. The sequence of events implied by Voorspoed HDF compositional and nitrogen aggregation differences show affinities with other occurrences in South Africa (e.g. Kimberley, Finsch and Koffiefontein) and may reflect thermal and lithological variation between the central and southwest Kaapvaal lithosphere.