Plume-related diamond formation in reworked Archaean mantle: Sm–Nd age constraints from Voorspoed peridotitic and eclogitic diamonds (Kaapvaal Craton)

Abstract

Here we study silicate inclusion-bearing diamonds from the Voorspoed Group II kimberlite (or carbonate-rich olivine lamproite—CROL) in the central Kaapvaal Craton. Based on major and trace elements, along with Sm–Nd isotope characteristics, the peridotitic garnet and clinopyroxene inclusions define three compositional groups that are contemporaneous with lithospheric refertilisation and re-healing related to 2.79–2.68 Ga Ventersdorp-related plume magmatism across the central Kaapvaal craton. Three harzburgitic (G10) inclusions define an isochron of 2877 ± 249 Ma, with a low initial ε_Nd =  − 15 ± 10 that overlaps the oldest Kaapvaal enriched components as represented by crust in the 3.66–3.22 Ga Ancient Gneiss Complex. This finding suggests that harzburgitic diamond formation occurred in the oldest > 3.2 Ga Kaapvaal lithospheric mantle. Five lherzolitic (G9) garnet and 11 Cr-diopside inclusions with La_N > 1 (where N refers to chondrite normalisation) and low ¹⁴⁷Sm/¹⁴⁴Nd (0.11–0.41), define a 2606 ± 95 Ma Sm–Nd isochron with an initial ε_Nd =  − 6.9 ± 5.4 that is consistent with diamond formation in enriched > 3.0 Ga lithospheric mantle. Four lherzolitic (G9) garnet and five Cr-diopside inclusions with La_N < 1 and higher ¹⁴⁷Sm/¹⁴⁴Nd (0.45–1.51), define a 2560 ± 51 Ma Sm–Nd isochron with an initial ε_Nd = 9.8 ± 7.8. This initial ratio overlaps with depleted mantle indicating rapid re-healing of the lithospheric mantle after 2.79–2.68 Ga Ventersdorp-related plume magmatism. Sixteen eclogitic inclusions (10 garnets and six omphacites) define an isochron of 2196 ± 61 Ma, with initial ε_Nd =  − 9.3 ± 3.9. Growth of Voorspoed eclogitic diamonds is interpreted to be related to magmatism and failed rifting at the time of the 2.25–2.23 Ga Hekpoort LIP and associated magmatism across the Transvaal Basin. Low-pressure protoliths for the eclogite host rocks are supported by the presence of omphacite that is not stable as a high-pressure liquidus phase, along with Eu anomalies in the majority of eclogitic inclusions. The likely host rocks for eclogitic diamond formation are subduction-related eclogites in the lithospheric mantle that may be similar to those sampled nearby at Lace. The Voorspoed peridotitic and eclogitic diamonds document multiple episodes of melt infiltration and modification of older lithospheric mantle, and have allowed us to place diamond formation and preservation in the context of the long history of plume impingement on the Kaapvaal lithospheric mantle.