Age and origin of peridotitic diamonds from Venetia, Limpopo Belt, Kaapvaal–Zimbabwe craton

The 520 Ma old Venetia kimberlite cluster is located in the Central Zone of the Limpopo Belt where Archean crust has experienced at least two major tectonothermal events at ca. 2.6 and 2.0 Ga, the second of which closely follows the 2.054 Ga emplacement of the Bushveld Complex. Peridotitic garnet inclusions in Venetia diamonds are harzburgitic to lherzolitic in composition with low Ca and high Cr contents spanning the entire G10 garnet field. The related garnet macrocrysts generally have less extreme Ca and Cr contents and represent the re-equilibrated mantle host rocks of at least some of the diamonds. The garnets encapsulated in diamonds have low Sm/Nd and 143Nd/144Nd directly correlated with Ca and moderate 87Sr/86Sr (0.704–0.706) inversely correlated with Ca. The garnet macrocrysts also show low though more scattered Sm/Nd and 143Nd/144Nd but much higher 87Sr/86Sr (up to 0.720). Three of four inclusion groups give a nominal Sm–Nd isochron age of 2.30 ± 0.04 Ga with an unradiogenic initial (εNd = − 8). However, 143Nd/144Nd and 87Sr/86Sr are also correlated with reciprocal Nd and Sr concentration, consistent with mixing between a low Ca, low Sm/Nd, harzburgitic end member with radiogenic Sr (≤ 0.707) and a higher Ca, higher Sm/Nd, ‘basaltic’ end member with unradiogenic Sr (≤ 0.702), which raised the initial Nd isotope ratios of the inclusions in proportion to the degree of mixing. Therefore, 2.3 Ga is a maximum age for the diamonds. The initial Nd composition and characteristics of the mixing array indicate a > 3 Ga continental mantle harzburgite precursor to which a basaltic component was added at ca. 2 Ga, as suggested by the Re–Os isotope systematics of single sulfide inclusions in Venetia diamonds. In particular, four Venetia eclogitic sulfide inclusions describe a ca. 2.05 Ga Re–Os array with elevated initial 187Os/188Os ratio even more radiogenic than that of Bushveld PGE mineralization. Combined silicate Sm–Nd and Rb–Sr and sulfide Re–Os isotope compositions indicate variable interaction of original convecting mantle magmas with harzburgitic and eclogitic SCLM components during genesis of both the diamonds and the Bushveld Complex. In this model, the Venetia peridotitic diamonds crystallized (or recrystallized) at ca. 2 Ga following modification of Archean harzburgitic SCLM by Bushveld type magmas beneath the Limpopo Belt.