Trace element and Sr-Nd-Pb isotope compositions of diamond-forming fluids in Voorspoed diamonds

Trace element and Sr-Nd-Pb isotope compositions of high-density fluids (HDFs) trapped in diamonds provide key insights into mantle processes and diamond formation. This study focuses on diamonds containing different HDF types from the Voorspoed carbonate-rich olivine lamproite (CROL) in the Kroonstad cluster, South Africa. Their trace elements reveal signatures varying between primitive mantle-normalized incompatible enriched fractionated patterns mostly characterizing saline HDFs, and overall flatter patterns for silicic-carbonatitic compositions. The HDFs Sr-Nd-Pb isotope compositions vary markedly; ⁸⁷Sr/⁸⁶Sr = 0.70647–0.71556, ¹⁴³Nd/¹⁴⁴Nd = 0.5113–0.5122, ²⁰⁶Pb/²⁰⁴Pb = 17.36–18.77, ²⁰⁷Pb/²⁰⁴Pb = 15.41–15.71 and ²⁰⁸Pb/²⁰⁴Pb = 37.47–39.39. A Rb–Sr age of 780 ± 220 Ma recorded by the saline HDFs does not correspond with the timing of their host diamonds formation (~ 160–220 Ma; based on nitrogen aggregation estimates). The age records an earlier metasomatic event associated with formation of the silicic-carbonatitic HDFs and diamond (~ 330–730 Ma; based on nitrogen aggregation estimates), that likely took place during the Pan-African Orogeny. We suggest that Neoproterozoic subduction-related saline fluids infiltrated different lithologies in the Kroonstad lithospheric mantle. Upon interaction with eclogite, melting occurred and diamonds crystallized, forming the older silicic-carbonatitic HDF-bearing diamonds with lower alkalis and La/Nb, Th/Nb, La/Sm ratios. Concurrently saline fluids that penetrated harzburgite had little interaction with the host rock and were stored as metasomes. These metasomes were locally re-melted during subsequent thermal event/s, potentially the Karoo flood basalt volcanism (~ 180 Ma), to form saline HDFs and their host diamonds. Later metasomatism that involved high-Mg carbonatitic HDFs was smaller in scale than the previous diamond-forming events and took place at < 160 Ma (< 30 Myr before the Voorspoed CROL erupted). The similarities in trace element and isotope compositions between Voorspoed HDFs and Kroonstad CROLs, support some degree of shared lithospheric origin or similar metasomatic processes that controlled their compositions.