Type Ib diamond formation and preservation in the West African lithospheric mantle: Re–Os age constraints from sulphide inclusions in Zimmi diamonds

• 10 sulphides from 3 Zimmi diamonds have Re–Os isotopic compositions that fall along 650 Ma age arrays.
• Diamond formation is temporally related to Gondwana assembly, recorded in the Rokelide orogen.
• Sulphides have radiogenic 187Os/188Os and low Re/Os that is inconsistent with a syngenetic origin.
• Zimmi sulphides are protogenetic, residing in the lithosphere prior to 650 Ma diamond formation.
• Isolated nitrogen in the diamonds suggests they were tectonically exhumed shortly after 650 Ma.

Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe–Cu–Ni–Co contents and Re–Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850 °C for any extended period. Such diamonds make up less than 0.1% of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic 187Os/188Os, all features characteristic of eclogitic sulphides. Each diamond has 3–4 individual inclusions. 187Re/188Os and 187Os/188Os data fall along three individual ∼650 Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond – unambiguously dating the time of diamond formation. The ∼650 Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial 187Os/188Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650 Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.