Sulphide survival and diamond genesis during formation and evolution of Archaean subcontinental lithosphere: A comparison between the Slave and Kaapvaal cratons

Sulphide inclusions from 35 eclogitic and 7 peridotitic diamonds from the Diavik kimberlites in the central Slave craton have been characterized to address questions of diamond age and craton formation. Eclogitic sulphide inclusions occur in diamonds with mantle-like δ¹³C (- 4.94 ± 0.72 1σ) and low N aggregation states (%N as B = 8.2 ± 10.0, average N contents of 720 ppm) indicative of relatively low mantle residence temperatures. A 1.86 ± 0.19 Ga Re-Os age array for eclogitic sulphides with suprachondritic initial ¹⁸⁷Os/¹⁸⁸Os of 0.13 (± 0.10) indicates a close temporal link between eclogitic diamond formation, eclogite emplacement and collisional events affecting the Slave craton. Sulphides in peridotitic diamonds plot on older, previously established 3.3 and 3.5 Ga isochrons, consistent with higher average N aggregation states (~ 20%) despite lower N contents (~ 230 ppm) for their host diamonds compared to eclogitic diamonds. Two intriguing observations emerge from a comparison of diamond populations and formation ages between the Slave and Kaapvaal that indicate fundamentally different and common diamond formation mechanisms, respectively: (1) Despite the general abundance of peridotitic silicate inclusions, peridotitic sulphide inclusions are rare in the Kaapvaal and occur in relatively young diamonds whereas in the central Slave there is a sizable Archaean population. (2) Compared to the distribution of silicate inclusions in diamonds, both cratons have an overabundance of eclogitic relative to peridotitic sulphide inclusions. During Kaapvaal lithospheric mantle formation, large melting intervals, as gauged by extremely depleted silicate inclusions in diamonds, led to exhaustion of sulphide in the residue. Formation of peridotitic sulphide inclusion-bearing diamonds occurred only significantly later, after re-sulphidation accompanying metasomatism. By contrast, the less depleted deep lithospheric mantle beneath the central Slave craton may have formed during plume subcretion, leading to smaller melting intervals due to the presence of a pre-existing lithospheric mantle lid, thereby allowing for coeval precipitation of sulphide inclusion-bearing diamonds. Abundant eclogitic sulphide inclusion-bearing diamonds that can be related to accretionary processes along the edges of the Slave and Kaapvaal craton indicate that sulphide-saturated eclogite is a fertile source for diamond formation. Reduced fluids from dehydration of underlying seawater-altered peridotite may react with the overlying oceanic crust to precipitate eclogitic sulphide-bearing diamonds penecontemporaneously with metamorphism and tectonic emplacement of eclogite into the subcratonic lithosphere.