Chalcophile and siderophile elements in sulphide inclusions in eclogitic diamonds and metal cycling in a Paleoproterozoic subduction zone

Abstract

In the central Slave craton, eclogitic diamonds are suggested to have formed during Paleoproterozoic subduction in a meta-gabbroic substrate representing former lower oceanic crust that interacted with serpentinite-derived fluids at high fluid–rock ratios. In order to assess the behaviour of chalcophile and siderophile elements (CSE) during this process, we measured trace-element concentrations of sulphide inclusions in diamonds from the Diavik mine by laser ablation ICPMS. The nitrogen systematics of the diamonds (average N concentration of ∼600ppm and aggregation state 4% N as B-aggregates) indicate average mantle residence temperatures of ∼1050°C for a 1.85Ga formation age, corresponding ∼170km depth. Based on the available evidence from natural samples and experiments, we suggest that the highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pd, Pt, Re) except Au behaved largely conservatively during fluid-induced metamorphism, which may point to a reducing and Cl-poor nature of the fluid. The abundances of the moderately siderophile and chalcophile elements Cr, Co, Ni, Cu, Ag, Sn, Mo and W may also have changed little, whereas As, Sb, Tl, Pb and Bi may have been mobilised from the subducting lower oceanic crust. The partitioning of CSE in eclogite and geochemical behaviour during oceanic crust formation was assessed for inferred conservative elements. Assuming an average sulphide mode of 0.3wt.% for the oceanic crust, its abundances of HSE, Cu, Mo, Se and Te can mostly be accounted for by sulphide minerals alone. Lithophile behaviour of W, Cd, In and Sn and enrichment in residual melts may explain their lower abundances in the gabbroic eclogitic sulphide inclusions compared to MORB sulphide. These elements, as well as Cr, Co, Ni, Zn and Ga require additional host phases both in eclogite, where rutile partitions significant amounts of Cr, Zn, W, Ga and Sn, and in the oceanic crust.