In situ Re‐Os analysis of sulfide inclusions in kimberlitic olivine: New constraints on depletion events in the Siberian lithospheric mantle

Abstract

Os and other highly siderophile elements in mantle‐derived peridotites are strongly concentrated into trace amounts of sulfide minerals. We have used a laser‐ablation microprobe coupled to a multicollector ICPMS (LAM‐MC‐ICPMS) to determine the Os isotope composition, Re/Os and Pt/Os of 92 sulfide inclusions in olivine macrocrysts, derived from mantle peridotites, from the Udachnaya kimberlite in the Siberian craton. 26 of these have also been analyzed for Platinum Group Elements and other trace elements by LAM‐ICPMS. The sulfides are mixtures of Ni‐rich and Fe‐rich monosulfide solid solutions (MSS), pentlandite and chalcopyrite, exsolved from MSS bulk compositions. They can be divided into five populations (1, 2, 3A–3C) on the basis of Os content, Os/Pt and Re/Os. The genetic relationships of these groups can be constrained by comparison with published experimental data on element partitioning between MSS and sulfide melts. Group 1 sulfides can be modeled as the MSS residual after low degrees of melting of a primitive mantle source, under sulfur‐saturated conditions. Group 2 sulfides are best modeled as mixtures of MSS and alloy phases, formed at low degrees of melting under sulfur‐undersaturated conditions; many contain Pt‐rich micronuggets. Group 3C sulfides can be modeled as sulfide liquids, or as MSS crystallized from very evolved sulfide liquids. Sulfides of Groups 3A and 3B are interpreted as the products of reaction between MSS of Groups 1 and 2, and liquids of Group 3C. Inclusions of different groups may occur within single olivine grains, suggesting repeated introduction of sulfide melts, followed by annealing and grain growth. Sulfides of Groups 1 and 2, and some 3A sulfides, give Os model ages (TMA) that are geologically reasonable (0–4 Ga). Most Group 3 sulfides contain unsupported 187Os, implying a two‐stage history. A negative correlation between TMA and Re/Os is consistent with mixing between residual MSS of Groups 1 and 2, and liquids (3C) derived from a source with a high Re/Os, such as the ca 3.0 Ga eclogites described from Udachnnaya [Pearson et al., 1995c]. Our modeling suggests that sulfides with 187Re/188Os < 0.07 are unlikely to have been disturbed. Fifty‐two grains satisfy this criterion; 45 of these give TMA ages between 2.5 and 3.6 Ga, and 35 are >2.8 Ga. The data suggest that most of the lithospheric mantle beneath the Daldyn kimberlite field formed during the period 3–3.5 Ga, and that lithosphere formation culminated in a major event at ca 2.9 Ga, which may have involved remelting of older eclogites. There is little evidence in the Re‐Os data for significant additions to the lithosphere after this time. In situ Re‐Os analysis of single sulfide inclusions removes some of the ambiguity involved in the analysis of whole rock peridotite samples (or even separated olivine grains), where several generations of sulfides may be present. In situ analysis, combined with careful petrographic and chemical study of the sulfide populations, thus can provide more precise temporal constraints on the evolution of lithospheric mantle.