Mantle formation and evolution, Slave Craton: constraints from HSE abundances and Re-Os isotope systematics of sulfide inclusions in mantle xenocrysts

Major elements, trace elements including highly siderophile elements (HSE) and Re-Os isotope ratios were analysed in situ on individual sulfide grains and one Fe metal grain enclosed in fresh xenocrystic olivine and pyroxenes from kimberlites in the Lac de Gras area, central Slave Craton. Reconstruction of low-temperature assemblages to high-temperature precursor sulfides shows that there are two major sulfide populations: nickeliferous monosulfide solid solution (mss), and a distinct group of S-deficient Ni- and Co-rich sulfides ((Ni,Co,Fe)(3-x)S-2). The inclusion suite is unusual because all (Ni,Co,Fe)(3-x)S-2, a subgroup of mss, and Fe metal inclusion have high W concentrations (up to 3400, 2600 and 24,000 ppm, respectively) and because (Ni,Co,Fe)(3-x)S-2 have higher Ni and Co concentrations (mean 47 and 6.8 wt.%, respectively) combined with higher metal/sulfur (mean Me/S = 1.1) than the great majority of mantle-derived sulfides reported in the literature. Ca-in-olivine temperatures indicate that the host olivine and included sulfides are derived from the deep layer of the stratified lithosphere beneath Lac de Gras. Re-Os isotope data were collected for 24 mss inclusions. Re-187/Os-188 ranges from 0.006 to 2.71, and Os-187/Os-188 from 0.1002 to 0.4732, corresponding to gamma(Os) from - 21 to 272. The sulfides show a major mode of T-RD at similar to 2.8 Ga, extending to 3.9 Ga, and of T-CHUR at similar to 2.9 Ga. Eleven samples define an isochron at t = 3.27 +/- 0.34 Ga, with an enriched initial Os-187/Os-188 at t of 0.10725 +/- 0.00014 (gamma(Os,i) = 2.53 +/- 0.13; MSWD = 0.75). Weak covariations of Os contents and Re-187/Os-188 in mss with Mg/(Mg + Fe) of host olivine are consistent with variable degrees of melt depletion of sulfide-bearing peridotite. The incoherent behaviour of incompatible element abundances, the unrelated enrichment in the fluid-mobile elements W, As and Zn, and the unsupported radiogenic Os and recent Re-enrichment in some sulfides suggest repeated metasomatic processes subsequent to melting. The formation of the unusual (Ni,Co,Fe)(3-x)S-2 could be related to metasomatism or alteration of mantle sulfide. However, the major element compositions of mss and (Ni,Co,Fe)(3-x)S-2 would also be consistent with an igneous origin, by fractionation from extremely Ni-rich sulfide melts that unmix from S-bearing metal melts expected in the lower mantle. Despite the evidence for interaction with fluids, some sulfides were able to preserve ancient melt-depletion signatures. Model ages and the isochron age in the deep lithosphere beneath Lac de Gras exceed the age of the overlying crust of the Contwoyto terrane. This "age paradox" may be reconciled if the lithospheric mantle beneath the adjacent older Central Slave Basement Complex (CSBC) was thrust beneath the younger Contwoyto terrane during collision of these two entities, resulting in a translithospheric northeast-dipping suture. (C) 2004 Elsevier B.V. All rights reserved.