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Figure 1 shows the mantle age spectra obtained from the sulfides compared to known crust formation and tectonic events in the Slave craton. & Makovicky, M, 1996, Partitioning of nickel, copper, iridium, rhenium, platinum, and palladium between monosulfide solid solution and sulfide liquid: Effects of composition and temperature, Geochim.

The isochron age coincides with a period of crust formation between 3.3 and 3.4 Ga in the western craton, a continental complex.

Therefore, this system is ideal to for tracinge lithosphere forming events where extraction of a melt volume strips depletes the initially primitive mantle source of itsin Re-content and leaves behind a residue enriched in Os, with which retardsed the ingrowth of the radiogenic Os.

Moreover, due to the high concentration of Os in the residue and the typically low Os concentrations in mantle-derived melts, this system is thought to be robust during subsequent metasomatism.

Image analysis and major element data reveal that of 72 samples, 47 are Fe-rich mono-sulfide solid solution (mss), 12 Ni-Co-rich mss.

Fe-rich mss have Me/S (metal-sulfur ratios) = 0.78 1.17, Ni/(Ni Fe) = 0.06 0.45 and Co-contents of generally = -7.29 - -9.88, with a mean at 7.92.

Four samples of the Ni-Co-rich suite yielded very high chondrite-normalised abundances of the compatible PGE (10,000's to 100,000's), whereas incompatible elements trend towards values an order of magnitude lower values. Ni-Co-rich mss tend to have lower f O = -8.88 - -13.54, mean = -10.46) than Fe-rich mss.

(2) A group of Fe-rich mss (N=4) which have high Re/Os yet lowbut unradiogenic Os isotopic compositions, indicating that the Re-enrichment is very recent and that the compatible trace element budget has not been disturbed. Also, Re-Os data obtained for many different Archaean mantle sections should reveal whether there have been major mantle formation events in the early Earth history that led to the generation and stabilisation of continental lithosphere, or whether the formation and growth of continents has proceeded continuously.Samples, Analytical Techniques, Results 72 sulfide samples were picked from heavy mineral concentrate, mounted in situ in their host in aluminium sleeves and polished.Both appear to have been derived from a moderately depleted deep layer of the stratified lithosphere identified by Pearson et al. Age data for primary sulfides indicate that at least a portionsome parts of the mantle is are significantly older than the overlying arc-related crust, but coincide withsimilar ages are found in the adjacent older terrane (the Central Slave Basement Complex, CSBC), which lies just 100 km to the west., This may suggesting that ancient mantle was subducted beneath the Lac de Gras area during the collision of the terranes.Introduction The Re-Os isotope system is unique among the systems commonly used in geochronology in that the parent isotope is mildly incompatible whereas the daughter isotope behaves compatibly during mantle melting (Morgan 1986).

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