The geological record of base metal sulfides in the cratonic mantle: A microscale Os-187/Os-188 study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

We report detailed petrographic investigations along with Os-187/Os-188 data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. When devoid of alteration, BMS grains mainly consist of pentlandite + pyrrhotite +/- chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement. The Os-187/Os-188 measured in the twenty analysed BMS grains vary from 0.1084 to > 0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in Os-187/Os-188 is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock T-RD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution. The T-RD ages determined in BMS grains are clustered around 2.8-2.7, similar to 2.2 and similar to 1.9 Ga. The 2.8-2.7 Ga T-RD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic T ages can be explained by addition of metasomatic BMS during (i) major lithospheric rifting at similar to 2.2 Ga and (ii) the Taltson-Thelon orogeny at similar to 1.9 Ga. The data suggest that even metasomatic BMS can inherit Os-187/Os-188 from their original mantle source. The lack of isotopic equilibration, even at the micro-scale, allowed the preservation of different populations of BMS grains with distinct Os-187/Os-188, providing age information on multiple magmatic events that affected the SCLM. (C) 2017 Elsevier Ltd. All rights reserved