|Title||The geological record of base metal sulfides in the cratonic mantle: a microscale 187 Os/188 Os study of peridotite xenoliths from Somerset island, Rae craton (Canada)|
|Author||Bragagni, A; Luguet, A; Fonseca, R O C; Pearson, D G; Lorand, J P; Nowell, G M; Kjarsgaard, B A|
|Source||Geochimica et Cosmochimica Acta vol. 216, 2017 p. 264-285, https://doi.org/10.1016/j.gca.2017.04.015|
|Alt Series||Natural Resources Canada, Contribution Series 20170072|
|Media||paper; on-line; digital|
|Lat/Long WENS|| -94.0000 -90.0000 74.0000 72.5000|
|Subjects||xenoliths; petrographic analyses; sulphides; pentlandite; pyrrhotite; chalcopyrite; kimberlites; metasomatism; metasomatic deposits; Base Metal Sulfide (BMS); Rae Craton; Sub Continental Lithospheric
|Illustrations||geological sketch maps; photomicrographs; tables; plots|
Geo-mapping for Energy and Minerals|
|Abstract||We report detailed petrographic investigations along with 187Os/188Os data in Base Metal Sulfide (BMS) on cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on
the processes affecting the Re-Os systematic and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton. |
The highest BMS abundance is observed in the two investigated peridotites with
the highest bulk Pd/Ir and Pt/Pd, attesting a more extensive sulfide metasomatism. Overall, in the four investigated xenoliths, the majority of the BMS grains consist of pentlandite-pyrrhotite-chalcopyrite, which likely formed during melt/fluid
percolation in the Sub Continental Lithospheric Mantle (SCLM). Metasomatic sulfides were also introduced during infiltration of the host kimberlite magma, which resulted in the crystallization of djerfisherite around older Fe-Ni-sulfides along with
the formation of clinopyroxene rims around orthopyroxene. On the whole-rock scale, the interaction with the kimberlite is visible in a subset of bulk xenoliths as a Re-Os errorchron that dates the host magma emplacement. The latest modifications of
the sulfide mineralogy occurred during serpentinization, when heazwloodite replaced pentlandite, triggering the formation of platinum group minerals.
Twenty BMS grains were extracted from thin sections and successfully analysed for 187Os/188Os. No
correlation is observed between 187Os/188Os and sulfide assemblage, except for the radiogenic signature (187Os/188Os=0.172) of a djerfisherite-rich grain. The largest range in 187Os/188Os is observed in BMS grains from the two xenoliths with the
highest PPGE/IPGE (i.e. Pt and Ir over Os, Ir, and Ru) content and BMS modal abundance. The whole-rock TRD ages of these two samples underestimate the melting age obtained from BMS, stressing caution in the interpretation of Re-Os model ages from
samples with clear evidence of metasomatism.
The TRD ages obtained from BMS grains are clustered around 2.8-2.7, ~2.2 and ~1.9 Ga. The 2.8-2.7 Ga TRD ages document the main SCLM formation in the Rae craton, which is likely related to the
emplacement of the local greenstone belts in a continental rift setting. The Paleoproterozoic TRD ages can be explained by addition of metasomatic BMS during major lithospheric rifting at ~2.2 Ga and via subduction associated with the Taltson-Thelon
orogeny at ~1.9 Ga.
The data depicted here suggest that even metasomatic BMS can inherit 187Os/188Os from their original asthenospheric source. The lack of isotopic equilibration, even at the micro-scale, allows the preservation of different
populations of BMS grains with distinct 187Os/188Os, providing age information on multiple events that affected the SCLM.
|Summary||(Plain Language Summary, not published)|
Age determinations on individual sulphide mineral grains are compared with age determinations from whole rock sulphides to better understand lithospheric
mantle formation, and subsequent processes.