|Titre||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)|
|Auteur||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|
|Séries alt.||Ressources naturelles Canada, Contribution externe 20170072|
|Document||publication en série|
|Media||papier; en ligne; numérique|
|Lat/Long OENS|| -94.0000 -90.0000 74.0000 72.5000|
|Sujets||xénolites; analyses pétrographiques; sulfures; pentlandite; pyrrhotite; chalcopyrite; kimberlites; métasomatose; gisements métasomatiques; Craton de Rae |
|Illustrations||geological sketch maps; photomicrographs; tables; plots|
|Programme||Diamands, GEM : La géocartographie de l'énergie et des minéraux|
|Résumé||(disponible en anglais seulement)|
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.
|Résumé||(Résumé en langage clair et simple, non publié)|
Les determinations d'âge sur des grains individuels de mineraux de sulfures sont comparées aux roches de sulfure entières pour mieux comprendre la
formation du manteaux lithosphérique, et les processus subsequent.