Daniele L. Pinti¹, Lucille Daver¹, Hélène Bureau3

(¹Geotop & Département des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, Succ. Centre-Ville, Montréal H3C 3P8, QC, Canada

²Consortium de recherche en exploration minérale (CONSOREM), Université du Québec à Chicoutimi, 

Chicoutimi, Québec, G7H 2B1, Canada

³Institut de Minéralogie de Physique des Matériaux et de Cosmochimie, Sorbonne Université, UMR CNRS 

7590, 75005 Paris, France) 

Abstract: The Diavik diamond mine on the Slave craton contains gem-quality monocrystalline diamonds (83% of peridotitic and 12% of eclogitic paragenesis). Here, we focus on the less-coveted polycrystalline and coated diamonds, which are enriched in fluid inclusions that record diamond growth conditions and metasomatic events that affected the Slave Craton, Canada. Previous studies suggested metasomatic events at 1.8 Ga during the Wopmay collisional event (Weiss et al., 2023, Goldschmidt Conf.); around 350 Ma, corresponding roughly to the age of clouded diamonds (Weiss et al., 2015, Nature); and at the kimberlite eruption (55 Ma ago; Brooker et al., 2024, GCA). A complete set of 7 polycrystalline and 12 coated fibrous diamonds, together with one olivine xenolith, was vacuum-crushed and fluid extracted for He, Ne, and Ar isotopes. Results were compared with noble gas data from diamonds and xenoliths of the nearby Ekati mine (Burgess et al., 2009, GCA; Pinti and Fedortchouk, unpublished). Helium isotopic ratios (³He/⁴He) do not correlate with ⁴He abundance or the nitrogen aggregation state, suggesting no in situ radiogenic He addition and thus reflecting pristine fluidcompositions. The ⁴He abundance is higher in Ekati than in Diavik diamonds, with the lowest values measured in olivine xenoliths. Most ³He/⁴He ratios fall within the range of sub-continental mantle (SCLM) or lower. The ⁴⁰Ar/³⁶Ar vs ⁴He/³⁶Ar ratio shows binary mixing between the mantle and seawater (reflecting subducting slab-derived fluids) or between the mantle and entrapped air during kimberlite eruption. However, air-corrected ³He/⁴He ratios plotted against terrigenic ⁴⁰Ar*/⁴He* ratios suggest three sources: undegassed mantle; degassed melts (mainly recorded by xenoliths), and a crustal component which may record subducting fluids. 

Keywords: Helium isotopes; Diamonds; metasomatic fluids; Slave Craton; Canada.

Author Profile (first or corresponding author): 

Daniele Pinti, Male, Full Professor; noble gas geochemist involved in groundwater, geothermal, and critical metals resources. E-mail: pinti.daniele@uqam.ca