James Kelly Russell

Carbonated mantle is a significant source for many alkaline magmas, yet carbonate minerals are rarely preserved in samples of mantle lithosphere. Here, we report the first occurrence of mantle-equilibrated (“primary”) carbonate within spinel peridotite xenoliths from the Pacific Coast Ranges of North America. The xenoliths are hosted in a 19 Ma basanite dyke at Mt. Preston, British Columbia near the boundary between the Intermontane and Coast Belts of the Canadian Cordillera. Magnesian calcite (X_Ca ~0.90) occurs in all samples as: i) intergranular grains in textural equilibrium with surrounding minerals, ii) inclusions within mantle silicates; and as iii) intergranular or fracture-filling veins. Sulphides (pentlandite and chalcopyrite) occur in association with carbonate. Two-pyroxene geothermometry on carbonate-bearing mantle xenoliths return temperatures of ~815 to 1120 °C, corresponding to depths of ~32 to 55 km on a model geotherm for warm, thin Cordillera-style lithosphere. C–O isotopic compositions of the xenoliths vary as δ¹³C = -3 to -6 ‰, and δ¹⁸O = 10 to 12 ‰. Collectively, the textural, geochemical and isotopic evidence suggest that a carbonate melt with associated monosulphide solution metasomatized and enriched previously depleted mantle lithosphere. The metasomatic fluids sourced from the subduction of oceanic crust beneath North America during Coast Plutonic Belt magmatism when Mt. Preston was in an arc to back arc position. The host basanite dyke contains magmatic groundmass calcite (δ¹⁸O = 14±0.2‰, δ¹³C = -4±0.2‰, ⁸⁷Sr/⁸⁶Sr = 0.704018) indicating a high intrinsic P_CO2 that inhibited thermal decomposition of mantle carbonate within the xenoliths during ascent.