James Kelly Russell

Carbonated mantle is widely considered a significant source for many alkaline magmas, yet carbonate minerals are rarely preserved in mantle samples due to their tendency to volatilize under typical magmatic and surface conditions. Here, we report the first occurrence of mantle-equilibrated (“primary”) carbonate within spinel peridotite xenoliths from the Pacific Coast Ranges in North America. The xenoliths are hosted in a 19 Ma basanite dyke near the boundary between the Intermontane and Coast Belts of the Canadian Cordillera (Mt. Preston, British Columbia). Magnesian calcite (Ca ratio ~0.90) occurs in all samples as: i) intergranular grains in textural equilibrium with surrounding minerals (granoblastic texture,  0.4 mm grains; < 2 vol%), ii) inclusions within mantle silicates; and as iii) intergranular or fracture-filling veins. Sulphides (pentlandite and chalcopyrite), also occur in equilibrium with the carbonate. Geothermometry on carbonate-bearing mantle xenoliths record paleo-equilibrium temperatures of ~815 to 1120 °C, corresponding to depths of ~32 to 55 km on a model geotherm for warm, thin Cordilleran-style lithosphere. Bulk-rock C–O isotopes of the xenolith suite 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±, δ13C = -4±, 87Sr/86Sr = 0.7040±) indicating a high intrinsic PCO2 that inhibited thermal decomposition of mantle carbonate within the xenoliths during ascent.