Mantle xenocrysts from the Arkhangelskaya kimberlite (Lomonosov mine, NW Russia): Constraints on the composition and thermal state of the diamondiferous lithospheric mantle

The Arkhangelskaya kimberlite pipe belongs to the Zolotitsa kimberlite field in the Arkhangelsk region, NW Russia. It is the first pipe of the Lomonosov diamond mine to be put into production, with 2 million tons of ore already extracted. In this study major and trace element compositions of garnet, clinopyroxene (Cpx), Mg-ilmenite and chromite xenocrysts from the Arkhangelskaya pipe have been used to infer information about the compositional variability of the mantle underlying the Zolotitsa field. Single-grain thermobarometry of peridotitic Cpx xenocrysts yields a cool cratonic geotherm that follows a ca. 36 mW/m2 conductive model. Equilibration temperatures of garnet and chromite grains based on Ni- and Zn-thermometry, respectively, indicate a sampling interval of ca. 70–230 km of the lithospheric mantle when projected onto the Cpx-derived geotherm. The major element chemistry of Mg-ilmenite xenocrysts suggests that almost optimal redox conditions for diamond preservation prevailed in the mantle during the time of emplacement of the host kimberlite magmas. Garnet major and trace element compositions combined with the Cpx-geotherm indicate that the peridotitic diamond window extends from 130 to 210 km under Zolotitsa and that the deeper parts of the lithosphere have been affected by metasomatic events. Arkhangelskaya seems to have sampled the bulk of its diamonds from the deepest portion of the diamond stability field, between 190 and 210 km. In comparison, the neighbouring Lomonosova and Pionerskaya pipes are known to have collected their diamonds from 130–160 km. The comparable grade of the three pipes suggests that diamondiferous material is generously distributed within the diamond stability field. The remarkable difference evidenced by garnet composition and thermobarometry between Arkhangelskaya and the two other Zolotitsa pipes probably derives from differences in rheology and eruption rates of the rising kimberlite magmas.