Composition of primary kimberlite melt in a garnet lherzolite mantle source: constraints from melting phase relations in anhydrous Udachnaya-East kimberlite with variable CO2 content at 6.5 GPa

Abstract

The critical issue in the study of kimberlites, known as principal host rocks of diamonds, is the reconstruction of their primary melt composition, which is poorly constrained due to contamination by xenogenic materials, significant loss of volatiles during eruption, and post-magmatic alteration. It is generally accepted that the last equilibration of primary kimberlite melt with surrounding mantle (garnet lherzolite) occurred beneath cratons at 5–7GPa (150–230km depths). However, the subliquidus mineral assemblages obtained in kimberlite melting experiments at mantle pressures differ from lherzolite, probably owing to unaccounted loss of CO2. Here we present experiments at 6.5GPa and 1200–1600°C on unaltered kimberlite with an addition of 2–22mol% CO2 over its natural abundance in the rock (13mol%), but keeping proportions of other components identical to those in an exceptionally fresh anhydrous kimberlite from Udachnaya-East pipe in Siberia. We found that the partial melt achieves equilibrium with garnet lherzolite at 1500°C and 19–23mol% CO2 in the system. Under these conditions this melt contains (mol%): SiO2=9, FeO=6–7, MgO=23–26, CaO=16, Na2O=4, K2O=1, and CO2=30–35. We propose, therefore, the alkali-rich carbonatitic composition of primary kimberlite melt and loss of 34–45mol% (34–46wt%) CO2 during ascent of the kimberlite magma to the surface.