Kimberlite: Rapid Ascent of Lithospherically Modified Carbonatitic Melts

Abstract

Kimberlite deposits commonly contain in excess of 25 vol. % solid material in the form of mantle-derived macrocrysts and xenoliths, implying that the original magma was enriched in solids and, thus, dense. Numerous studies on this mantle-derived cargo, including diamond, suggest that the parental magmas derive from depths C200 km and that they are transported rapidly to the Earth’s surface. Virtually, all models for kimberlite ascent invoke the presence of an exsolved fluid (i.e. CO2 and H2O) phase for buoyancy, yet the cause and nature of fluid exsolution remain elusive. Here, we present high-temperature analogue melting experiments to demonstrate a new mechanism for the efficient, continuous, and spontaneous production of a volatile phase within ascending kimberlite. We suggest that the parent melts to kimberlite range in composition from carbonatitic to carbonate-rich silica-undersaturated melts. In transit through the cratonic mantle lithosphere (CML), these silica-undersaturated melts assimilate mantle minerals, especially orthopyroxene, driving the melt to more silicic compositions and causing a marked drop in CO2 solubility. The solubility drop manifests itself immediately by continuous and vigorous exsolution of a fluid phase, thereby reducing magma density, increasing buoyancy, and driving the rapid and accelerating ascent of the resultant kimberlite magma. Ultimately, continued orthopyroxene assimilation drives the melt towards silicate saturation allowing for late-stage magmatic crystallization of olivine as overgrowths on macrocrysts and as (micro-)phenocrysts.