Experimental constraints on orthopyroxene dissolution in alkali-carbonate melts in the lithospheric mantle: Implications for kimberlite melt composition and magma ascent

Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150-250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na2CO3 + 0.3K2CO3 + 2CaMg(CO3)2), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1-6.5 GPa and T = 1200-1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg2Si2O6 (Opx) + CaMg(CO3)2 (CL) = CaMgSi2O6 (clinopyroxene) + 2MgCO3 (CL) and Mg2Si2O6 (Opx) = Mg2SiO4 (olivine) + SiO2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO2 in the experiments because the CO2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO2 fluid at 3.1-6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100-250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition.