Lherzolite - carbonatite melt interaction in the presence of additive CO2 and H2O: Experimental data at 5.5 GPa and 1200-1450°C

We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO₂ and/or H₂O in experiments at 5.5 GPa and 1200-1450°C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the CO₂ contents in the melt, which impedes immediate separation of CO₂ fluid from melt equilibrated with the peridotite source. The solubility of molecular CO₂ in melt decreases from 20-25 wt.% at 4.5-6.8 wt.% SiO₂ typical of carbonatite to 7-12 wt.% in more silicic kimberlite-like melts with 26-32 wt.% SiO₂. Interaction of garnet lherzolite with carbonatitic melt (2:1) in the presence of 2-3 wt.% H₂O and 9-13 wt.% molecular CO₂ at 1200-1450°С yields low SiO₂ (<10 wt.%) alkali‐carbonatite melts, which shows multiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can originate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM).

Having separated from the source, carbonatitic magma enriched in molecular CO₂ and H₂O can rapidly acquire a kimberlitic composition with >25 wt.% SiO₂ by dissolution and carbonation of entrapped peridotite. Furthermore, interaction of garnet lherzolite with carbonatitic melt rich in K, CO₂, and H₂O at 1350°С produces immiscible kimberlite-like carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops lamelli of foam-like vesicular glass. Differentiation of immiscible melts early during ascent may equalize the compositions of kimberlite magmas generated in different CLM sources. The fluid phase can release explosively from ascending magma at lower pressures as a result of SiO₂ increase which reduces the solubility of CO₂ due to decarbonation reaction of magnesite and orthopyroxene.

The research was performed by a grant of the Russian Science Foundation (19-77-10023).