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Russian Geology and Geophysics

2005 year, number 12


I.D. Ryabchikov and A.V. Girnis
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry,
Russian Academy of Sciences, 35 Staromonetny per., Moscow, 119017, Russia
Keywords: Kimberlite, peridotite, experiment, melting of mantle, phase equilibria, solubility of CO2
Pages: 1202-1212


Experimental data on melting of carbonatized mantle peridotites, phase equilibria during the high-pressure crystallization of kimberlite melts, and solubility of CO2 in kimberlite-like melts are analyzed. Melting of lherzolite at ~5 GPa in the presence of CO2 yields a wide spectrum of compositions depending on the content of the latter: from high-magnesium picritic magmas in CO2-free systems to dolomitic melts containing <5 wt.% CO2. Low-calcium melts of kimberlite composition from this series contain ~20 wt.% CO2. Experimental studies showed that the solubility of CO2 in kimberlite melts drastically increases at pressures of >4.5 GPa and reaches 20 wt.% at 5 GPa. At ~6 GPa, there is a stable garnet + orthopyroxene + magnesite association on the liquidus of CO2-saturated kimberlite melt. Experimental results indicate that the melting of magnesite-bearing harzburgite at ~6 GPa produces CO2-saturated kimberlite-like melts. Analysis of geochemical data showed that the ratios of CO2 to elements of a close degree of incompatibility (e.g., Th) in such hypothetic melts are nearly the same as in the primitive and depleted mantle. Thus, deep metasomatism of the mantle source is not necessary for the formation of kimberlite magmas, and high contents of incompatible elements might be the result of the extremely low degrees of rock melting. The proposed model for the formation of kimberlite magmas implies the interaction of melts from the asthenospheric mantle with garnet harzburgite in the lower continental lithosphere. This process can lead to CO2 saturation of the melts at a depth of ~200 km, which will favor a rapid magma ascent and transport of deep-seated minerals.