| Реферат: | eng: According to the existing models of kimberlite origin, free exsolution CO2 may be an important agent in the evo-lution of primary kimberlite magma and initiation of crack propagation. We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO2 and H2O in experiments at 5.5 GPa and 1200-1450 degrees C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the contents of molecular CO2 in the melt, which impedes immediate separation of CO2 fluid from melt equilibrated with the peridotite source. The solubility of molecular CO2 in the melt decreases from 20-25 wt% at 4.5-6.8 wt% SiO2 typical of carbonatite to below 7-12 wt% in more silicic melts with 26-32 wt% SiO2. Interac-tion of garnet lherzolite with carbonatitic melt (at a weight proportion of 2:1) in the presence of 2-3 wt% H2O and 17-24 wt% of total CO2 at 1200-1450 degrees C yields low-SiO2 (<10 wt%) alkali-carbonated melts, which shows mul-tiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can orig-inate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM). Excessive volatiles may be present in carbonatitic melts not equilibrated with the peridotitic source due to the formation of metasomatic reaction zones. Having separated from the source, carbonatitic magma enriched in molecular CO2 and H2O can rapidly become more silicic (>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 degrees C produces immiscible carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops globules of foam-like vesicular glass. Differentiation of immiscible melts early during their 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 and due to the decarbonation re-action of magnesite and orthopyroxene. (C) 2021 Elsevier B.V. All rights reserved.
|