Experimental study of eclogite melting with participation of the H2O-CO2-KCl fluid at 5 GPa

Abstract

In order to model the processes of formation of the highly alkaline (potassic) melts during the par- tial melting of the eclogite nodules in kimberlites, experiments on the melting of the model and natural eclogites in presence of the H 2 O-CO 2 and H 2 O-CO 2 -KCl fluids at 5 GPa and 1200 and 1300 ° C are performed. A com- parative analysis of the phase relations in the systems with H 2 O-CO 2 and H 2 O-CO 2 -KCl demonstrate that KCl in the fluid equilibrated with eclogites intensifies their melting. It is related to both high Cl concentration in the forming silicate melt (2.0-5.5 wt %) and its enrichment in K 2 O owing to the K-Na exchange reactions with the immiscible chloride melt. Because of these reactions, the K 2 O/Cl ratio in the melts increases with the KCl con- tent in the system and reaches 2.5-3.5 in the silicate melts coexisting with the immiscible chloride liquid. How- ever, the ratio KCl/(H 2 O + CO 2 + KCl) in the fluid does not influence on the ratio K 2 O/Cl in the melts. Thus, the solubility KCl in the melts, apparently, does not depend on presence of the H 2 O-CO 2 fluid, at least, within the concentration range used in the experiments (up to 20 wt %). The experiments show that the deliberated chloride liquid is necessary to form the potassium-rich chlorine-bearing silicate melts during the eclogite melt- ing. It corresponds to the KCl content in the system above 5 wt %.