Experimental Study of Eclogite Melting with Participation of the H_2O-CO_2-KC1 Fluid at 5 GPa

摘要

In order to model the processes of formation of the highly alkaline (potassic) melts during the partial melting of the eclogile nodules in kimberlites, experiments on the melting of the model and natural eclogites in presence of the H_2O-CO_2 and H_2O-CO_2-KCI fluids at 5 GPa and 1200 and 1300℃ are performed. A comparative analysis of the phase relations in the systems with H_2O-CO_2 and H_2O-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_2O owing to the K-Na exchange reactions with the immiscible chloride melt. Because of these reactions, the K_2O/Cl ratio in the melts increases with the KCl content in the system and reaches 2.5-3.5 in the silicate melts coexisting with the immiscible chloride liquid. However, the ratio KC1/(H_2O + CO_2 + KCl) in the fluid does not influence on the ratio K_2O/Cl in the melts. Thus, the solubility KCl in the melts, apparently, does not depend on presence of the H_2O-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 melting. It corresponds to the KCl content in the system above 5 wt %.