Experimental Investigation into Interaction between Amphibole and Highly Saline H2O–NaCl–KCl Fluid at 750°C, 700 MPa: Implications to Alkaline Metasomatism of Amphibole Rocks

The paper presents experimental data on the interaction of amphibole with NaCl–H₂O and (K, Na)Cl–H₂O solutions at varying salt content. When interacting with H₂O–NaCl fluid, amphibole remains the predominant mineral in all experiments, and the newly formed minerals are Na-phlogopite, plagioclase, and nepheline/sodalite. At \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) > 0.6, the amphibole melts. When amphibole interacts with H₂O–NaCl–KCl fluid at \({{X}_{{{{{\text{H}}}_{2}}{\text{O}}}}}\) < 0.40 and Х_KCl/(Х_KCl + Х_NaCl) in the fluid, defined as Х_NaCl = 0.506 – 0.84Х_KCl, the amphibole is replaced by the association of nepheline with sodic plagioclase, sodalite, and biotite. At Х_KCl/(Х_KCl + Х_NaCl) > 0.3, nepheline, sodalite, and plagioclase become unstable, K-feldspar is formed, and biotite, clinopyroxene, and amphibole remain stable. At Х_KCl/(Х_KCl + Х_NaCl) > 0.5, the association Cpx + Bt + Kfs + Grt (grossular–andradite) is stable. Thus, grossular–andradite garnet is an indicator of a high potassium activity in the fluid, whereas nepheline testifies that the sodium activity was high. Na → K exchange is typical of the amphibole and biotite, and Ca → Na exchange occurs in the clinopyroxene, and all of these minerals (but neither nepheline nor garnet) remain generally stable within a wide range of the K/Na ratio in the fluid. Clinopyroxene in the experiments spans Ca–Fe–Mg compositions with a varying, sometimes high, Al content, and the amphiboles belong to the pargasite–hastingsite series. With an increase in \({{a}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) (\({{X}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) > 0.57), i.e., a decrease in the gross salinity of the fluid, melts are generated, and their composition varies from trachyte to phonolite. An increase in the Х_KCl/(Х_KCl + Х_NaCl) ratio in the fluids leads to a decrease in alumina content of the melts. An increase in the total salinity of the fluid leads to an increase in the content of potassium in the melt and a decrease in the content of chlorine in it. The experiments have shown that interaction between amphibole and fluids containing high NaCl and KCl concentrations results in mineral associations typically produced by alkaline metasomatism of amphibole-bearing rocks and concomitant HCl enrichment in the fluid phase. The substitution of highly saline fluids for highly acidic ones leads to the leaching of Ca, Mg, Fe from the metamorphic rocks, and the transport and redeposition of these components. It follows that significant removal of FeO, MgO, CaO from rocks is sometimes a consequence of the interaction of the host rocks with saline aqueous solutions.