Experimental Study of Pargasite NaCa2(Mg4Al)[Si6Al2O22](OH)2 Stability at T = 1000–1100°C and Pressure up to \({{P}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) = 5 Kbar

Abstract

Pargasite stability was experimentally studied in IHPV at \({{P}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\) = 2 kbar and temperatures of 1000 to 1100^oC, with equilibrium approached from above and below. Calcic amphibole was used to experimentally model processes that occur in a volcanic chamber at pressures up to 5 kbar. The phase diagram of pargasite has been refined. It has been established that the stability of pargasite is controlled by three reactions. (1) At low water pressures of less than 1 kbar, the dehydration reaction Prg = Fo + Sp + Di + Ne + An + H₂O proceeds. (2) At water pressures higher than 1.2–1.5 kbar and a temperature of about 1100°C, the decomposition of pargasite is controlled by its incongruent melting Prg = Fo + Sp + {Di + Ne + An}^L + H₂O. (3) The third reaction Prg + L = Fo + Sp + Di + {Ne + Pl}^L + H₂O occurs within the same pressure range as the previous one but at lower temperatures of about ~1050°C. The reaction controls the pargasite liquidus and is caused by interaction between amphibole and coexisting melt. The liquidus of pargasite seems to most strongly depend on the activity of silica \({{a}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}\) in the melt.