Melting Relationships in the System NaAlSi_3O_8-NaCl-H_2O at One Kilobar Pressure, with Petrological Applications

Abstract

Phase relationships in the system NaAlSi_3O_8-NaCl-H_2O appear to be ternary between 850° C and 950° C at 1 kbar pressure. A two-phase field containing NaCl-rich liquid and H_2O-rich vapor extends from the system NaCl-H_2O into the ternary system, although this field was not intersected by the joins investigated. In the system NaAlSi_3O_8-H_2O a wide miscibility gap occurs between liquid and vapor; in the system NaAlS_2O_8-NaCl a wide miscibility gap extends between silicate-rich liquid and NaCl-rich liquid at temperatures above 1,100° C at 1 bar pressure and also by inference at 1 kbar pressure. Both miscibility gaps are connected through the ternary system, separating a silicate liquid from a fluid phase with composition close to the join NaCl-H_2O and containing a small, unknown proportion of dissolved silicates. A temperature minimum is present on the ternary liquidus at 873 ± 5° C, representing the reaction: Albite+Fluid (H_2O-Nacl) ⇔ Liquid_(Albite) (1) The liquid composition, in terms of the anhydrous components, is approximately 99.7 wt percent NaAlSi_3O_8, 0.3 wt percent NaCl; its H_2O content is about 10 wt percent. It is expected that at lower pressures two additional reactions will occur: Albite+Vapor=Liquid_(Albite)+Liquid_(Nacl) (2) and Albite+Liquid_(Nacl)+Vapor; ⇔ Liquid_(Albite). (3) The results contrast with those in the system NaAlSi_3O_8-NaF-H_2O and confirm previous conclusions from the systems NaAlSi_3O_8-HCL-H_2O and NaAlSi_3O_8-HF-H_2O. Whereas fluoride (or NaF) tends to remain in the liquid (magma), chlorine (or NaCl) passes preferentially into the vapor or fluid phase. The solubility of H2O in a silicate melt increases when a small quantity of chlorine is present. Coexistence of H_2O-rich liquid inclusions and NaCl-rich liquid inclusions in crystalline phases in igneous rocks may indicate low-pressure conditions (<1 kbar) during capture of the inclusions.

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