Melting Reactions in the System NaAlSi_3O_8-KAlSi_3O_8-SiO_2-H_2O to 20 Kilobars Compared with Results for Other Feldspar-Quartz-H_2O and H_2O Systems

Abstract

Synthetic feldspar with composition Ab_(60)Or_(40) was crystallized from gel and a portion mixed with 10 wt percent natural quartz. These starting materials were reacted with excess water in sealed platinum capsules, using half-inch-diameter piston-cylinder apparatus. Univariant melting curves were located between 10 and 20 kbar pressure for the systems NaAlSi_3O_8(Ab)-KAlSi_3O_8(Or)-SiO_2(Qz)-H_2O and Ab-Or-H_2O. Points on the reaction curves are: (1) Ab_(ss)+Or_(ss)+Qz+V⇔L; 10 kbar at 615° C, 15 kbar at 605 °C. (2) Ab_(ss)+Or_(ss)+V⇔L; 10 kbar at 655°C, 15 kbar at 615° C, 16.75 kbar at 605 °C. (3) A reaction common to both systems, Or_(ss)+Jd+Qz+V⇔L (Jd is jadeite); 19.5 kbar at 630° C.. These three reaction curves meet at an invariant point at 16.8 kbar and 605° C. Reactions (1) and (2) represent the curves for the beginning of melting of granite and syenite, respectively. A review of previous results provides our best estimates for melting curves from 1 to 20 kbar (or higher) in these systems and additional systems Or-Qz-H_2O and CaAl_2Si_2O_8(An)-Qz-H_2O and rock-H_2O systems. Curves for the beginning of melting of major rock types in the presence of excess water have recently been extended to mantle pressures, and the influence of feldspar and quartz in granite, tonalite, syenite, and gabbro melting is evident to about 15 kbar, from comparison of the rock-H_2O curves with the feldspar-quartz-H_2O curves. The melting pattern changes above the pressure where plagioclase feldspar breaks down, and the slope (dP/dT) of the melting curves then becomes positive and roughly parallel with curves for dry systems. Data for excess-water systems are almost complete; these will provide guides for the study of water-deficient systems in the vapor-absent region, which is more relevant for problems of magma genesis and magma evolution.

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