LOW-PRESSURE PHASE RELATIONSHIPS IN THE SYSTEM NA2O–CAO–FEO–MGO–AL2O3–SIO2 AT 1100°C, WITH IMPLICATIONS FOR THE DIFFERENTIATION OF BASALTIC MAGMAS

Abstract

Experiments were performed on the system Na2O–CaO–FeO–MgO–Al2O3–SiO2 at 1100°C, with the interest focused on the assemblage Liq+Aug+Pl+Ol and its boundaries. Glass synthesized in a very reducing atmosphere was used as starting material. To avoid sodium loss during the experiment, the starting material was loaded into iron capsules, and the experiments were carried out in evacuated silica glass tubes. All phases in the products were identified and analysed with an electron microprobe. The probe analyses indicate that the assemblage Liq+Aug+Pl+Ol is stable over a wide range of compositions, and is bounded by the appearance of pigeonite in the silica-rich compositions. In the silica-poor compositions, the assemblage is successively bounded by the appearance of kirschsteinite, melilite, and nepheline with increasing sodium content. Owing to the isothermal and ‘isobaric’ divariant nature of the assemblage Liq+Aug+Pl+Ol in the studied system, a numerical method has been used to describe the phase compositions with Si and Na contents in the liquid as two arbitrarily chosen independent variables. This procedure results in quantitative characterization of the assemblage Liq+Aug+Pl+Ol over a range of compositions.