Low-Pressure Phase Relationships in the System Na2O--CaO--FeO--MgO--Al2O3--SiO2 at 1100C, with Implications for the Differentiation of Basaltic Magmas

Experiments were performed on the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂at 1100C, with the interest focused on the assemblage Liq+Aug+Pl+Oland its boundaries. Glass synthesized in a very reducing atmospherewas used as starting material. To avoid sodium loss during theexperiment, the starting material was loaded into iron capsules,and the experiments were carried out in evacuated silica glasstubes. All phases in the products were identified and analysedwith an electron microprobe. The probe analyses indicate thatthe assemblage Liq+Aug+Pl+Ol is stable over a wide range ofcompositions, and is bounded by the appearance of pigeonitein the silica-rich compositions. In the silica-poor compositions,the assemblage is successively bounded by the appearance ofkirschsteinite, melilite, and nepheline with increasing sodiumcontent. Owing to the isothermal and ‘isobaric’divariant nature of the assemblage Liq+Aug+Pl+Ol in the studiedsystem, a numerical method has been used to describe the phasecompositions with Si and Na contents in the liquid as two arbitrarilychosen independent variables. This procedure results in quantitativecharacterization of the assemblage Liq+Aug+Pl+Ol over a rangeof compositions. ^*Present address: Geochemistry Group, Geology Dept., Beijing University, Beijing, 100871, P.R. China.