SILICA-UNDERSATURATED PORTION OF THE SYSTEM NEPHELINE - KALSILITE - SiO2 AT 2 GPa [P(H2O) = P(Total)

Abstract

The nepheline (Ne) – kalsilite (Kls) – sanidine (Sa) – albite (Ab) quadrilateral representing petrogeny’s residua system for silica-undersaturated alkalic magmas has been studied at 2 GPa [P(H 2 O) = P(Total)] and various temperatures. The composition of the eutectics, their assemblages and the equilibrated temperatures of the various binary joins studied at 2 GPa in presence of excess H 2 O are as follows. Along the Kls–Sa join, the eutectic occurs at Kls 13 Sa 87 (all compositions are expressed in wt% unless specified) and 700°C, where Kls, Sa ss , liquid and vapor are in equilibrium. Along the Kls–Ne binary join, the eutectic is located at Kls 44 Ne 56 and 650°C, where Kls and Ne ss coexist with liquid and vapor. The eutectic point of the binary join Ne–Ab occurs at Ne 87 Ab 13 and 670°C, where Ne ss , Jd ss , liquid and vapor are in equilibrium. In the case of the Ab–Kfs binary join, the eutectic lies at Ab 73 Kfs 27 and 630°C, where Jd ss , Sa ss , quartz, liquid and vapor are the equilibrium assemblage. There are two eutectics in the silica-undersaturated portion of the Ne–Kls–SiO 2 system, studied at 2 GPa in presence of excess H 2 O: 1) E 1 occurs at Kls 18 Ne 52 Qtz 30 and 620°C, where Jd ss , Sa ss , Ne ss , liquid and vapor are in equilibrium, and 2) E 2 is located at Kls 49 Ne 25 Qtz 26 and 615°C, where Ne ss , Kls, Sa ss coexist with the liquid. In this system, the maximum (M) produced by the intersection of the join nepheline–sanidine with the nepheline–feldspar cotectic line occurs at Kls 35 Ne 38 Qtz 27 and 635°C, where Ne ss + Sa ss + Gl + V are in equilibrium. In contrast to the phase-equilibria studies made at 1 atm, 0.1, 0.2 and 0.5 GPa in presence of excess H 2 O, there is no field of leucite ss at 2 GPa. The join Ab–Sa no longer acts as a thermal barrier as at low pressure. Instead, Jd (jadeite)–Sa becomes the stable thermal barrier; because of this, all compositions within the triangle Ab–Sa–Jd yield Sa ss , Jd ss and quartz at 2 GPa isobaric polythermal conditions in presence of excess H 2 O. The same compositions at low pressure produce feldspathoid– feldspar-bearing assemblages. The occurrence of jadeite along with feldspathoid and feldspar in nepheline syenites arises because the bulk composition of the magma lies in the silica-deficient portion of the system Jd–Sa–Ne, whereas the occurrence of Jd–Sa– SiO 2 observed in association with granitic rocks is related to crystallization of the magma at a high pressure.