Phase equilibrium constraints on the petrogenesis of nephelinites with reference to the system diopside-nepheline under variable P-T conditions in the presence or absence of water.

Abstract

The join diopside (Di)-nepheline (Ne) was studied to determine the upper stability limit of melilite (Mel) in nephelinitic magmas under both dry and wet conditions. Experiments at 28 kbar under dry condition show that the join Di-Ne has a pseudoeutectic at 1420°C and Ne70Di30. The solidus is attained at 1220±10°C in the compositional range of Di100-Di40Ne60, but beyond the composition Di40Ne60 toward the nepheline-rich side, the solidus is reached at 1200±10°C. Melilite is absent in the 28 kbar polythermal join. In order to explore the upper stability of melilite in the join Di-Ne under dry condition, subsolidus runs were performed between 15 and 25 kbar at 1000°C. In the run at 15 kbar, melilite coexists with nepheline and diopside but this phase disappears in the runs above 20 kbar. Investigation at 10 kbar in the presence H2O [P(H2O)=P(Total)] shows that the join Di-Ne cuts through the primary phase volumes of diopsidess, nepheline and amphiboless (Amph) with a pseudoeutectic at Di5Ne95 and 680°C. Melilite is not stable in the 10 kbar isobaric-polythermal H2O-bearing join. Variation of phase assemblages between 1 to 10 kbars and under the presence of excess water for an arbitrarily chosen composition Di40Ne60 was also described. The study shows that rocks having melilite nephelinite compositions are represented at pressures above about 18 kbar (under dry condition) by nephelinite, but in the presence of excess water by an amphibole-bearing nephelinites. The study also demonstrates that an olivine-melilite nephelinite and a melilite nephelinite crystallize from an aqueous magma only under volcanic to subvolcanic conditions.