The nepheline syenite on Stjernöy is characterized by uniformly low U and Th contents. This observation, when regarded in conjunction with the high K contents of the rock, indicates that the nepheline syenite has not resulted from a liquid fractionation process governed by a liquid - solid equilibrium. If the nepheline syenite is magmatic, a separate vapor phase capable of selectively concentrating U and Th should have been active during the crystallization. The hypothesis is suggested that this vapor is related to the fluid which formed the carbonatite around the nepheline syenite, but the data are inconclusive. The carbonatite is higher than the nepheline syenite in both U and Th, and lower in K, however, the K content is higher than that typically associated with similar U and Th concentrations in magmatic rocks.
A note on the U, Th, and K contents in the nepheline syenite and carbonatite on Stjernöy, North Norway.