K,U and Th distribution between dry and wet facies of a syenitic intrusion and the role of fluid content

Syenitic intrusions of the Laramie anorthosite complex consist of a biotite-hornblende syenite and a pyroxene syenite. Rocks of the two related types of syenite show practically the same major element chemistry, but concentrations of U and Th are much higher in the “wet” biotite-hornblende syenite than in the “dry” pyroxene syenite. A strong empirical relationship between hydrous mineralogy and higher U and Th concentrations is established. If U and Th are strongly concentrated upward in granitic intrusions this cannot be caused by differentiation affecting major elements; such and upward concentration of U and Th could be caused by upward differentiation of H₂O.