Experimental incorporation of U into xenotime at mid to lower crustal pressures and temperatures utilizing alkali-bearing fluids under reducing and oxidizing conditions

A natural gem quality, inclusion-free, U-poor xenotime has been metasomatically altered at 900 °C and 500–1000 MPa in a series of alkali-bearing fluids including 2 N KOH, 2 N NaOH, Na₂Si₂O₅ + H₂O, and NaF + H₂O to which UO₂ and SiO₂ have been added. All experiments were buffered to CO₂-CO-graphite with the exception of the NaF + H₂O experiment at 900 °C and 500 MPa, which was buffered to magnetite-hematite. With the exception of the Na₂Si₂O₅ + H₂O experiment, the xenotime reacted with the fluid tried in all of the remaining experiments to varying degrees. The xenotime showed the greatest degree of reactivity in the two NaF + H₂O experiments tried. This reactivity takes the form of a coupled dissolution-reprecipitation reaction, which involves enriching reacted areas of the xenotime in U + Si as well as remobilizing the HREE in these areas inherent to the original xenotime. One result was the formation of alternating bands of Y-rich and HREE- + U-rich bands in the NaF + H₂O experiment when the system was oxidized, which are presumed to have formed due a combination of simple chemical zoning coupled with diffusion and coupled dissolution-reprecipitation in the altered areas of the xenotime. A total lack of fluid and mineral inclusions on the nano-scale, as well as no disturbances in the crystal lattice in the reacted areas of the xenotime under HRTEM imaging, indicated that total recrystallization of the reacted areas took place during the course of the NaF + H₂O experiment. The results of these experiments has important implications for the metasomatic resetting of the xenotime geochromometer in the presence of alkali-bearing fluids, which would potentially allow metasomatised xenotime to time and record metasomatic events affecting the rock.