Resetting the uraninite geochronometer metasomatically: The role of Na-Ca-F-OH-bearing fluids at 450 and 600 °C and 200 MPa

Abstract

Uraninite [UO2] is an increasingly recognized accessory mineral for geochronological studies of the mid to upper crust. Similar to what is seen for zircon and monazite, the U-Pb system of uraninite can become reset under relatively low temperatures in certain domains via the action of fluids through the process of coupled dissolution-reprecipitation. Whether or not the uraninite geochronometer is reset will be dependent on the chemistry of the fluid it interacts with as opposed to being purely dependent on P-T. This makes uraninite a mineral of interest for the dating of low- to mid-temperature, fluid-controlled geological processes. In order to better understand which factors cause the recrystallization and/or metasomatic alteration of uraninite, a set of 5 metasomatism experiments have been performed in cold seal autoclaves on a hydrothermal line involving a natural uraninite from Příbram, Czech Republic and a series of Na-, Ca-, OH-, and F-bearing fluids at 600 °C and 200 MPa for 21 days. A second set of the same 5 experiments, to which elemental sulfur was added, were subsequently run at 450 °C and 200 MPa for 66 days. Generally, little textural alteration of the starting material was observed in any of the experiments, which was independent of the fluid chemistry and temperature, except for an increase in the apparent porosity of the reacted grains. In the second set of experiments galena formed as small grains in four of the runs, indicating that Pb had migrated out from the uraninite into the solution and reacted with the sulfur to form galena. The excessive depletion of Pb in the metasomatized uraninite to negligible amounts in some of these fluids is especially evident if the solutions were NaF + H2O and 2 M NaOH. This suggests that interaction of uraninite with F- or high pH Na-bearing fluids can metasomatically reset the uraninite geochronometer at 450 °C and mid to upper crustal pressures and by analogy to even lower temperatures given sufficient time.