Oxygen Isotope Fractionation in U3O8 during Thermal Processing in Humid Atmospheres

The incorporation of oxygen isotopes from water into uranium oxides during industrial processing presents a pathway for determining a material's geographical origin. This study is founded on the hypothesis that oxygen isotopes from atmospheric water vapor will exchange with isotopes of oxygen in solid uranium oxides during thermal processing or calcination. Using a commonly encountered oxide, U3O8, the exchange kinetics and equilibrium fractionation with water vapor (in a concentration range of 50-55% relative humidity) were investigated using processing temperatures of 400, 600, and 800 degrees C. In an atmosphere containing only water vapor diluted in N-2, oxygen isotope equilibration in U3O8 occurred within 12 h at 400 degrees C and within 2 h at 600 and 800 degrees C. Fractionation factors (1000In alpha, U3O8-H2O) between the water and oxide were -12.1, -11.0, and -8.0 at 400, 600, and 800 degrees C, respectively. With both humidity and O-2 present in the calcining atmosphere, isotopic equilibration is attained within 2 h at and above 400 degrees C. In this mixed atmosphere, which was designed to emulate Earth's troposphere, isotopes are incorporated preferentially from water vapor at 400 degrees C and from O-2 at 600 and 800 degrees C. Rapid and temperature/species-dependent isotope exchange also elucidated the impact of retrograde exchange in humid air, showing a shift from O-2-dependent to H2O-dependent fractionation as U3O8 cooled from 800 degrees C. These results confirm that uranium oxides inherit oxygen isotopes from humidity during thermal processing, illuminating an important mechanism in the formation of this forensic signature.