Calculation of oxygen isotope fractionation between uranium dioxide, uranium trioxide and water

Abstract

Reduced partition function ratios (r.p.f.r) for uranium dioxide and uranium trioxide are calculated using selected crystal field models for optical modes and Debye functions for acoustic modes at temperatures ranging from 0 to 1000°C. The contribution of the acoustic modes is negligibly small in the calculated values of r.p.f.r. in all temperature ranges. The uncertainty of the r.p.f.r. is calculated from the standard deviation of the force constant used for the calculation, which is about ± 5%. at 0°C, ±2%. at 200°C and ±0.2%. at 1000°C for both oxides. The r.p.f.r. for both uranium oxides are lower than that of water, implying that uraninite is depleted in 18O with respect to associated fluids at almost all temperature ranges. The fractionation factors between uranium oxides and water are expressed as: 10 3 ln α = 3.29 10 6 T 2 −13.63 10 3 T + 4.65 , and 10 3 ln α = 3.63 10 6 T 2 −13.29 10 3 T + 4.42 for α-UO 3 and UO 2, respectively. The values of r.p.f.r. for uranium oxides are low compared to those for common rock-forming minerals, however they are similar to the values for other metal oxides, such as magnetite, rutile and probably cassiterite.