Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University

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2188 M. Fayek and T. K. KyserTable 3. Uraninite–CO 2 equilibration results. aRun #Temp.(°C)Time(h) 18 OCO 2b(‰)mol CO 2(final) 18 OUr b(‰)Yield (Ur)(mol/mg)M Bal c(‰) (int.) d(‰) (fin.) d(‰)1 100 212 107.8 72.4 26.7 4.6 0.76 91.8 87.02 100 708 104.2 57.4 32.0 4.6 0.20 91.8 77.53 100 4800 101.5 71.6 34.5 5.0 1.68 91.8 71.94 100 7200 103.0 77.0 32.5 4.7 1.11 91.8 75.76 100 212 31.2 75.2 24.7 4.7 0.05 56.5 55.77 100 708 31.2 73.3 25.1 4.7 0.22 56.5 56.18 100 4800 30.1 75.7 22.0 5.4 0.39 56.5 51.99 100 7200 31.7 67.7 23.0 4.6 1.01 56.5 54.510 200 20 102.8 74.2 30.3 4.6 0.04 88.9 77.711 200 68 97.2 56.4 32.8 4.7 1.08 88.9 68.912 200 212 95.6 66.3 36.5 4.9 1.76 88.9 63.313 200 379 94.2 60.1 39.4 4.7 0.47 88.9 58.714 200 1512 83.5 69.7 53.6 4.9 0.87 88.9 32.115 200 4500 74.2 69.1 61.5 4.6 0.36 88.9 13.616 200 20 31.0 70.8 24.2 4.7 0.13 56.5 55.017 200 68 29.3 74.2 22.4 4.8 0.10 56.5 51.518 200 212 28.4 73.9 20.9 5.0 0.34 56.5 49.119 200 379 29.0 73.5 21.2 4.8 0.37 56.5 50.020 200 1512 23.3 70.2 18.9 4.6 0.83 56.5 42.121 200 4500 23.4 70.3 16.9 4.7 0.56 56.5 40.222 300 15 94.5 75.9 44.2 4.9 0.50 88.9 54.123 300 40 86.7 65.2 49.6 4.6 0.06 88.9 39.824 300 86 88.7 79.8 51.0 4.7 0.63 88.9 40.525 300 240 77.1 72.2 64.2 4.5 1.14 88.9 13.926 300 398 77.4 69.3 58.2 4.5 0.55 88.9 20.627 300 732 74.3 83.2 68.5 4.5 2.18 88.9 6.228 300 15 25.0 73.7 19.0 4.7 0.11 56.5 43.929 300 40 23.8 73.9 17.2 4.7 1.52 56.5 40.930 300 86 23.7 76.3 15.8 4.7 0.64 56.5 39.331 300 240 19.0 70.7 12.2 4.6 0.74 56.5 31.132 300 398 20.7 72.2 13.4 4.7 1.69 56.5 34.033 300 732 19.9 67.5 13.7 4.5 0.03 56.5 33.534 100 7200 61.9 61.9 33.5 5.0 0.32 52.2 29.835 200 4500 47.2 71.2 35.6 6.0 6.86 52.2 12.136 300 732 65.2 78.2 36.4 4.8 0.68 52.2 30.337 300 15 60.9 81.0 42.1 4.8 0.85 52.2 19.8a Abbreviations: Temp. temperature; h hours; Ur uraninite; int. initial; fin. final.b 18 O of the products.c Calculated mass balance: (X 18 O) (reactants) .d 10 3 ln Ur–CO2 .3. RESULTSThe results of the exchange experiments are shown in Tables3 and 4. Oxygen-isotope mass balance for each run was calculatedby summing the product of the oxygen mass fraction andthe 18 O/ 16 O ratio for each reactant and subtracting the resultfrom a similar sum from the product. A random distribution ofpositive and negative mass balance values were calculated asexpected. Propagation of analytical errors using the method ofRees (1984) suggests that mass balance should be 0.0 1.1‰.Most runs fall within this bound, but some do not. Therandom, trace impurities in the uraninite and UO 3 startingmaterial may account for the largest discrepancies. Naturaluraninite used in these experiments contains minor amounts ofCA, Si, REEs, and Th (Pagel and Ruhlmann, 1985; Kotzer andKyser, 1993; Fayek and Kyser, 1997). High concentration (1–2wt%) of these elements may affect the rate of oxygen isotopeexchange as well as form stable oxide phases which are difficultto detect by XRD, electron microprobe or SEM becausethey are fine grained. The synthetically prepared UO 3 containedsome residual gas impurities. Heating the UO 3 powder at200°C and 300°C for 15 hours caused the sample to changecolor from bright yellow to mustard yellow. This was likely dueto the release of the residual gas. These impurities also resultedin rather variable yields (0.5 mol O 2 /mg) (Tables 3 and 4).Experiments where CO 2 gas was sealed in empty silica tubesand held at 100–300°C for up to 6 months showed that reactionbetween the CO 2 gas and silica tubes was minimal. Only smallshifts in the 18 O values of the CO 2 gas (0.8‰) occurred insamples where the 18 O values of the CO 2 gas greatly differed(100‰) from that of the silica tubes (Table 2). Moreover,diffusion of oxygen through silica glass is considerably slower(Stolper and Epstein, 1991) than diffusion of oxygen throughuraninite or UO 2 (Auskern and Belle, 1961). Therefore, CO 2loss did not occur as confirmed by our manometric measurements(Table 2).Some of these potential sources of error were difficult toquantify and increased the overall analytical error of the experiments.However, after careful consideration of thesesources of experimental error and their effect on the measureduraninite–CO 2 and UO 3 –CO 2 fractionations, only runs with a
Low temperature oxygen isotopic fractionation2189Table 4. UO 3 -CO 2 equilibration results. aRun #Temp.(°C)Time(h) 18 OCO 2b(‰)mol CO 2(final) 18 OUO 3b(‰)Yield (UO 3 )(mol/mg)M Bal c(‰) (int.) d(‰) (fin.) d(‰)1a 100 20 71.0 69.2 19.4 5.2 0.87 87.1 54.12a 100 39 83.1 72.4 15.8 5.2 2.89 87.1 70.83a 100 136 68.8 68.5 19.6 5.2 1.40 87.1 51.54a 100 176 76.6 69.2 17.9 5.2 0.02 87.1 61.65a 100 349 69.3 69.9 20.6 5.3 0.42 87.1 51.06a 100 525 73.4 70.5 23.1 5.2 1.11 87.1 52.97a 100 20 33.6 76.5 1.8 5.3 3.91 33.6 34.88a 100 39 30.2 78.3 5.8 5.2 0.12 33.6 35.69a 100 136 29.5 79.2 6.1 5.2 0.37 33.6 35.210a 100 176 29.5 78.7 5.0 5.7 0.23 33.6 34.111a 100 349 29.7 80.4 4.7 5.7 0.49 33.6 34.012a 100 525 29.5 80.4 5.1 5.4 0.19 33.6 34.213a 200 3.5 49.3 70.6 43.7 5.5 1.20 101.5 5.914a 200 7.5 46.2 71.7 42.3 5.2 3.69 101.5 4.115a 200 25 57.2 74.9 57.4 5.3 0.66 101.5 0.216a 200 384 53.2 78.4 60.6 5.2 0.04 101.5 7.817a 200 525 50.4 72.1 54.6 5.4 0.44 101.5 4.418a 200 3.5 24.2 76.7 4.0 5.5 0.53 30.7 27.919a 200 7.5 23.3 76.4 2.0 5.4 0.07 30.7 25.020a 200 25 22.8 78.2 2.8 5.3 0.58 30.7 25.321a 200 384 22.5 77.2 1.5 5.2 0.24 30.7 23.822a 200 525 22.8 79.0 1.9 5.7 0.23 30.7 24.423a 300 3.5 37.8 58.3 42.0 5.4 2.73 87.1 4.424a 300 7.5 48.2 60.6 49.6 5.2 3.78 87.1 1.525a 300 15 41.1 62.2 40.9 5.9 0.09 87.1 0.226a 300 36 37.5 61.6 42.7 5.3 0.31 87.1 5.427a 300 133 41.0 64.6 47.5 5.4 0.41 87.1 6.828a 300 173 46.7 63.2 43.9 5.8 0.36 87.1 2.929a 300 346 46.6 76.7 51.7 5.8 3.46 87.1 5.430a 300 3.5 22.9 79.2 0.5 5.5 0.96 31.7 22.131a 300 7.5 27.7 78.4 5.5 5.2 5.32 31.7 21.832a 300 15 23.4 84.4 0.3 5.6 0.09 31.7 23.433a 300 36 23.4 85.8 1.3 5.2 0.44 31.7 24.434a 300 133 24.3 103.5 0.6 5.8 0.75 31.7 23.435a 300 173 23.2 85.5 0.4 5.6 0.37 31.7 22.536a 300 346 23.1 86.3 0.3 5.5 0.08 31.7 23.137a 100 525 35.3 50.0 24.5 5.5 0.95 63.3 11.138a 200 525 28.1 64.9 42.1 5.2 0.98 63.3 14.539a 300 346 27.1 66.1 39.5 5.6 0.75 63.3 12.8a Abbreviations: Temp. temperature; h hours; int. initial; fin. final.b 18 O of the products.c Calculated mass balance: (X 18 O) (products) –(X 18 O) (reactants) .d 10 3 ln (UO3 –CO 2 ).mass balance within 1.5‰ of 0‰, the majority of our data,were included.At each temperature there are at least one set of pairedexperiments of similar duration where the mineral–CO 2 gasfractionation of the starting material was opposite in sign.Using the equations of Criss et al. (1987) and Northorp andClayton (1966), equilibrium uraninite–CO 2 and UO 3 –CO 2fractionation and percent exchange for each set of runs at agiven temperature were calculated (Table 5). The oxygen isotopefractionation factors determined experimentally for uraninite–CO2 and UO 3 –CO 2 in conjunction with the CO 2 –H 2 Ofractionation factor of Truesdell (1974) were used to calculateuraninite– and UO 3 –H 2 O fractionation factors (Table 5).The oxygen isotopic composition of uraninite and CO 2changes with time and are shown to approached equilibriumfrom two opposite directions at relatively equal rates (Figs.1a,b,c). The UO 3 –CO 2 experiments show similar trends (Figs.2a,b,c), although the UO 3 –CO 2 experiments show a higherdegree of exchange relative to uraninite–CO 2 experiments (Table5). This likely occurred because the grain size of thesynthetic UO 3 powder was much smaller (5–15 m) relative tothe uraninite (60–74 m), and decreasing the grain size generallydecreases the time required to achieve a given degree ofexchange (Cole and Ohmoto, 1986).3.1. Fractionation FactorTheoretical fractionation factors for UO 3 –H 2 O and UO 2 –H 2 O of Hattori and Halas (1982) (1) and (2), and Zheng (1995)(3) and (4) are nonlinear over the temperature range of 100°Cto 300°C (Fig. 3). Regression of these curves gives1000 ln UO2–H 2O 3.6310 6 /T 2 13.2910 3 /T 4.42,(1)1000 ln UO3–H 2O 3.2910 6 /T 2 13.6310 3 /T 4.65,(2)
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