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OXYGEN-ISOTOPE FRACTIONATION BETWEEN ALUMINUM-HYDROXIDE PHASES AND WATER AT <60°C: RESULTS OF DECADE-LONG SYNTHESIS EXPERIMENTS

¹ Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7 Canada
² Research School of Earth Sciences, The Australian National University, Canberra, A.C.T. 0200, Australia

E-mail of corresponding author: fvitali{at}julian.uwo.ca

Oxygen-isotope data were obtained for synthetic aluminum-hydroxide phases precipitated over 65–125 mo and have been compared to results from similar experiments conducted for 3–56 mo. The Al(OH)₃ polymorphs, gibbsite, nordstrandite, and bayerite, were synthesized, but gibbsite was dominant in most samples, and commonly the only phase present. Using pure gibbsite samples, the following oxygen-isotope fractionation factors, _gibbsite-H2O, were obtained: 1.0167 ± 0.0003 (9 ± 1°C), 1.0147 ± 0.0007 (24 ± 2°C), 1.0120 ± 0.0003 (51 ± 2°C). These values, and the associated equation for an oxygen-isotope geothermometer for the interval 0–60°C, 10³ln _gibbsite-H2O = 2.04 x 10⁶/T² – 3.61 x 10³/T + 3.65 (T in K), are not significantly different from those obtained from experiments of much shorter duration. These results, and the good agreement with _gibbsite-H2O values obtained for well-constrained natural systems, suggest that the experimentally determined fractionation factors describe equilibrium conditions for gibbsite that has precipitated directly from solution.

As also proposed by others using a modified-increment calculation, our synthesis experiments suggest that _Al(OH)3-H2O is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of _{Al(OH) 3-H2O}. However, previously calculated fractionation factors obtained using the modified-increment method are higher than those obtained from the experiments, with this discrepancy becoming larger as temperature decreases.

Key Words: Aluminum Hydroxides • Fractionation Factor • Gibbsite • Oxygen Isotope • Synthesis

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