Oxygen-isotope fractionation between aluminum-hydroxide phases and water at <60°C:: Results of decade-long synthesis experiments

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 Ino. 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, alpha(gibbsite-H2O) were obtained: 1.0167 +/- 0.0003 (9 +/- 1 degrees C), 1.0147 +/- 0.0007 (24 +/- 2 degrees C), 1.0120 +/- 0.0003 (51 +/- 2 degrees C). These values, and the associated equation for an oxygen-isotope geothermometer for the interval 0-60 degrees C, 10(3)In alpha(gibbsite . H2O) = 2.04 x 10(6)/T-2 - 3.61 x 10(3)/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 alpha(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 alpha(Al(OH)3 . H2O) is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of alpha(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.