Oxygen-isotope fractionation between aluminium hydroxides phases and water at <60�C: results of decade-long synthesis experiments

Abstract

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, α(gibbsite-H(2)O), 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, 1031n α(gibbsite-H(2)O) = 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 α(gibbsite-H(2)O) 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 α(Al((OH))(3)-H(2)O) is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of α(Al((OH))(3)-H(2)O). 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.