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Clays and Clay Minerals

, Volume 48, Issue 2, pp 230–237 | Cite as

Oxygen-Isotope Fractionation between Aluminum-Hydroxide Phases and Water at <60°C: Results of Decade-Long Synthesis Experiments

  • Frédéric VitaliEmail author
  • Fred J. Longstaffe
  • Michael I. Bird
  • W. Glen E. Caldwell
Article

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, \(\alpha_{gibbsite-H_2{O}}\), 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 103ln \(\alpha_{gibbsite-H_2{O}} = 2.04 \times 10^{6}/T^{2} - 3.61 \times 10^{3}/T + 3.65\) (T in K), are not significantly different from those obtained from experiments of much shorter duration. These results, and the good agreement with \(\alpha_{gibbsite-H_2{O}}\) 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 \(\alpha_{Al(OH)_{3}-H_2{O}}\) is polymorph-dependent at low temperatures and that a significant temperature-dependent trend exists in the values of \(\alpha_{Al(OH)_{3}-H_2{O}}\). 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.

Key Words

Aluminum Hydroxides Fractionation Factor Gibbsite Oxygen Isotope Synthesis 

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Copyright information

© The Clay Minerals Society 2000

Authors and Affiliations

  • Frédéric Vitali
    • 1
    Email author
  • Fred J. Longstaffe
    • 1
  • Michael I. Bird
    • 2
  • W. Glen E. Caldwell
    • 1
  1. 1.Department of Earth SciencesThe University of Western OntarioLondonCanada
  2. 2.Research School of Earth SciencesThe Australian National UniversityCanberraAustralia

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