Oxygen-isotope fractionation in gibbsite: Synthesis experiments versus natural samples

Abstract

The following oxygen-isotope fractionation factors for the gibbsite-water system ( α g-w ) have been obtained from a series of twenty-two synthesis experiments: 1.0119 ± 0.0005(51 ± 2°C); 1.0144 ± 0.0003(25 ± 0.1°C); 1.0139 ± 0.0003(22°C ± 2°C); and 1.0162 ± 0.0012(8 ± 2°C). Isotopic equilibrium is considered to have been reached in the majority of cases because of the coincidence of results for syntheses using several different techniques. All three polymorphs of Al(OH) 3, namely, gibbsite, bayerite and nordstrandite, were identified in the synthesis products. Partial dissolution techniques have been applied to samples from twelve bauxite deposits to obtain the δ 18O value of both gibbsite and kaolinite. These data, plus previously published results, have been used to calculate α g-w values, using two independent methods: 1. (1) by comparison with the δ 18O value of associated kaolinite and 2. (2) by comparison with the δ 18O value of local meteoric waters. The results show a spread of values from those obtained by synthesis experiments to values up to ~6%. higher. It is concluded that the spread of values is real and that the δ 18O value of gibbsite is dependent upon its formation mechanism. It is suggested that the gibbsite samples which exhibit high α g-w values have either obtained their isotopic signature by inheritance during the desilication of kaolinite, or from exchange with oxygen-bearing anions such as sulphate, during the transport of aluminum as Al-OH complexes. The possibility that the high values are the result of evaporative modification of the fluid prior to gibbsite precipitation cannot be ruled out, but is unlikely, given that humid conditions are required for bauxite formation. Such conclusions suggest that the oxygen isotope composition of gibbsite is unsuitable as a palaeoclimatic indicator but may prove useful in providing genetic information on bauxite formation.