A stable-isotope study of lateritic bauxites

Stable-isotope analyses of gibbsitic samples from a number of lateritic bauxites suggest that the accepted oxygen- and hydrogen-isotope fractionation factors for the mineral at surficial temperatures are incorrect. A new α^ox of 1.0143 ± 0.002 and α^hy of 0.998 ± 0.015 is proposed for gibbsite, and, in addition, an α^ox of 1.0175 ± 0.001 and α^hy of 0.975 ± 0.008 is proposed for boehmite at surficial temperatures. Application of these fractionation factors suggests that gibbsite from many of the equatorial bauxites analyzed formed in equilibrium with meteoric waters similar to those which commonly accompany monsoon-type rainfall. These waters are generally depleted in ¹⁸O and D compared to precipitation derived from tradewinds. The isotopic composition of gibbsite from the mid-Tertiary Weipa bauxite in north Queensland indicates equilibrium with waters similar in isotopic composition to modern groundwaters in the region. This suggests that a monsoon-type climate, similar to that of the present, has prevailed in the region since at least the mid-Tertiary. The oxygen- and hydrogen-isotope composition of gibbsite from the Paragominas deposit in Brazil is markedly out of isotopic equilibrium with modern tradewind-derived regional rainfall but in equilibrium with waters comparable to those derived from monsoonal rains. This suggests that, since the formation of the deposit, the climate has changed and the monsoons no longer operate in the region. Thus, the link between the isotopic composition of bauxite minerals and the nature of the climate prevailing during their formation can be used to define tropical regions which have previously experienced a monsoonal climate, providing valuable information on the nature of atmospheric circulation in the past.