Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite ( -Al(OH3)): CO2 abundance and C values

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO2 and H2O. “Plateau” F (CO2/H2O ratios) and C values of the CO2 derived from gibbsite were attained over the dehydration interval, Xv(H2) 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 0.0003, while the corresponding C value of evolved CO2 was 16.0‰ 0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of C-depleted CO2 ( 46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or C for CO2 evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO2 occluded in gibbsite may preserve information on C values of CO2 in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO2 occluded in gibbsite and CO2 in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems. Copyright 0016-7037/05 $30.00 .00