Stable-isotope evidence for low-temperature kaolinitic weathering and post-formational hydrogen-isotope exchange in permian kaolinites

Abstract

Depleted δ 18O- and δD-values of +6.1 to +9.4‰ and −107 to −94‰, respectively, have been obtained from kaolinite samples of early Permian (Sakmarian-Artinskian) age from the Gunnedah Basin, eastern Australia. The samples come from kaolinite clayrocks representing both in situ kaolinitic weathering profiles and the eroded and re-deposited products of this weathering developed in the Quirindi-Wingen and Boggabri-Gunnedah areas of New South Wales, Australia. The clayrocks were produced by the intense surficial weathering of an underlying volcanic sequence, with subsequent erosion and re-deposition in basins flanking the volcanic pile. The calculated isotopic composition of meteoric water in equilibrium with kaolinite from the clayrocks had δ 18O of ⩽ −17‰ and δD of ⩽ −125‰, indicating polar or sub-polar temperatures of condensation. As kaolinite formation cannot occur below the freezing point of water, the highly depleted isotopic results obtained from the samples suggest equilibrium with waters partly derived from deglaciation of the Gondwanan landmass, an event which consequently must have been well underway in eastern Australia by the Sakmarian-Artinskian. The climate prevailing at the time of kaolinite formation is unlikely to have been warmer than cool temperate, a conclusion which is difficult to reconcile with the classical interpretation of kaolinitic weathering as a tropical or subtropical phenomenon. This suggests that high rates of infiltration, efficient leaching of soluble cations, and good drainage are of more importance to kaolinite formation than the temperature at which the process operates. Oxygen-isotope exchange in the kaolinite samples since formation is thought to have been negligible, however, marked post-formational hydrogen-isotope exchange is demonstrable at temperatures not exceeding ∼ 80°C and probably considerably lower in some cases.